diff --git a/CHANGELOG.md b/CHANGELOG.md
index d861cea..36aa1d7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,16 @@
+# TBD
+
+ - **implement `@use` and the module system**
+ - support the filter syntax for function arguments, e.g. `alpha(opacity=1)`
+ - disallow certain at-rules in functions, resolving several panics
+ - allow vendor-prefixed special CSS functions, e.g. `-webkit-calc(...)`
+ - allow decimal percent selectors inside `@keyframes`
+ - allow vendor-prefixed `@keyframes`
+ - resolve parsing bug for maps involving silent comments
+ - allow escaped `!` in selectors
+ - allow multiline comments in functions
+ - resolve several panics on malformed input when parsing bracketed lists
+
 # 0.10.0
 
  - bugfixes for `@media` query regressions
diff --git a/README.md b/README.md
index cf3ed26..531cf72 100644
--- a/README.md
+++ b/README.md
@@ -25,7 +25,7 @@ The large features remaining are
 ```
 indented syntax
 css imports
-@use, @forward, and the module system
+@forward
 compressed output
 ```
 
@@ -77,6 +77,13 @@ cargo b --release
 
 These numbers come from a default run of the Sass specification as shown above.
 
+```
+2020-08-07
+PASSING: 3375
+FAILING: 1718
+TOTAL: 5093
+```
+
 ```
 2020-07-24
 PASSING: 2935
diff --git a/src/atrule/mixin.rs b/src/atrule/mixin.rs
index a85bcf8..b89d2f7 100644
--- a/src/atrule/mixin.rs
+++ b/src/atrule/mixin.rs
@@ -1,21 +1,67 @@
-use crate::{args::FuncArgs, Token};
+use std::fmt;
+
+use crate::{
+    args::{CallArgs, FuncArgs},
+    error::SassResult,
+    parse::{Parser, Stmt},
+    Token,
+};
+
+pub(crate) type BuiltinMixin = fn(CallArgs, &mut Parser<'_>) -> SassResult<Vec<Stmt>>;
+
+#[derive(Clone)]
+pub(crate) enum Mixin {
+    UserDefined(UserDefinedMixin),
+    Builtin(BuiltinMixin),
+}
+
+impl fmt::Debug for Mixin {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::UserDefined(u) => f
+                .debug_struct("UserDefinedMixin")
+                .field("args", &u.args)
+                .field("body", &u.body)
+                .field("accepts_content_block", &u.accepts_content_block)
+                .field("declared_at_root", &u.declared_at_root)
+                .finish(),
+            Self::Builtin(..) => f.debug_struct("BuiltinMixin").finish(),
+        }
+    }
+}
+
+impl Mixin {
+    pub fn new_user_defined(
+        args: FuncArgs,
+        body: Vec<Token>,
+        accepts_content_block: bool,
+        declared_at_root: bool,
+    ) -> Self {
+        Mixin::UserDefined(UserDefinedMixin::new(
+            args,
+            body,
+            accepts_content_block,
+            declared_at_root,
+        ))
+    }
+}
 
 #[derive(Debug, Clone)]
-pub(crate) struct Mixin {
+pub(crate) struct UserDefinedMixin {
     pub args: FuncArgs,
     pub body: Vec<Token>,
     pub accepts_content_block: bool,
     pub declared_at_root: bool,
 }
 
-impl Mixin {
+impl UserDefinedMixin {
     pub fn new(
         args: FuncArgs,
         body: Vec<Token>,
         accepts_content_block: bool,
         declared_at_root: bool,
     ) -> Self {
-        Mixin {
+        Self {
             args,
             body,
             accepts_content_block,
diff --git a/src/atrule/mod.rs b/src/atrule/mod.rs
index 7c0bda1..80304d0 100644
--- a/src/atrule/mod.rs
+++ b/src/atrule/mod.rs
@@ -1,6 +1,5 @@
 pub(crate) use function::Function;
 pub(crate) use kind::AtRuleKind;
-pub(crate) use mixin::{Content, Mixin};
 pub(crate) use supports::SupportsRule;
 pub(crate) use unknown::UnknownAtRule;
 
@@ -8,6 +7,6 @@ mod function;
 pub mod keyframes;
 mod kind;
 pub mod media;
-mod mixin;
+pub mod mixin;
 mod supports;
 mod unknown;
diff --git a/src/builtin/color/hsl.rs b/src/builtin/functions/color/hsl.rs
similarity index 81%
rename from src/builtin/color/hsl.rs
rename to src/builtin/functions/color/hsl.rs
index 64c07a6..b97ef24 100644
--- a/src/builtin/color/hsl.rs
+++ b/src/builtin/functions/color/hsl.rs
@@ -35,7 +35,8 @@ fn inner_hsl(name: &'static str, mut args: CallArgs, parser: &mut Parser<'_>) ->
         }
 
         let lightness = match channels.pop() {
-            Some(Value::Dimension(n, ..)) => n / Number::from(100),
+            Some(Value::Dimension(Some(n), ..)) => n / Number::from(100),
+            Some(Value::Dimension(None, ..)) => todo!(),
             Some(v) => {
                 return Err((
                     format!("$lightness: {} is not a number.", v.inspect(args.span())?),
@@ -47,7 +48,8 @@ fn inner_hsl(name: &'static str, mut args: CallArgs, parser: &mut Parser<'_>) ->
         };
 
         let saturation = match channels.pop() {
-            Some(Value::Dimension(n, ..)) => n / Number::from(100),
+            Some(Value::Dimension(Some(n), ..)) => n / Number::from(100),
+            Some(Value::Dimension(None, ..)) => todo!(),
             Some(v) => {
                 return Err((
                     format!("$saturation: {} is not a number.", v.inspect(args.span())?),
@@ -59,7 +61,8 @@ fn inner_hsl(name: &'static str, mut args: CallArgs, parser: &mut Parser<'_>) ->
         };
 
         let hue = match channels.pop() {
-            Some(Value::Dimension(n, ..)) => n,
+            Some(Value::Dimension(Some(n), ..)) => n,
+            Some(Value::Dimension(None, ..)) => todo!(),
             Some(v) => {
                 return Err((
                     format!("$hue: {} is not a number.", v.inspect(args.span())?),
@@ -78,7 +81,8 @@ fn inner_hsl(name: &'static str, mut args: CallArgs, parser: &mut Parser<'_>) ->
         ))))
     } else {
         let hue = match args.get_err(0, "hue")? {
-            Value::Dimension(n, ..) => n,
+            Value::Dimension(Some(n), ..) => n,
+            Value::Dimension(None, ..) => todo!(),
             v if v.is_special_function() => {
                 let saturation = args.get_err(1, "saturation")?;
                 let lightness = args.get_err(2, "lightness")?;
@@ -105,7 +109,8 @@ fn inner_hsl(name: &'static str, mut args: CallArgs, parser: &mut Parser<'_>) ->
             }
         };
         let saturation = match args.get_err(1, "saturation")? {
-            Value::Dimension(n, ..) => n / Number::from(100),
+            Value::Dimension(Some(n), ..) => n / Number::from(100),
+            Value::Dimension(None, ..) => todo!(),
             v if v.is_special_function() => {
                 let lightness = args.get_err(2, "lightness")?;
                 let mut string = format!(
@@ -134,7 +139,8 @@ fn inner_hsl(name: &'static str, mut args: CallArgs, parser: &mut Parser<'_>) ->
             }
         };
         let lightness = match args.get_err(2, "lightness")? {
-            Value::Dimension(n, ..) => n / Number::from(100),
+            Value::Dimension(Some(n), ..) => n / Number::from(100),
+            Value::Dimension(None, ..) => todo!(),
             v if v.is_special_function() => {
                 let mut string = format!(
                     "{}({}, {}, {}",
@@ -164,10 +170,11 @@ fn inner_hsl(name: &'static str, mut args: CallArgs, parser: &mut Parser<'_>) ->
         let alpha = match args.default_arg(
             3,
             "alpha",
-            Value::Dimension(Number::one(), Unit::None, true),
+            Value::Dimension(Some(Number::one()), Unit::None, true),
         )? {
-            Value::Dimension(n, Unit::None, _) => n,
-            Value::Dimension(n, Unit::Percent, _) => n / Number::from(100),
+            Value::Dimension(Some(n), Unit::None, _) => n,
+            Value::Dimension(Some(n), Unit::Percent, _) => n / Number::from(100),
+            Value::Dimension(None, ..) => todo!(),
             v @ Value::Dimension(..) => {
                 return Err((
                     format!(
@@ -205,18 +212,18 @@ fn inner_hsl(name: &'static str, mut args: CallArgs, parser: &mut Parser<'_>) ->
     }
 }
 
-fn hsl(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn hsl(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     inner_hsl("hsl", args, parser)
 }
 
-fn hsla(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn hsla(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     inner_hsl("hsla", args, parser)
 }
 
-fn hue(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn hue(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "color")? {
-        Value::Color(c) => Ok(Value::Dimension(c.hue(), Unit::Deg, true)),
+        Value::Color(c) => Ok(Value::Dimension(Some(c.hue()), Unit::Deg, true)),
         v => Err((
             format!("$color: {} is not a color.", v.inspect(args.span())?),
             args.span(),
@@ -225,10 +232,10 @@ fn hue(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     }
 }
 
-fn saturation(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn saturation(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "color")? {
-        Value::Color(c) => Ok(Value::Dimension(c.saturation(), Unit::Percent, true)),
+        Value::Color(c) => Ok(Value::Dimension(Some(c.saturation()), Unit::Percent, true)),
         v => Err((
             format!("$color: {} is not a color.", v.inspect(args.span())?),
             args.span(),
@@ -237,10 +244,10 @@ fn saturation(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value>
     }
 }
 
-fn lightness(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn lightness(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "color")? {
-        Value::Color(c) => Ok(Value::Dimension(c.lightness(), Unit::Percent, true)),
+        Value::Color(c) => Ok(Value::Dimension(Some(c.lightness()), Unit::Percent, true)),
         v => Err((
             format!("$color: {} is not a color.", v.inspect(args.span())?),
             args.span(),
@@ -249,7 +256,7 @@ fn lightness(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     }
 }
 
-fn adjust_hue(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn adjust_hue(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(2)?;
     let color = match args.get_err(0, "color")? {
         Value::Color(c) => c,
@@ -262,7 +269,8 @@ fn adjust_hue(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value>
         }
     };
     let degrees = match args.get_err(1, "degrees")? {
-        Value::Dimension(n, ..) => n,
+        Value::Dimension(Some(n), ..) => n,
+        Value::Dimension(None, ..) => todo!(),
         v => {
             return Err((
                 format!(
@@ -290,7 +298,8 @@ fn lighten(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
         }
     };
     let amount = match args.get_err(1, "amount")? {
-        Value::Dimension(n, u, _) => bound!(args, "amount", n, u, 0, 100) / Number::from(100),
+        Value::Dimension(Some(n), u, _) => bound!(args, "amount", n, u, 0, 100) / Number::from(100),
+        Value::Dimension(None, ..) => todo!(),
         v => {
             return Err((
                 format!(
@@ -318,7 +327,8 @@ fn darken(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
         }
     };
     let amount = match args.get_err(1, "amount")? {
-        Value::Dimension(n, u, _) => bound!(args, "amount", n, u, 0, 100) / Number::from(100),
+        Value::Dimension(Some(n), u, _) => bound!(args, "amount", n, u, 0, 100) / Number::from(100),
+        Value::Dimension(None, ..) => todo!(),
         v => {
             return Err((
                 format!(
@@ -346,7 +356,8 @@ fn saturate(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     }
 
     let amount = match args.get_err(1, "amount")? {
-        Value::Dimension(n, u, _) => bound!(args, "amount", n, u, 0, 100) / Number::from(100),
+        Value::Dimension(Some(n), u, _) => bound!(args, "amount", n, u, 0, 100) / Number::from(100),
+        Value::Dimension(None, ..) => todo!(),
         v => {
             return Err((
                 format!(
@@ -360,7 +371,7 @@ fn saturate(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     };
     let color = match args.get_err(0, "color")? {
         Value::Color(c) => c,
-        Value::Dimension(n, u, _) => {
+        Value::Dimension(Some(n), u, _) => {
             return Ok(Value::String(
                 format!("saturate({}{})", n, u),
                 QuoteKind::None,
@@ -390,7 +401,8 @@ fn desaturate(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value>
         }
     };
     let amount = match args.get_err(1, "amount")? {
-        Value::Dimension(n, u, _) => bound!(args, "amount", n, u, 0, 100) / Number::from(100),
+        Value::Dimension(Some(n), u, _) => bound!(args, "amount", n, u, 0, 100) / Number::from(100),
+        Value::Dimension(None, ..) => todo!(),
         v => {
             return Err((
                 format!(
@@ -405,11 +417,11 @@ fn desaturate(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value>
     Ok(Value::Color(Box::new(color.desaturate(amount))))
 }
 
-fn grayscale(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn grayscale(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     let color = match args.get_err(0, "color")? {
         Value::Color(c) => c,
-        Value::Dimension(n, u, _) => {
+        Value::Dimension(Some(n), u, _) => {
             return Ok(Value::String(
                 format!("grayscale({}{})", n, u),
                 QuoteKind::None,
@@ -426,7 +438,7 @@ fn grayscale(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     Ok(Value::Color(Box::new(color.desaturate(Number::one()))))
 }
 
-fn complement(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn complement(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     let color = match args.get_err(0, "color")? {
         Value::Color(c) => c,
@@ -441,14 +453,15 @@ fn complement(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value>
     Ok(Value::Color(Box::new(color.complement())))
 }
 
-fn invert(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn invert(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(2)?;
     let weight = match args.default_arg(
         1,
         "weight",
-        Value::Dimension(Number::from(100), Unit::Percent, true),
+        Value::Dimension(Some(Number::from(100)), Unit::Percent, true),
     )? {
-        Value::Dimension(n, u, _) => bound!(args, "weight", n, u, 0, 100) / Number::from(100),
+        Value::Dimension(Some(n), u, _) => bound!(args, "weight", n, u, 0, 100) / Number::from(100),
+        Value::Dimension(None, ..) => todo!(),
         v => {
             return Err((
                 format!(
@@ -462,9 +475,10 @@ fn invert(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     };
     match args.get_err(0, "color")? {
         Value::Color(c) => Ok(Value::Color(Box::new(c.invert(weight)))),
-        Value::Dimension(n, Unit::Percent, _) => {
+        Value::Dimension(Some(n), Unit::Percent, _) => {
             Ok(Value::String(format!("invert({}%)", n), QuoteKind::None))
         }
+        Value::Dimension(None, ..) => todo!(),
         Value::Dimension(..) => Err((
             "Only one argument may be passed to the plain-CSS invert() function.",
             args.span(),
diff --git a/src/builtin/color/mod.rs b/src/builtin/functions/color/mod.rs
similarity index 76%
rename from src/builtin/color/mod.rs
rename to src/builtin/functions/color/mod.rs
index 42bb71d..55c6425 100644
--- a/src/builtin/color/mod.rs
+++ b/src/builtin/functions/color/mod.rs
@@ -1,9 +1,9 @@
 use super::{Builtin, GlobalFunctionMap};
 
-mod hsl;
-mod opacity;
-mod other;
-mod rgb;
+pub mod hsl;
+pub mod opacity;
+pub mod other;
+pub mod rgb;
 
 pub(crate) fn declare(f: &mut GlobalFunctionMap) {
     hsl::declare(f);
diff --git a/src/builtin/color/opacity.rs b/src/builtin/functions/color/opacity.rs
similarity index 79%
rename from src/builtin/color/opacity.rs
rename to src/builtin/functions/color/opacity.rs
index 313ee60..2686a23 100644
--- a/src/builtin/color/opacity.rs
+++ b/src/builtin/functions/color/opacity.rs
@@ -5,35 +5,18 @@ use crate::{
     value::Value,
 };
 
+/// Check if `s` matches the regex `^[a-zA-Z]+\s*=`
 fn is_ms_filter(s: &str) -> bool {
-    let mut chars = s.chars();
+    let mut bytes = s.bytes();
 
-    if let Some(c) = chars.next() {
-        if !matches!(c, 'a'..='z' | 'A'..='Z') {
-            return false;
-        }
-    } else {
+    if !bytes.next().map_or(false, |c| c.is_ascii_alphabetic()) {
         return false;
     }
 
-    for c in &mut chars {
-        match c {
-            ' ' | '\t' | '\n' => break,
-            'a'..='z' | 'A'..='Z' => continue,
-            '=' => return true,
-            _ => return false,
-        }
-    }
-
-    for c in chars {
-        match c {
-            ' ' | '\t' | '\n' => continue,
-            '=' => return true,
-            _ => return false,
-        }
-    }
-
-    false
+    bytes
+        .skip_while(u8::is_ascii_alphabetic)
+        .find(|c| !matches!(c, b' ' | b'\t' | b'\n'))
+        == Some(b'=')
 }
 
 #[cfg(test)]
@@ -52,10 +35,10 @@ mod test {
     }
 }
 
-fn alpha(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn alpha(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     if args.len() <= 1 {
         match args.get_err(0, "color")? {
-            Value::Color(c) => Ok(Value::Dimension(c.alpha(), Unit::None, true)),
+            Value::Color(c) => Ok(Value::Dimension(Some(c.alpha()), Unit::None, true)),
             Value::String(s, QuoteKind::None) if is_ms_filter(&s) => {
                 Ok(Value::String(format!("alpha({})", s), QuoteKind::None))
             }
@@ -86,14 +69,15 @@ fn alpha(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     }
 }
 
-fn opacity(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn opacity(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "color")? {
-        Value::Color(c) => Ok(Value::Dimension(c.alpha(), Unit::None, true)),
-        Value::Dimension(num, unit, _) => Ok(Value::String(
+        Value::Color(c) => Ok(Value::Dimension(Some(c.alpha()), Unit::None, true)),
+        Value::Dimension(Some(num), unit, _) => Ok(Value::String(
             format!("opacity({}{})", num, unit),
             QuoteKind::None,
         )),
+        Value::Dimension(None, ..) => todo!(),
         v => Err((
             format!("$color: {} is not a color.", v.inspect(args.span())?),
             args.span(),
@@ -102,6 +86,7 @@ fn opacity(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     }
 }
 
+// todo: unify `opacify` and `fade_in`
 fn opacify(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(2)?;
     let color = match args.get_err(0, "color")? {
@@ -115,7 +100,8 @@ fn opacify(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
         }
     };
     let amount = match args.get_err(1, "amount")? {
-        Value::Dimension(n, u, _) => bound!(args, "amount", n, u, 0, 1),
+        Value::Dimension(Some(n), u, _) => bound!(args, "amount", n, u, 0, 1),
+        Value::Dimension(None, ..) => todo!(),
         v => {
             return Err((
                 format!("$amount: {} is not a number.", v.inspect(args.span())?),
@@ -140,7 +126,8 @@ fn fade_in(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
         }
     };
     let amount = match args.get_err(1, "amount")? {
-        Value::Dimension(n, u, _) => bound!(args, "amount", n, u, 0, 1),
+        Value::Dimension(Some(n), u, _) => bound!(args, "amount", n, u, 0, 1),
+        Value::Dimension(None, ..) => todo!(),
         v => {
             return Err((
                 format!("$amount: {} is not a number.", v.inspect(args.span())?),
@@ -152,6 +139,7 @@ fn fade_in(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     Ok(Value::Color(Box::new(color.fade_in(amount))))
 }
 
+// todo: unify with `fade_out`
 fn transparentize(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(2)?;
     let color = match args.get_err(0, "color")? {
@@ -165,7 +153,8 @@ fn transparentize(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Val
         }
     };
     let amount = match args.get_err(1, "amount")? {
-        Value::Dimension(n, u, _) => bound!(args, "amount", n, u, 0, 1),
+        Value::Dimension(Some(n), u, _) => bound!(args, "amount", n, u, 0, 1),
+        Value::Dimension(None, ..) => todo!(),
         v => {
             return Err((
                 format!("$amount: {} is not a number.", v.inspect(args.span())?),
@@ -190,7 +179,8 @@ fn fade_out(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
         }
     };
     let amount = match args.get_err(1, "amount")? {
-        Value::Dimension(n, u, _) => bound!(args, "amount", n, u, 0, 1),
+        Value::Dimension(Some(n), u, _) => bound!(args, "amount", n, u, 0, 1),
+        Value::Dimension(None, ..) => todo!(),
         v => {
             return Err((
                 format!("$amount: {} is not a number.", v.inspect(args.span())?),
diff --git a/src/builtin/color/other.rs b/src/builtin/functions/color/other.rs
similarity index 90%
rename from src/builtin/color/other.rs
rename to src/builtin/functions/color/other.rs
index af1367d..5b0b66e 100644
--- a/src/builtin/color/other.rs
+++ b/src/builtin/functions/color/other.rs
@@ -15,7 +15,8 @@ use crate::{
 macro_rules! opt_rgba {
     ($args:ident, $name:ident, $arg:literal, $low:literal, $high:literal) => {
         let $name = match $args.default_named_arg($arg, Value::Null)? {
-            Value::Dimension(n, u, _) => Some(bound!($args, $arg, n, u, $low, $high)),
+            Value::Dimension(Some(n), u, _) => Some(bound!($args, $arg, n, u, $low, $high)),
+            Value::Dimension(None, ..) => todo!(),
             Value::Null => None,
             v => {
                 return Err((
@@ -31,9 +32,10 @@ macro_rules! opt_rgba {
 macro_rules! opt_hsl {
     ($args:ident, $name:ident, $arg:literal, $low:literal, $high:literal) => {
         let $name = match $args.default_named_arg($arg, Value::Null)? {
-            Value::Dimension(n, u, _) => {
+            Value::Dimension(Some(n), u, _) => {
                 Some(bound!($args, $arg, n, u, $low, $high) / Number::from(100))
             }
+            Value::Dimension(None, ..) => todo!(),
             Value::Null => None,
             v => {
                 return Err((
@@ -46,7 +48,7 @@ macro_rules! opt_hsl {
     };
 }
 
-fn change_color(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn change_color(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     if args.positional_arg(1).is_some() {
         return Err((
             "Only one positional argument is allowed. All other arguments must be passed by name.",
@@ -81,7 +83,8 @@ fn change_color(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value
     }
 
     let hue = match args.default_named_arg("hue", Value::Null)? {
-        Value::Dimension(n, ..) => Some(n),
+        Value::Dimension(Some(n), ..) => Some(n),
+        Value::Dimension(None, ..) => todo!(),
         Value::Null => None,
         v => {
             return Err((
@@ -113,7 +116,7 @@ fn change_color(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value
     }))
 }
 
-fn adjust_color(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn adjust_color(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     let color = match args.get_err(0, "color")? {
         Value::Color(c) => c,
         v => {
@@ -140,7 +143,8 @@ fn adjust_color(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value
     }
 
     let hue = match args.default_named_arg("hue", Value::Null)? {
-        Value::Dimension(n, ..) => Some(n),
+        Value::Dimension(Some(n), ..) => Some(n),
+        Value::Dimension(None, ..) => todo!(),
         Value::Null => None,
         v => {
             return Err((
@@ -175,8 +179,8 @@ fn adjust_color(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value
 
 #[allow(clippy::cognitive_complexity)]
 // todo: refactor into rgb and hsl?
-fn scale_color(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
-    fn scale(val: Number, by: Number, max: Number) -> Number {
+pub(crate) fn scale_color(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+    pub(crate) fn scale(val: Number, by: Number, max: Number) -> Number {
         if by.is_zero() {
             return val;
         }
@@ -198,9 +202,10 @@ fn scale_color(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value>
     macro_rules! opt_scale_arg {
         ($args:ident, $name:ident, $arg:literal, $low:literal, $high:literal) => {
             let $name = match $args.default_named_arg($arg, Value::Null)? {
-                Value::Dimension(n, Unit::Percent, _) => {
+                Value::Dimension(Some(n), Unit::Percent, _) => {
                     Some(bound!($args, $arg, n, Unit::Percent, $low, $high) / Number::from(100))
                 }
+                Value::Dimension(None, ..) => todo!(),
                 v @ Value::Dimension(..) => {
                     return Err((
                         format!(
@@ -288,7 +293,7 @@ fn scale_color(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value>
     }))
 }
 
-fn ie_hex_str(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn ie_hex_str(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     let color = match args.get_err(0, "color")? {
         Value::Color(c) => c,
diff --git a/src/builtin/color/rgb.rs b/src/builtin/functions/color/rgb.rs
similarity index 82%
rename from src/builtin/color/rgb.rs
rename to src/builtin/functions/color/rgb.rs
index 3249f44..9bf36ab 100644
--- a/src/builtin/color/rgb.rs
+++ b/src/builtin/functions/color/rgb.rs
@@ -38,10 +38,11 @@ fn inner_rgb(name: &'static str, mut args: CallArgs, parser: &mut Parser<'_>) ->
         }
 
         let blue = match channels.pop() {
-            Some(Value::Dimension(n, Unit::None, _)) => n,
-            Some(Value::Dimension(n, Unit::Percent, _)) => {
+            Some(Value::Dimension(Some(n), Unit::None, _)) => n,
+            Some(Value::Dimension(Some(n), Unit::Percent, _)) => {
                 (n / Number::from(100)) * Number::from(255)
             }
+            Some(Value::Dimension(None, ..)) => todo!(),
             Some(v) if v.is_special_function() => {
                 let green = channels.pop().unwrap();
                 let red = channels.pop().unwrap();
@@ -67,10 +68,11 @@ fn inner_rgb(name: &'static str, mut args: CallArgs, parser: &mut Parser<'_>) ->
         };
 
         let green = match channels.pop() {
-            Some(Value::Dimension(n, Unit::None, _)) => n,
-            Some(Value::Dimension(n, Unit::Percent, _)) => {
+            Some(Value::Dimension(Some(n), Unit::None, _)) => n,
+            Some(Value::Dimension(Some(n), Unit::Percent, _)) => {
                 (n / Number::from(100)) * Number::from(255)
             }
+            Some(Value::Dimension(None, ..)) => todo!(),
             Some(v) if v.is_special_function() => {
                 let string = match channels.pop() {
                     Some(red) => format!(
@@ -95,10 +97,11 @@ fn inner_rgb(name: &'static str, mut args: CallArgs, parser: &mut Parser<'_>) ->
         };
 
         let red = match channels.pop() {
-            Some(Value::Dimension(n, Unit::None, _)) => n,
-            Some(Value::Dimension(n, Unit::Percent, _)) => {
+            Some(Value::Dimension(Some(n), Unit::None, _)) => n,
+            Some(Value::Dimension(Some(n), Unit::Percent, _)) => {
                 (n / Number::from(100)) * Number::from(255)
             }
+            Some(Value::Dimension(None, ..)) => todo!(),
             Some(v) if v.is_special_function() => {
                 return Ok(Value::String(
                     format!(
@@ -148,8 +151,9 @@ fn inner_rgb(name: &'static str, mut args: CallArgs, parser: &mut Parser<'_>) ->
             }
         };
         let alpha = match args.get_err(1, "alpha")? {
-            Value::Dimension(n, Unit::None, _) => n,
-            Value::Dimension(n, Unit::Percent, _) => n / Number::from(100),
+            Value::Dimension(Some(n), Unit::None, _) => n,
+            Value::Dimension(Some(n), Unit::Percent, _) => n / Number::from(100),
+            Value::Dimension(None, ..) => todo!(),
             v @ Value::Dimension(..) => {
                 return Err((
                     format!(
@@ -184,8 +188,11 @@ fn inner_rgb(name: &'static str, mut args: CallArgs, parser: &mut Parser<'_>) ->
         Ok(Value::Color(Box::new(color.with_alpha(alpha))))
     } else {
         let red = match args.get_err(0, "red")? {
-            Value::Dimension(n, Unit::None, _) => n,
-            Value::Dimension(n, Unit::Percent, _) => (n / Number::from(100)) * Number::from(255),
+            Value::Dimension(Some(n), Unit::None, _) => n,
+            Value::Dimension(Some(n), Unit::Percent, _) => {
+                (n / Number::from(100)) * Number::from(255)
+            }
+            Value::Dimension(None, ..) => todo!(),
             v @ Value::Dimension(..) => {
                 return Err((
                     format!(
@@ -222,8 +229,11 @@ fn inner_rgb(name: &'static str, mut args: CallArgs, parser: &mut Parser<'_>) ->
             }
         };
         let green = match args.get_err(1, "green")? {
-            Value::Dimension(n, Unit::None, _) => n,
-            Value::Dimension(n, Unit::Percent, _) => (n / Number::from(100)) * Number::from(255),
+            Value::Dimension(Some(n), Unit::None, _) => n,
+            Value::Dimension(Some(n), Unit::Percent, _) => {
+                (n / Number::from(100)) * Number::from(255)
+            }
+            Value::Dimension(None, ..) => todo!(),
             v @ Value::Dimension(..) => {
                 return Err((
                     format!(
@@ -259,8 +269,11 @@ fn inner_rgb(name: &'static str, mut args: CallArgs, parser: &mut Parser<'_>) ->
             }
         };
         let blue = match args.get_err(2, "blue")? {
-            Value::Dimension(n, Unit::None, _) => n,
-            Value::Dimension(n, Unit::Percent, _) => (n / Number::from(100)) * Number::from(255),
+            Value::Dimension(Some(n), Unit::None, _) => n,
+            Value::Dimension(Some(n), Unit::Percent, _) => {
+                (n / Number::from(100)) * Number::from(255)
+            }
+            Value::Dimension(None, ..) => todo!(),
             v @ Value::Dimension(..) => {
                 return Err((
                     format!(
@@ -297,10 +310,11 @@ fn inner_rgb(name: &'static str, mut args: CallArgs, parser: &mut Parser<'_>) ->
         let alpha = match args.default_arg(
             3,
             "alpha",
-            Value::Dimension(Number::one(), Unit::None, true),
+            Value::Dimension(Some(Number::one()), Unit::None, true),
         )? {
-            Value::Dimension(n, Unit::None, _) => n,
-            Value::Dimension(n, Unit::Percent, _) => n / Number::from(100),
+            Value::Dimension(Some(n), Unit::None, _) => n,
+            Value::Dimension(Some(n), Unit::Percent, _) => n / Number::from(100),
+            Value::Dimension(None, ..) => todo!(),
             v @ Value::Dimension(..) => {
                 return Err((
                     format!(
@@ -336,18 +350,18 @@ fn inner_rgb(name: &'static str, mut args: CallArgs, parser: &mut Parser<'_>) ->
     }
 }
 
-fn rgb(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn rgb(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     inner_rgb("rgb", args, parser)
 }
 
-fn rgba(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn rgba(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     inner_rgb("rgba", args, parser)
 }
 
-fn red(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn red(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "color")? {
-        Value::Color(c) => Ok(Value::Dimension(c.red(), Unit::None, true)),
+        Value::Color(c) => Ok(Value::Dimension(Some(c.red()), Unit::None, true)),
         v => Err((
             format!("$color: {} is not a color.", v.inspect(args.span())?),
             args.span(),
@@ -356,10 +370,10 @@ fn red(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     }
 }
 
-fn green(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn green(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "color")? {
-        Value::Color(c) => Ok(Value::Dimension(c.green(), Unit::None, true)),
+        Value::Color(c) => Ok(Value::Dimension(Some(c.green()), Unit::None, true)),
         v => Err((
             format!("$color: {} is not a color.", v.inspect(args.span())?),
             args.span(),
@@ -368,10 +382,10 @@ fn green(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     }
 }
 
-fn blue(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn blue(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "color")? {
-        Value::Color(c) => Ok(Value::Dimension(c.blue(), Unit::None, true)),
+        Value::Color(c) => Ok(Value::Dimension(Some(c.blue()), Unit::None, true)),
         v => Err((
             format!("$color: {} is not a color.", v.inspect(args.span())?),
             args.span(),
@@ -380,7 +394,7 @@ fn blue(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     }
 }
 
-fn mix(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn mix(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(3)?;
     let color1 = match args.get_err(0, "color1")? {
         Value::Color(c) => c,
@@ -407,9 +421,10 @@ fn mix(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     let weight = match args.default_arg(
         2,
         "weight",
-        Value::Dimension(Number::from(50), Unit::None, true),
+        Value::Dimension(Some(Number::from(50)), Unit::None, true),
     )? {
-        Value::Dimension(n, u, _) => bound!(args, "weight", n, u, 0, 100) / Number::from(100),
+        Value::Dimension(Some(n), u, _) => bound!(args, "weight", n, u, 0, 100) / Number::from(100),
+        Value::Dimension(None, ..) => todo!(),
         v => {
             return Err((
                 format!(
diff --git a/src/builtin/list.rs b/src/builtin/functions/list.rs
similarity index 87%
rename from src/builtin/list.rs
rename to src/builtin/functions/list.rs
index 9dbb483..56ac391 100644
--- a/src/builtin/list.rs
+++ b/src/builtin/functions/list.rs
@@ -11,20 +11,21 @@ use crate::{
     value::{Number, Value},
 };
 
-fn length(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn length(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     Ok(Value::Dimension(
-        Number::from(args.get_err(0, "list")?.as_list().len()),
+        Some(Number::from(args.get_err(0, "list")?.as_list().len())),
         Unit::None,
         true,
     ))
 }
 
-fn nth(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn nth(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(2)?;
     let mut list = args.get_err(0, "list")?.as_list();
     let n = match args.get_err(1, "n")? {
-        Value::Dimension(num, ..) => num,
+        Value::Dimension(Some(num), ..) => num,
+        Value::Dimension(None, ..) => todo!(),
         v => {
             return Err((
                 format!("$n: {} is not a number.", v.inspect(args.span())?),
@@ -61,7 +62,7 @@ fn nth(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     }))
 }
 
-fn list_separator(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn list_separator(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     Ok(Value::String(
         match args.get_err(0, "list")? {
@@ -73,7 +74,7 @@ fn list_separator(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Val
     ))
 }
 
-fn set_nth(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn set_nth(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(3)?;
     let (mut list, sep, brackets) = match args.get_err(0, "list")? {
         Value::List(v, sep, b) => (v, sep, b),
@@ -81,7 +82,8 @@ fn set_nth(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
         v => (vec![v], ListSeparator::Space, Brackets::None),
     };
     let n = match args.get_err(1, "n")? {
-        Value::Dimension(num, ..) => num,
+        Value::Dimension(Some(num), ..) => num,
+        Value::Dimension(None, ..) => todo!(),
         v => {
             return Err((
                 format!("$n: {} is not a number.", v.inspect(args.span())?),
@@ -120,7 +122,7 @@ fn set_nth(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     Ok(Value::List(list, sep, brackets))
 }
 
-fn append(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn append(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(3)?;
     let (mut list, sep, brackets) = match args.get_err(0, "list")? {
         Value::List(v, sep, b) => (v, sep, b),
@@ -158,7 +160,7 @@ fn append(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     Ok(Value::List(list, sep, brackets))
 }
 
-fn join(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn join(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(4)?;
     let (mut list1, sep1, brackets) = match args.get_err(0, "list1")? {
         Value::List(v, sep, brackets) => (v, sep, brackets),
@@ -225,7 +227,7 @@ fn join(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     Ok(Value::List(list1, sep, brackets))
 }
 
-fn is_bracketed(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn is_bracketed(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     Ok(Value::bool(match args.get_err(0, "list")? {
         Value::List(.., brackets) => match brackets {
@@ -236,7 +238,7 @@ fn is_bracketed(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value
     }))
 }
 
-fn index(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn index(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(2)?;
     let list = args.get_err(0, "list")?.as_list();
     let value = args.get_err(1, "value")?;
@@ -244,10 +246,10 @@ fn index(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
         Some(v) => Number::from(v + 1),
         None => return Ok(Value::Null),
     };
-    Ok(Value::Dimension(index, Unit::None, true))
+    Ok(Value::Dimension(Some(index), Unit::None, true))
 }
 
-fn zip(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn zip(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     let lists = args
         .get_variadic()?
         .into_iter()
diff --git a/src/builtin/macros.rs b/src/builtin/functions/macros.rs
similarity index 100%
rename from src/builtin/macros.rs
rename to src/builtin/functions/macros.rs
diff --git a/src/builtin/map.rs b/src/builtin/functions/map.rs
similarity index 87%
rename from src/builtin/map.rs
rename to src/builtin/functions/map.rs
index 068d4af..5572b12 100644
--- a/src/builtin/map.rs
+++ b/src/builtin/functions/map.rs
@@ -8,7 +8,7 @@ use crate::{
     value::{SassMap, Value},
 };
 
-fn map_get(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn map_get(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(2)?;
     let key = args.get_err(1, "key")?;
     let map = match args.get_err(0, "map")? {
@@ -26,7 +26,7 @@ fn map_get(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     Ok(map.get(&key)?.unwrap_or(Value::Null))
 }
 
-fn map_has_key(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn map_has_key(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(2)?;
     let key = args.get_err(1, "key")?;
     let map = match args.get_err(0, "map")? {
@@ -44,7 +44,7 @@ fn map_has_key(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value>
     Ok(Value::bool(map.get(&key)?.is_some()))
 }
 
-fn map_keys(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn map_keys(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     let map = match args.get_err(0, "map")? {
         Value::Map(m) => m,
@@ -65,7 +65,7 @@ fn map_keys(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     ))
 }
 
-fn map_values(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn map_values(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     let map = match args.get_err(0, "map")? {
         Value::Map(m) => m,
@@ -86,7 +86,7 @@ fn map_values(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value>
     ))
 }
 
-fn map_merge(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn map_merge(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(2)?;
     let mut map1 = match args.get_err(0, "map1")? {
         Value::Map(m) => m,
@@ -116,7 +116,7 @@ fn map_merge(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     Ok(Value::Map(map1))
 }
 
-fn map_remove(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn map_remove(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     let mut map = match args.get_err(0, "map")? {
         Value::Map(m) => m,
         Value::List(v, ..) if v.is_empty() => SassMap::new(),
diff --git a/src/builtin/math.rs b/src/builtin/functions/math.rs
similarity index 72%
rename from src/builtin/math.rs
rename to src/builtin/functions/math.rs
index 94104ec..470937b 100644
--- a/src/builtin/math.rs
+++ b/src/builtin/functions/math.rs
@@ -13,10 +13,11 @@ use crate::{
     value::{Number, Value},
 };
 
-fn percentage(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn percentage(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     let num = match args.get_err(0, "number")? {
-        Value::Dimension(n, Unit::None, _) => n * Number::from(100),
+        Value::Dimension(Some(n), Unit::None, _) => n * Number::from(100),
+        Value::Dimension(None, ..) => todo!(),
         v @ Value::Dimension(..) => {
             return Err((
                 format!(
@@ -35,13 +36,14 @@ fn percentage(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value>
                 .into())
         }
     };
-    Ok(Value::Dimension(num, Unit::Percent, true))
+    Ok(Value::Dimension(Some(num), Unit::Percent, true))
 }
 
-fn round(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn round(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "number")? {
-        Value::Dimension(n, u, _) => Ok(Value::Dimension(n.round(), u, true)),
+        Value::Dimension(Some(n), u, _) => Ok(Value::Dimension(Some(n.round()), u, true)),
+        Value::Dimension(None, ..) => todo!(),
         v => Err((
             format!("$number: {} is not a number.", v.inspect(args.span())?),
             args.span(),
@@ -50,10 +52,11 @@ fn round(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     }
 }
 
-fn ceil(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn ceil(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "number")? {
-        Value::Dimension(n, u, _) => Ok(Value::Dimension(n.ceil(), u, true)),
+        Value::Dimension(Some(n), u, _) => Ok(Value::Dimension(Some(n.ceil()), u, true)),
+        Value::Dimension(None, ..) => todo!(),
         v => Err((
             format!("$number: {} is not a number.", v.inspect(args.span())?),
             args.span(),
@@ -62,10 +65,11 @@ fn ceil(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     }
 }
 
-fn floor(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn floor(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "number")? {
-        Value::Dimension(n, u, _) => Ok(Value::Dimension(n.floor(), u, true)),
+        Value::Dimension(Some(n), u, _) => Ok(Value::Dimension(Some(n.floor()), u, true)),
+        Value::Dimension(None, ..) => todo!(),
         v => Err((
             format!("$number: {} is not a number.", v.inspect(args.span())?),
             args.span(),
@@ -74,10 +78,11 @@ fn floor(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     }
 }
 
-fn abs(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn abs(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "number")? {
-        Value::Dimension(n, u, _) => Ok(Value::Dimension(n.abs(), u, true)),
+        Value::Dimension(Some(n), u, _) => Ok(Value::Dimension(Some(n.abs()), u, true)),
+        Value::Dimension(None, ..) => todo!(),
         v => Err((
             format!("$number: {} is not a number.", v.inspect(args.span())?),
             args.span(),
@@ -86,7 +91,7 @@ fn abs(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     }
 }
 
-fn comparable(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn comparable(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(2)?;
     let unit1 = match args.get_err(0, "number1")? {
         Value::Dimension(_, u, _) => u,
@@ -114,14 +119,15 @@ fn comparable(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value>
 
 // TODO: write tests for this
 #[cfg(feature = "random")]
-fn random(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn random(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     let limit = match args.default_arg(0, "limit", Value::Null)? {
-        Value::Dimension(n, ..) => n,
+        Value::Dimension(Some(n), ..) => n,
+        Value::Dimension(None, ..) => todo!(),
         Value::Null => {
             let mut rng = rand::thread_rng();
             return Ok(Value::Dimension(
-                Number::from(rng.gen_range(0.0, 1.0)),
+                Some(Number::from(rng.gen_range(0.0, 1.0))),
                 Unit::None,
                 true,
             ));
@@ -136,7 +142,7 @@ fn random(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     };
 
     if limit.is_one() {
-        return Ok(Value::Dimension(Number::one(), Unit::None, true));
+        return Ok(Value::Dimension(Some(Number::one()), Unit::None, true));
     }
 
     if limit.is_decimal() {
@@ -164,20 +170,21 @@ fn random(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
 
     let mut rng = rand::thread_rng();
     Ok(Value::Dimension(
-        Number::from(rng.gen_range(0, limit) + 1),
+        Some(Number::from(rng.gen_range(0, limit) + 1)),
         Unit::None,
         true,
     ))
 }
 
-fn min(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn min(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.min_args(1)?;
     let span = args.span();
     let mut nums = args
         .get_variadic()?
         .into_iter()
         .map(|val| match val.node {
-            Value::Dimension(number, unit, _) => Ok((number, unit)),
+            Value::Dimension(Some(number), unit, _) => Ok((number, unit)),
+            Value::Dimension(None, ..) => todo!(),
             v => Err((format!("{} is not a number.", v.inspect(span)?), span).into()),
         })
         .collect::<SassResult<Vec<(Number, Unit)>>>()?
@@ -190,12 +197,12 @@ fn min(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
         if ValueVisitor::new(parser, span)
             .less_than(
                 HigherIntermediateValue::Literal(Value::Dimension(
-                    num.0.clone(),
+                    Some(num.0.clone()),
                     num.1.clone(),
                     true,
                 )),
                 HigherIntermediateValue::Literal(Value::Dimension(
-                    min.0.clone(),
+                    Some(min.0.clone()),
                     min.1.clone(),
                     true,
                 )),
@@ -205,17 +212,18 @@ fn min(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
             min = num;
         }
     }
-    Ok(Value::Dimension(min.0, min.1, true))
+    Ok(Value::Dimension(Some(min.0), min.1, true))
 }
 
-fn max(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn max(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.min_args(1)?;
     let span = args.span();
     let mut nums = args
         .get_variadic()?
         .into_iter()
         .map(|val| match val.node {
-            Value::Dimension(number, unit, _) => Ok((number, unit)),
+            Value::Dimension(Some(number), unit, _) => Ok((number, unit)),
+            Value::Dimension(None, ..) => todo!(),
             v => Err((format!("{} is not a number.", v.inspect(span)?), span).into()),
         })
         .collect::<SassResult<Vec<(Number, Unit)>>>()?
@@ -228,12 +236,12 @@ fn max(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
         if ValueVisitor::new(parser, span)
             .greater_than(
                 HigherIntermediateValue::Literal(Value::Dimension(
-                    num.0.clone(),
+                    Some(num.0.clone()),
                     num.1.clone(),
                     true,
                 )),
                 HigherIntermediateValue::Literal(Value::Dimension(
-                    max.0.clone(),
+                    Some(max.0.clone()),
                     max.1.clone(),
                     true,
                 )),
@@ -243,7 +251,7 @@ fn max(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
             max = num;
         }
     }
-    Ok(Value::Dimension(max.0, max.1, true))
+    Ok(Value::Dimension(Some(max.0), max.1, true))
 }
 
 pub(crate) fn declare(f: &mut GlobalFunctionMap) {
diff --git a/src/builtin/meta.rs b/src/builtin/functions/meta.rs
similarity index 60%
rename from src/builtin/meta.rs
rename to src/builtin/functions/meta.rs
index 9255de7..1bdc1b9 100644
--- a/src/builtin/meta.rs
+++ b/src/builtin/functions/meta.rs
@@ -7,7 +7,6 @@ use crate::{
     common::{Identifier, QuoteKind},
     error::SassResult,
     parse::Parser,
-    unit::Unit,
     value::{SassFunction, Value},
 };
 
@@ -20,7 +19,7 @@ fn if_(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     }
 }
 
-fn feature_exists(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn feature_exists(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "feature")? {
         #[allow(clippy::match_same_arms)]
@@ -50,7 +49,7 @@ fn feature_exists(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Val
     }
 }
 
-fn unit(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn unit(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     let unit = match args.get_err(0, "number")? {
         Value::Dimension(_, u, _) => u.to_string(),
@@ -65,23 +64,18 @@ fn unit(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     Ok(Value::String(unit, QuoteKind::Quoted))
 }
 
-fn type_of(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn type_of(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     let value = args.get_err(0, "value")?;
     Ok(Value::String(value.kind().to_owned(), QuoteKind::None))
 }
 
-fn unitless(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn unitless(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
-    #[allow(clippy::match_same_arms)]
-    Ok(match args.get_err(0, "number")? {
-        Value::Dimension(_, Unit::None, _) => Value::True,
-        Value::Dimension(..) => Value::False,
-        _ => Value::True,
-    })
+    Ok(Value::bool(args.get_err(0, "number")?.unitless()))
 }
 
-fn inspect(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn inspect(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     Ok(Value::String(
         args.get_err(0, "value")?.inspect(args.span())?.into_owned(),
@@ -89,7 +83,7 @@ fn inspect(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     ))
 }
 
-fn variable_exists(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn variable_exists(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "name")? {
         Value::String(s, _) => Ok(Value::bool(
@@ -103,33 +97,81 @@ fn variable_exists(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Va
     }
 }
 
-fn global_variable_exists(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
-    args.max_args(1)?;
-    match args.get_err(0, "name")? {
-        Value::String(s, _) => Ok(Value::bool(parser.global_scope.var_exists(s.into()))),
-        v => Err((
-            format!("$name: {} is not a string.", v.inspect(args.span())?),
-            args.span(),
-        )
-            .into()),
-    }
-}
-
-fn mixin_exists(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn global_variable_exists(
+    mut args: CallArgs,
+    parser: &mut Parser<'_>,
+) -> SassResult<Value> {
     args.max_args(2)?;
-    match args.get_err(0, "name")? {
-        Value::String(s, _) => Ok(Value::bool(
-            parser.scopes.mixin_exists(s.into(), parser.global_scope),
-        )),
-        v => Err((
-            format!("$name: {} is not a string.", v.inspect(args.span())?),
-            args.span(),
-        )
-            .into()),
-    }
+
+    let name: Identifier = match args.get_err(0, "name")? {
+        Value::String(s, _) => s.into(),
+        v => {
+            return Err((
+                format!("$name: {} is not a string.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    };
+
+    let module = match args.default_arg(1, "module", Value::Null)? {
+        Value::String(s, _) => Some(s),
+        Value::Null => None,
+        v => {
+            return Err((
+                format!("$module: {} is not a string.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    };
+
+    Ok(Value::bool(if let Some(module_name) = module {
+        parser
+            .modules
+            .get(module_name.into(), args.span())?
+            .var_exists(name)
+    } else {
+        parser.global_scope.var_exists(name)
+    }))
 }
 
-fn function_exists(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn mixin_exists(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+    args.max_args(2)?;
+    let name: Identifier = match args.get_err(0, "name")? {
+        Value::String(s, _) => s.into(),
+        v => {
+            return Err((
+                format!("$name: {} is not a string.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    };
+
+    let module = match args.default_arg(1, "module", Value::Null)? {
+        Value::String(s, _) => Some(s),
+        Value::Null => None,
+        v => {
+            return Err((
+                format!("$module: {} is not a string.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    };
+
+    Ok(Value::bool(if let Some(module_name) = module {
+        parser
+            .modules
+            .get(module_name.into(), args.span())?
+            .mixin_exists(name)
+    } else {
+        parser.scopes.mixin_exists(name, parser.global_scope)
+    }))
+}
+
+pub(crate) fn function_exists(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(2)?;
     match args.get_err(0, "name")? {
         Value::String(s, _) => Ok(Value::bool(
@@ -143,7 +185,7 @@ fn function_exists(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Va
     }
 }
 
-fn get_function(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn get_function(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(3)?;
     let name: Identifier = match args.get_err(0, "name")? {
         Value::String(s, _) => s.into(),
@@ -168,22 +210,26 @@ fn get_function(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value
         }
     };
 
-    if module.is_some() && css {
-        return Err((
-            "$css and $module may not both be passed at once.",
-            args.span(),
-        )
-            .into());
-    }
+    let func = match if let Some(module_name) = module {
+        if css {
+            return Err((
+                "$css and $module may not both be passed at once.",
+                args.span(),
+            )
+                .into());
+        }
 
-    let func = match parser.scopes.get_fn(
-        Spanned {
-            node: name,
-            span: args.span(),
-        },
-        parser.global_scope,
-    ) {
-        Some(f) => SassFunction::UserDefined(Box::new(f), name),
+        parser
+            .modules
+            .get(module_name.into(), args.span())?
+            .get_fn(Spanned {
+                node: name,
+                span: args.span(),
+            })?
+    } else {
+        parser.scopes.get_fn(name, parser.global_scope)
+    } {
+        Some(f) => f,
         None => match GLOBAL_FUNCTIONS.get(name.as_str()) {
             Some(f) => SassFunction::Builtin(f.clone(), name),
             None => return Err((format!("Function not found: {}", name), args.span()).into()),
@@ -193,7 +239,7 @@ fn get_function(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value
     Ok(Value::FunctionRef(func))
 }
 
-fn call(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn call(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     let func = match args.get_err(0, "function")? {
         Value::FunctionRef(f) => f,
         v => {
@@ -211,7 +257,7 @@ fn call(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
 }
 
 #[allow(clippy::needless_pass_by_value)]
-fn content_exists(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn content_exists(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(0)?;
     if !parser.flags.in_mixin() {
         return Err((
@@ -225,6 +271,13 @@ fn content_exists(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value>
     ))
 }
 
+#[allow(unused_variables)]
+pub(crate) fn keywords(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+    args.max_args(1)?;
+    drop(args);
+    todo!("builtin function `keywords` blocked on better handling of call args")
+}
+
 pub(crate) fn declare(f: &mut GlobalFunctionMap) {
     f.insert("if", Builtin::new(if_));
     f.insert("feature-exists", Builtin::new(feature_exists));
@@ -242,4 +295,5 @@ pub(crate) fn declare(f: &mut GlobalFunctionMap) {
     f.insert("get-function", Builtin::new(get_function));
     f.insert("call", Builtin::new(call));
     f.insert("content-exists", Builtin::new(content_exists));
+    f.insert("keywords", Builtin::new(keywords));
 }
diff --git a/src/builtin/functions/mod.rs b/src/builtin/functions/mod.rs
new file mode 100644
index 0000000..1c28974
--- /dev/null
+++ b/src/builtin/functions/mod.rs
@@ -0,0 +1,60 @@
+// A reference to the parser is only necessary for some functions
+#![allow(unused_variables)]
+
+use std::{
+    collections::HashMap,
+    sync::atomic::{AtomicUsize, Ordering},
+};
+
+use once_cell::sync::Lazy;
+
+use crate::{args::CallArgs, error::SassResult, parse::Parser, value::Value};
+
+#[macro_use]
+mod macros;
+
+pub mod color;
+pub mod list;
+pub mod map;
+pub mod math;
+pub mod meta;
+pub mod selector;
+pub mod string;
+
+pub(crate) type GlobalFunctionMap = HashMap<&'static str, Builtin>;
+
+static FUNCTION_COUNT: AtomicUsize = AtomicUsize::new(0);
+
+// TODO: impl Fn
+#[derive(Clone)]
+pub(crate) struct Builtin(
+    pub fn(CallArgs, &mut Parser<'_>) -> SassResult<Value>,
+    usize,
+);
+
+impl Builtin {
+    pub fn new(body: fn(CallArgs, &mut Parser<'_>) -> SassResult<Value>) -> Builtin {
+        let count = FUNCTION_COUNT.fetch_add(1, Ordering::Relaxed);
+        Self(body, count)
+    }
+}
+
+impl PartialEq for Builtin {
+    fn eq(&self, other: &Self) -> bool {
+        self.1 == other.1
+    }
+}
+
+impl Eq for Builtin {}
+
+pub(crate) static GLOBAL_FUNCTIONS: Lazy<GlobalFunctionMap> = Lazy::new(|| {
+    let mut m = HashMap::new();
+    color::declare(&mut m);
+    list::declare(&mut m);
+    map::declare(&mut m);
+    math::declare(&mut m);
+    meta::declare(&mut m);
+    selector::declare(&mut m);
+    string::declare(&mut m);
+    m
+});
diff --git a/src/builtin/selector.rs b/src/builtin/functions/selector.rs
similarity index 89%
rename from src/builtin/selector.rs
rename to src/builtin/functions/selector.rs
index dae2d00..f7d50e9 100644
--- a/src/builtin/selector.rs
+++ b/src/builtin/functions/selector.rs
@@ -9,7 +9,7 @@ use crate::{
     value::Value,
 };
 
-fn is_superselector(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn is_superselector(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(2)?;
     let parent_selector = args
         .get_err(0, "super")?
@@ -21,7 +21,7 @@ fn is_superselector(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<V
     ))
 }
 
-fn simple_selectors(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn simple_selectors(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     // todo: Value::to_compound_selector
     let selector = args
@@ -51,7 +51,7 @@ fn simple_selectors(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<V
     ))
 }
 
-fn selector_parse(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn selector_parse(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     Ok(args
         .get_err(0, "selector")?
@@ -59,7 +59,7 @@ fn selector_parse(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Val
         .into_value())
 }
 
-fn selector_nest(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn selector_nest(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     let span = args.span();
     let selectors = args.get_variadic()?;
     if selectors.is_empty() {
@@ -80,7 +80,7 @@ fn selector_nest(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
         .into_value())
 }
 
-fn selector_append(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn selector_append(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     let span = args.span();
     let selectors = args.get_variadic()?;
     if selectors.is_empty() {
@@ -138,7 +138,7 @@ fn selector_append(args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value>
         .into_value())
 }
 
-fn selector_extend(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn selector_extend(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(3)?;
     let selector = args
         .get_err(0, "selector")?
@@ -153,7 +153,7 @@ fn selector_extend(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Va
     Ok(Extender::extend(selector.0, source.0, target.0, args.span())?.to_sass_list())
 }
 
-fn selector_replace(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn selector_replace(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(3)?;
     let selector = args
         .get_err(0, "selector")?
@@ -167,7 +167,7 @@ fn selector_replace(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<V
     Ok(Extender::replace(selector.0, source.0, target.0, args.span())?.to_sass_list())
 }
 
-fn selector_unify(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn selector_unify(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(2)?;
     let selector1 = args
         .get_err(0, "selector1")?
diff --git a/src/builtin/string.rs b/src/builtin/functions/string.rs
similarity index 80%
rename from src/builtin/string.rs
rename to src/builtin/functions/string.rs
index 989d393..edcbbd7 100644
--- a/src/builtin/string.rs
+++ b/src/builtin/functions/string.rs
@@ -15,7 +15,7 @@ use crate::{
     value::{Number, Value},
 };
 
-fn to_upper_case(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn to_upper_case(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "string")? {
         Value::String(mut i, q) => {
@@ -30,7 +30,7 @@ fn to_upper_case(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Valu
     }
 }
 
-fn to_lower_case(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn to_lower_case(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "string")? {
         Value::String(mut i, q) => {
@@ -45,11 +45,11 @@ fn to_lower_case(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Valu
     }
 }
 
-fn str_length(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn str_length(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "string")? {
         Value::String(i, _) => Ok(Value::Dimension(
-            Number::from(i.chars().count()),
+            Some(Number::from(i.chars().count())),
             Unit::None,
             true,
         )),
@@ -61,7 +61,7 @@ fn str_length(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value>
     }
 }
 
-fn quote(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn quote(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "string")? {
         Value::String(i, _) => Ok(Value::String(i, QuoteKind::Quoted)),
@@ -73,7 +73,7 @@ fn quote(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     }
 }
 
-fn unquote(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn unquote(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
     match args.get_err(0, "string")? {
         i @ Value::String(..) => Ok(i.unquote()),
@@ -85,7 +85,7 @@ fn unquote(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     }
 }
 
-fn str_slice(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn str_slice(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(3)?;
     let (string, quotes) = match args.get_err(0, "string")? {
         Value::String(s, q) => (s, q),
@@ -99,17 +99,18 @@ fn str_slice(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     };
     let str_len = string.chars().count();
     let start = match args.get_err(1, "start-at")? {
-        Value::Dimension(n, Unit::None, _) if n.is_decimal() => {
+        Value::Dimension(Some(n), Unit::None, _) if n.is_decimal() => {
             return Err((format!("{} is not an int.", n), args.span()).into())
         }
-        Value::Dimension(n, Unit::None, _) if n.is_positive() => {
+        Value::Dimension(Some(n), Unit::None, _) if n.is_positive() => {
             n.to_integer().to_usize().unwrap_or(str_len + 1)
         }
-        Value::Dimension(n, Unit::None, _) if n.is_zero() => 1_usize,
-        Value::Dimension(n, Unit::None, _) if n < -Number::from(str_len) => 1_usize,
-        Value::Dimension(n, Unit::None, _) => (n.to_integer() + BigInt::from(str_len + 1))
+        Value::Dimension(Some(n), Unit::None, _) if n.is_zero() => 1_usize,
+        Value::Dimension(Some(n), Unit::None, _) if n < -Number::from(str_len) => 1_usize,
+        Value::Dimension(Some(n), Unit::None, _) => (n.to_integer() + BigInt::from(str_len + 1))
             .to_usize()
             .unwrap(),
+        Value::Dimension(None, ..) => todo!(),
         v @ Value::Dimension(..) => {
             return Err((
                 format!(
@@ -129,17 +130,18 @@ fn str_slice(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
         }
     };
     let mut end = match args.default_arg(2, "end-at", Value::Null)? {
-        Value::Dimension(n, Unit::None, _) if n.is_decimal() => {
+        Value::Dimension(Some(n), Unit::None, _) if n.is_decimal() => {
             return Err((format!("{} is not an int.", n), args.span()).into())
         }
-        Value::Dimension(n, Unit::None, _) if n.is_positive() => {
+        Value::Dimension(Some(n), Unit::None, _) if n.is_positive() => {
             n.to_integer().to_usize().unwrap_or(str_len + 1)
         }
-        Value::Dimension(n, Unit::None, _) if n.is_zero() => 0_usize,
-        Value::Dimension(n, Unit::None, _) if n < -Number::from(str_len) => 0_usize,
-        Value::Dimension(n, Unit::None, _) => (n.to_integer() + BigInt::from(str_len + 1))
+        Value::Dimension(Some(n), Unit::None, _) if n.is_zero() => 0_usize,
+        Value::Dimension(Some(n), Unit::None, _) if n < -Number::from(str_len) => 0_usize,
+        Value::Dimension(Some(n), Unit::None, _) => (n.to_integer() + BigInt::from(str_len + 1))
             .to_usize()
             .unwrap_or(str_len + 1),
+        Value::Dimension(None, ..) => todo!(),
         v @ Value::Dimension(..) => {
             return Err((
                 format!(
@@ -178,7 +180,7 @@ fn str_slice(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     }
 }
 
-fn str_index(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn str_index(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(2)?;
     let s1 = match args.get_err(0, "string")? {
         Value::String(i, _) => i,
@@ -203,12 +205,12 @@ fn str_index(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     };
 
     Ok(match s1.find(&substr) {
-        Some(v) => Value::Dimension(Number::from(v + 1), Unit::None, true),
+        Some(v) => Value::Dimension(Some(Number::from(v + 1)), Unit::None, true),
         None => Value::Null,
     })
 }
 
-fn str_insert(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn str_insert(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(3)?;
     let (s1, quotes) = match args.get_err(0, "string")? {
         Value::String(i, q) => (i, q),
@@ -233,10 +235,11 @@ fn str_insert(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value>
     };
 
     let index = match args.get_err(2, "index")? {
-        Value::Dimension(n, Unit::None, _) if n.is_decimal() => {
+        Value::Dimension(Some(n), Unit::None, _) if n.is_decimal() => {
             return Err((format!("$index: {} is not an int.", n), args.span()).into())
         }
-        Value::Dimension(n, Unit::None, _) => n,
+        Value::Dimension(Some(n), Unit::None, _) => n,
+        Value::Dimension(None, ..) => todo!(),
         v @ Value::Dimension(..) => {
             return Err((
                 format!(
@@ -305,7 +308,7 @@ fn str_insert(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value>
 
 #[cfg(feature = "random")]
 #[allow(clippy::needless_pass_by_value)]
-fn unique_id(args: CallArgs, _: &mut Parser<'_>) -> SassResult<Value> {
+pub(crate) fn unique_id(args: CallArgs, _: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(0)?;
     let mut rng = thread_rng();
     let string = std::iter::repeat(())
diff --git a/src/builtin/mod.rs b/src/builtin/mod.rs
index 4ec4354..7dd9b7d 100644
--- a/src/builtin/mod.rs
+++ b/src/builtin/mod.rs
@@ -1,60 +1,6 @@
-// A reference to the parser is only necessary for some functions
-#![allow(unused_variables)]
+mod functions;
+pub(crate) mod modules;
 
-use std::{
-    collections::HashMap,
-    sync::atomic::{AtomicUsize, Ordering},
+pub(crate) use functions::{
+    color, list, map, math, meta, selector, string, Builtin, GLOBAL_FUNCTIONS,
 };
-
-use once_cell::sync::Lazy;
-
-use crate::{args::CallArgs, error::SassResult, parse::Parser, value::Value};
-
-#[macro_use]
-mod macros;
-
-mod color;
-mod list;
-mod map;
-mod math;
-mod meta;
-mod selector;
-mod string;
-
-pub(crate) type GlobalFunctionMap = HashMap<&'static str, Builtin>;
-
-static FUNCTION_COUNT: AtomicUsize = AtomicUsize::new(0);
-
-// TODO: impl Fn
-#[derive(Clone)]
-pub(crate) struct Builtin(
-    pub fn(CallArgs, &mut Parser<'_>) -> SassResult<Value>,
-    usize,
-);
-
-impl Builtin {
-    pub fn new(body: fn(CallArgs, &mut Parser<'_>) -> SassResult<Value>) -> Builtin {
-        let count = FUNCTION_COUNT.fetch_add(1, Ordering::Relaxed);
-        Self(body, count)
-    }
-}
-
-impl PartialEq for Builtin {
-    fn eq(&self, other: &Self) -> bool {
-        self.1 == other.1
-    }
-}
-
-impl Eq for Builtin {}
-
-pub(crate) static GLOBAL_FUNCTIONS: Lazy<GlobalFunctionMap> = Lazy::new(|| {
-    let mut m = HashMap::new();
-    color::declare(&mut m);
-    list::declare(&mut m);
-    map::declare(&mut m);
-    math::declare(&mut m);
-    meta::declare(&mut m);
-    selector::declare(&mut m);
-    string::declare(&mut m);
-    m
-});
diff --git a/src/builtin/modules/color.rs b/src/builtin/modules/color.rs
new file mode 100644
index 0000000..f2cd754
--- /dev/null
+++ b/src/builtin/modules/color.rs
@@ -0,0 +1,27 @@
+use crate::builtin::{
+    color::{
+        hsl::{complement, grayscale, hue, invert, lightness, saturation},
+        opacity::alpha,
+        other::{adjust_color, change_color, ie_hex_str, scale_color},
+        rgb::{blue, green, mix, red},
+    },
+    modules::Module,
+};
+
+pub(crate) fn declare(f: &mut Module) {
+    f.insert_builtin("adjust", adjust_color);
+    f.insert_builtin("alpha", alpha);
+    f.insert_builtin("blue", blue);
+    f.insert_builtin("change", change_color);
+    f.insert_builtin("complement", complement);
+    f.insert_builtin("grayscale", grayscale);
+    f.insert_builtin("green", green);
+    f.insert_builtin("hue", hue);
+    f.insert_builtin("ie-hex-str", ie_hex_str);
+    f.insert_builtin("invert", invert);
+    f.insert_builtin("lightness", lightness);
+    f.insert_builtin("mix", mix);
+    f.insert_builtin("red", red);
+    f.insert_builtin("saturation", saturation);
+    f.insert_builtin("scale", scale_color);
+}
diff --git a/src/builtin/modules/list.rs b/src/builtin/modules/list.rs
new file mode 100644
index 0000000..16c7a6c
--- /dev/null
+++ b/src/builtin/modules/list.rs
@@ -0,0 +1,16 @@
+use crate::builtin::{
+    list::{append, index, is_bracketed, join, length, list_separator, nth, set_nth, zip},
+    modules::Module,
+};
+
+pub(crate) fn declare(f: &mut Module) {
+    f.insert_builtin("append", append);
+    f.insert_builtin("index", index);
+    f.insert_builtin("is-bracketed", is_bracketed);
+    f.insert_builtin("join", join);
+    f.insert_builtin("length", length);
+    f.insert_builtin("separator", list_separator);
+    f.insert_builtin("nth", nth);
+    f.insert_builtin("set-nth", set_nth);
+    f.insert_builtin("zip", zip);
+}
diff --git a/src/builtin/modules/map.rs b/src/builtin/modules/map.rs
new file mode 100644
index 0000000..13446ad
--- /dev/null
+++ b/src/builtin/modules/map.rs
@@ -0,0 +1,13 @@
+use crate::builtin::{
+    map::{map_get, map_has_key, map_keys, map_merge, map_remove, map_values},
+    modules::Module,
+};
+
+pub(crate) fn declare(f: &mut Module) {
+    f.insert_builtin("get", map_get);
+    f.insert_builtin("has-key", map_has_key);
+    f.insert_builtin("keys", map_keys);
+    f.insert_builtin("merge", map_merge);
+    f.insert_builtin("remove", map_remove);
+    f.insert_builtin("values", map_values);
+}
diff --git a/src/builtin/modules/math.rs b/src/builtin/modules/math.rs
new file mode 100644
index 0000000..ecc553b
--- /dev/null
+++ b/src/builtin/modules/math.rs
@@ -0,0 +1,622 @@
+use std::cmp::Ordering;
+
+use num_traits::{One, Signed, Zero};
+
+use crate::{
+    args::CallArgs,
+    builtin::{
+        math::{abs, ceil, comparable, floor, max, min, percentage, round},
+        meta::{unit, unitless},
+        modules::Module,
+    },
+    common::Op,
+    error::SassResult,
+    parse::Parser,
+    unit::Unit,
+    value::{Number, Value},
+};
+
+#[cfg(feature = "random")]
+use crate::builtin::math::random;
+
+fn clamp(mut args: CallArgs, _: &mut Parser<'_>) -> SassResult<Value> {
+    args.max_args(3)?;
+    let span = args.span();
+
+    let min = match args.get_err(0, "min")? {
+        v @ Value::Dimension(..) => v,
+        v => {
+            return Err((
+                format!("$min: {} is not a number.", v.inspect(args.span())?),
+                span,
+            )
+                .into())
+        }
+    };
+
+    let number = match args.get_err(1, "number")? {
+        v @ Value::Dimension(..) => v,
+        v => {
+            return Err((
+                format!("$number: {} is not a number.", v.inspect(span)?),
+                span,
+            )
+                .into())
+        }
+    };
+
+    let max = match args.get_err(2, "max")? {
+        v @ Value::Dimension(..) => v,
+        v => return Err((format!("$max: {} is not a number.", v.inspect(span)?), span).into()),
+    };
+
+    // ensure that `min` and `max` are compatible
+    min.cmp(&max, span, Op::LessThan)?;
+
+    let min_unit = match min {
+        Value::Dimension(_, ref u, _) => u,
+        _ => unreachable!(),
+    };
+    let number_unit = match number {
+        Value::Dimension(_, ref u, _) => u,
+        _ => unreachable!(),
+    };
+    let max_unit = match max {
+        Value::Dimension(_, ref u, _) => u,
+        _ => unreachable!(),
+    };
+
+    if min_unit == &Unit::None && number_unit != &Unit::None {
+        return Err((
+            format!(
+                "$min is unitless but $number has unit {}. Arguments must all have units or all be unitless.",
+                number_unit
+            ), span).into());
+    } else if min_unit != &Unit::None && number_unit == &Unit::None {
+        return Err((
+                format!(
+                    "$min has unit {} but $number is unitless. Arguments must all have units or all be unitless.",
+                    min_unit
+                ), span).into());
+    } else if min_unit != &Unit::None && max_unit == &Unit::None {
+        return Err((
+            format!(
+                "$min has unit {} but $max is unitless. Arguments must all have units or all be unitless.",
+                min_unit
+            ), span).into());
+    }
+
+    match min.cmp(&number, span, Op::LessThan)? {
+        Ordering::Greater => return Ok(min),
+        Ordering::Equal => return Ok(number),
+        Ordering::Less => {}
+    }
+
+    match max.cmp(&number, span, Op::GreaterThan)? {
+        Ordering::Less => return Ok(max),
+        Ordering::Equal => return Ok(number),
+        Ordering::Greater => {}
+    }
+
+    Ok(number)
+}
+
+fn hypot(args: CallArgs, _: &mut Parser<'_>) -> SassResult<Value> {
+    args.min_args(1)?;
+
+    let span = args.span();
+
+    let mut numbers = args.get_variadic()?.into_iter().map(|v| -> SassResult<_> {
+        match v.node {
+            Value::Dimension(n, u, ..) => Ok((n, u)),
+            v => Err((format!("{} is not a number.", v.inspect(span)?), span).into()),
+        }
+    });
+
+    let first: (Number, Unit) = match numbers.next().unwrap()? {
+        (Some(n), u) => (n.clone() * n, u),
+        (None, u) => return Ok(Value::Dimension(None, u, true)),
+    };
+
+    let rest = numbers
+        .enumerate()
+        .map(|(idx, val)| -> SassResult<Option<Number>> {
+            let (number, unit) = val?;
+            if first.1 == Unit::None {
+                if unit == Unit::None {
+                    Ok(number.map(|n| n.clone() * n))
+                } else {
+                    Err((
+                        format!(
+                            "Argument 1 is unitless but argument {} has unit {}. \
+                            Arguments must all have units or all be unitless.",
+                            idx + 2,
+                            unit
+                        ),
+                        span,
+                    )
+                        .into())
+                }
+            } else if unit == Unit::None {
+                Err((
+                    format!(
+                        "Argument 1 has unit {} but argument {} is unitless. \
+                        Arguments must all have units or all be unitless.",
+                        first.1,
+                        idx + 2,
+                    ),
+                    span,
+                )
+                    .into())
+            } else if first.1.comparable(&unit) {
+                Ok(number
+                    .map(|n| n.convert(&unit, &first.1))
+                    .map(|n| n.clone() * n))
+            } else {
+                Err((
+                    format!("Incompatible units {} and {}.", first.1, unit),
+                    span,
+                )
+                    .into())
+            }
+        })
+        .collect::<SassResult<Option<Vec<Number>>>>()?;
+
+    let rest = match rest {
+        Some(v) => v,
+        None => return Ok(Value::Dimension(None, first.1, true)),
+    };
+
+    let sum = first.0 + rest.into_iter().fold(Number::zero(), |a, b| a + b);
+
+    Ok(Value::Dimension(sum.sqrt(), first.1, true))
+}
+
+fn log(mut args: CallArgs, _: &mut Parser<'_>) -> SassResult<Value> {
+    args.max_args(2)?;
+
+    let number = match args.get_err(0, "number")? {
+        Value::Dimension(Some(n), Unit::None, ..) => n,
+        v @ Value::Dimension(Some(..), ..) => {
+            return Err((
+                format!(
+                    "$number: Expected {} to be unitless.",
+                    v.inspect(args.span())?
+                ),
+                args.span(),
+            )
+                .into())
+        }
+        v @ Value::Dimension(None, ..) => return Ok(v),
+        v => {
+            return Err((
+                format!("$number: {} is not a number.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    };
+
+    let base = match args.default_arg(1, "base", Value::Null)? {
+        Value::Null => None,
+        Value::Dimension(Some(n), Unit::None, ..) => Some(n),
+        v @ Value::Dimension(Some(..), ..) => {
+            return Err((
+                format!(
+                    "$number: Expected {} to be unitless.",
+                    v.inspect(args.span())?
+                ),
+                args.span(),
+            )
+                .into())
+        }
+        v @ Value::Dimension(None, ..) => return Ok(v),
+        v => {
+            return Err((
+                format!("$base: {} is not a number.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    };
+
+    Ok(Value::Dimension(
+        if let Some(base) = base {
+            if base.is_zero() {
+                Some(Number::zero())
+            } else {
+                (|| Some(number.ln()? / base.ln()?))()
+            }
+        } else if number.is_negative() {
+            None
+        } else if number.is_zero() {
+            todo!()
+        } else {
+            number.ln()
+        },
+        Unit::None,
+        true,
+    ))
+}
+
+fn pow(mut args: CallArgs, _: &mut Parser<'_>) -> SassResult<Value> {
+    args.max_args(2)?;
+
+    let base = match args.get_err(0, "base")? {
+        Value::Dimension(Some(n), Unit::None, ..) => n,
+        v @ Value::Dimension(Some(..), ..) => {
+            return Err((
+                format!(
+                    "$base: Expected {} to have no units.",
+                    v.inspect(args.span())?
+                ),
+                args.span(),
+            )
+                .into())
+        }
+        Value::Dimension(None, ..) => return Ok(Value::Dimension(None, Unit::None, true)),
+        v => {
+            return Err((
+                format!("$base: {} is not a number.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    };
+
+    let exponent = match args.get_err(1, "exponent")? {
+        Value::Dimension(Some(n), Unit::None, ..) => n,
+        v @ Value::Dimension(Some(..), ..) => {
+            return Err((
+                format!(
+                    "$exponent: Expected {} to have no units.",
+                    v.inspect(args.span())?
+                ),
+                args.span(),
+            )
+                .into())
+        }
+        Value::Dimension(None, ..) => return Ok(Value::Dimension(None, Unit::None, true)),
+        v => {
+            return Err((
+                format!("$exponent: {} is not a number.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    };
+
+    Ok(Value::Dimension(base.pow(exponent), Unit::None, true))
+}
+
+fn sqrt(mut args: CallArgs, _: &mut Parser<'_>) -> SassResult<Value> {
+    args.max_args(1)?;
+    let number = args.get_err(0, "number")?;
+
+    Ok(match number {
+        Value::Dimension(Some(n), Unit::None, ..) => Value::Dimension(n.sqrt(), Unit::None, true),
+        v @ Value::Dimension(Some(..), ..) => {
+            return Err((
+                format!(
+                    "$number: Expected {} to have no units.",
+                    v.inspect(args.span())?
+                ),
+                args.span(),
+            )
+                .into())
+        }
+        Value::Dimension(None, ..) => Value::Dimension(None, Unit::None, true),
+        v => {
+            return Err((
+                format!("$number: {} is not a number.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    })
+}
+
+macro_rules! trig_fn {
+    ($name:ident, $name_deg:ident) => {
+        fn $name(mut args: CallArgs, _: &mut Parser<'_>) -> SassResult<Value> {
+            args.max_args(1)?;
+            let number = args.get_err(0, "number")?;
+
+            Ok(match number {
+                Value::Dimension(Some(n), Unit::None, ..)
+                | Value::Dimension(Some(n), Unit::Rad, ..) => {
+                    Value::Dimension(n.$name(), Unit::None, true)
+                }
+                Value::Dimension(Some(n), Unit::Deg, ..) => {
+                    Value::Dimension(n.$name_deg(), Unit::None, true)
+                }
+                v @ Value::Dimension(Some(..), ..) => {
+                    return Err((
+                        format!(
+                            "$number: Expected {} to be an angle.",
+                            v.inspect(args.span())?
+                        ),
+                        args.span(),
+                    )
+                        .into())
+                }
+                Value::Dimension(None, ..) => Value::Dimension(None, Unit::None, true),
+                v => {
+                    return Err((
+                        format!("$number: {} is not a number.", v.inspect(args.span())?),
+                        args.span(),
+                    )
+                        .into())
+                }
+            })
+        }
+    };
+}
+
+trig_fn!(cos, cos_deg);
+trig_fn!(sin, sin_deg);
+trig_fn!(tan, tan_deg);
+
+fn acos(mut args: CallArgs, _: &mut Parser<'_>) -> SassResult<Value> {
+    args.max_args(1)?;
+    let number = args.get_err(0, "number")?;
+
+    Ok(match number {
+        Value::Dimension(Some(n), Unit::None, ..) => Value::Dimension(
+            if n > Number::from(1) || n < Number::from(-1) {
+                None
+            } else if n.is_one() {
+                Some(Number::zero())
+            } else {
+                n.acos()
+            },
+            Unit::Deg,
+            true,
+        ),
+        v @ Value::Dimension(Some(..), ..) => {
+            return Err((
+                format!(
+                    "$number: Expected {} to be unitless.",
+                    v.inspect(args.span())?
+                ),
+                args.span(),
+            )
+                .into())
+        }
+        Value::Dimension(None, ..) => Value::Dimension(None, Unit::Deg, true),
+        v => {
+            return Err((
+                format!("$number: {} is not a number.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    })
+}
+
+fn asin(mut args: CallArgs, _: &mut Parser<'_>) -> SassResult<Value> {
+    args.max_args(1)?;
+    let number = args.get_err(0, "number")?;
+
+    Ok(match number {
+        Value::Dimension(Some(n), Unit::None, ..) => {
+            if n > Number::from(1) || n < Number::from(-1) {
+                return Ok(Value::Dimension(None, Unit::Deg, true));
+            } else if n.is_zero() {
+                return Ok(Value::Dimension(Some(Number::zero()), Unit::Deg, true));
+            }
+
+            Value::Dimension(n.asin(), Unit::Deg, true)
+        }
+        v @ Value::Dimension(Some(..), ..) => {
+            return Err((
+                format!(
+                    "$number: Expected {} to be unitless.",
+                    v.inspect(args.span())?
+                ),
+                args.span(),
+            )
+                .into())
+        }
+        Value::Dimension(None, ..) => Value::Dimension(None, Unit::Deg, true),
+        v => {
+            return Err((
+                format!("$number: {} is not a number.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    })
+}
+
+fn atan(mut args: CallArgs, _: &mut Parser<'_>) -> SassResult<Value> {
+    args.max_args(1)?;
+    let number = args.get_err(0, "number")?;
+
+    Ok(match number {
+        Value::Dimension(Some(n), Unit::None, ..) => {
+            if n.is_zero() {
+                return Ok(Value::Dimension(Some(Number::zero()), Unit::Deg, true));
+            }
+
+            Value::Dimension(n.atan(), Unit::Deg, true)
+        }
+        v @ Value::Dimension(Some(..), ..) => {
+            return Err((
+                format!(
+                    "$number: Expected {} to be unitless.",
+                    v.inspect(args.span())?
+                ),
+                args.span(),
+            )
+                .into())
+        }
+        Value::Dimension(None, ..) => Value::Dimension(None, Unit::Deg, true),
+        v => {
+            return Err((
+                format!("$number: {} is not a number.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    })
+}
+
+fn atan2(mut args: CallArgs, _: &mut Parser<'_>) -> SassResult<Value> {
+    args.max_args(2)?;
+    let (y_num, y_unit) = match args.get_err(0, "y")? {
+        Value::Dimension(n, u, ..) => (n, u),
+        v => {
+            return Err((
+                format!("$y: {} is not a number.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    };
+
+    let (x_num, x_unit) = match args.get_err(1, "x")? {
+        Value::Dimension(n, u, ..) => (n, u),
+        v => {
+            return Err((
+                format!("$x: {} is not a number.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    };
+
+    let (x_num, y_num) = if x_unit == Unit::None && y_unit == Unit::None {
+        let x = match x_num {
+            Some(n) => n,
+            None => return Ok(Value::Dimension(None, Unit::Deg, true)),
+        };
+
+        let y = match y_num {
+            Some(n) => n,
+            None => return Ok(Value::Dimension(None, Unit::Deg, true)),
+        };
+
+        (x, y)
+    } else if y_unit == Unit::None {
+        return Err((
+            format!(
+                "$y is unitless but $x has unit {}. \
+            Arguments must all have units or all be unitless.",
+                x_unit
+            ),
+            args.span(),
+        )
+            .into());
+    } else if x_unit == Unit::None {
+        return Err((
+            format!(
+                "$y has unit {} but $x is unitless. \
+                Arguments must all have units or all be unitless.",
+                y_unit
+            ),
+            args.span(),
+        )
+            .into());
+    } else if x_unit.comparable(&y_unit) {
+        let x = match x_num {
+            Some(n) => n,
+            None => return Ok(Value::Dimension(None, Unit::Deg, true)),
+        };
+
+        let y = match y_num {
+            Some(n) => n,
+            None => return Ok(Value::Dimension(None, Unit::Deg, true)),
+        };
+
+        (x, y.convert(&y_unit, &x_unit))
+    } else {
+        return Err((
+            format!("Incompatible units {} and {}.", y_unit, x_unit),
+            args.span(),
+        )
+            .into());
+    };
+
+    Ok(
+        match (
+            NumberState::from_number(&x_num),
+            NumberState::from_number(&y_num),
+        ) {
+            (NumberState::Zero, NumberState::FiniteNegative) => {
+                Value::Dimension(Some(Number::from(-90)), Unit::Deg, true)
+            }
+            (NumberState::Zero, NumberState::Zero) | (NumberState::Finite, NumberState::Zero) => {
+                Value::Dimension(Some(Number::zero()), Unit::Deg, true)
+            }
+            (NumberState::Zero, NumberState::Finite) => {
+                Value::Dimension(Some(Number::from(90)), Unit::Deg, true)
+            }
+            (NumberState::Finite, NumberState::Finite)
+            | (NumberState::FiniteNegative, NumberState::Finite)
+            | (NumberState::Finite, NumberState::FiniteNegative)
+            | (NumberState::FiniteNegative, NumberState::FiniteNegative) => Value::Dimension(
+                y_num
+                    .atan2(x_num)
+                    .map(|n| (n * Number::from(180)) / Number::pi()),
+                Unit::Deg,
+                true,
+            ),
+            (NumberState::FiniteNegative, NumberState::Zero) => {
+                Value::Dimension(Some(Number::from(180)), Unit::Deg, true)
+            }
+        },
+    )
+}
+
+enum NumberState {
+    Zero,
+    Finite,
+    FiniteNegative,
+}
+
+impl NumberState {
+    fn from_number(num: &Number) -> Self {
+        match (num.is_zero(), num.is_positive()) {
+            (true, _) => NumberState::Zero,
+            (false, true) => NumberState::Finite,
+            (false, false) => NumberState::FiniteNegative,
+        }
+    }
+}
+
+pub(crate) fn declare(f: &mut Module) {
+    f.insert_builtin("ceil", ceil);
+    f.insert_builtin("floor", floor);
+    f.insert_builtin("max", max);
+    f.insert_builtin("min", min);
+    f.insert_builtin("round", round);
+    f.insert_builtin("abs", abs);
+    f.insert_builtin("compatible", comparable);
+    f.insert_builtin("is-unitless", unitless);
+    f.insert_builtin("unit", unit);
+    f.insert_builtin("percentage", percentage);
+    f.insert_builtin("clamp", clamp);
+    f.insert_builtin("sqrt", sqrt);
+    f.insert_builtin("cos", cos);
+    f.insert_builtin("sin", sin);
+    f.insert_builtin("tan", tan);
+    f.insert_builtin("acos", acos);
+    f.insert_builtin("asin", asin);
+    f.insert_builtin("atan", atan);
+    f.insert_builtin("log", log);
+    f.insert_builtin("pow", pow);
+    f.insert_builtin("hypot", hypot);
+    f.insert_builtin("atan2", atan2);
+    #[cfg(feature = "random")]
+    f.insert_builtin("random", random);
+
+    f.insert_builtin_var(
+        "e",
+        Value::Dimension(Some(Number::from(std::f64::consts::E)), Unit::None, true),
+    );
+    f.insert_builtin_var(
+        "pi",
+        Value::Dimension(Some(Number::from(std::f64::consts::PI)), Unit::None, true),
+    );
+}
diff --git a/src/builtin/modules/meta.rs b/src/builtin/modules/meta.rs
new file mode 100644
index 0000000..fb0b1ab
--- /dev/null
+++ b/src/builtin/modules/meta.rs
@@ -0,0 +1,127 @@
+use codemap::Spanned;
+
+use crate::{
+    args::CallArgs,
+    builtin::{
+        meta::{
+            call, content_exists, feature_exists, function_exists, get_function,
+            global_variable_exists, inspect, keywords, mixin_exists, type_of, variable_exists,
+        },
+        modules::{Module, ModuleConfig},
+    },
+    error::SassResult,
+    parse::{Parser, Stmt},
+    value::Value,
+};
+
+fn load_css(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Vec<Stmt>> {
+    args.max_args(2)?;
+
+    let span = args.span();
+
+    // todo: https://github.com/sass/dart-sass/issues/1054
+    let url = match args.get_err(0, "module")? {
+        Value::String(s, ..) => s,
+        v => {
+            return Err((
+                format!("$module: {} is not a string.", v.inspect(span)?),
+                span,
+            )
+                .into())
+        }
+    };
+
+    let with = match args.default_arg(1, "with", Value::Null)? {
+        Value::Map(map) => Some(map),
+        Value::Null => None,
+        v => return Err((format!("$with: {} is not a map.", v.inspect(span)?), span).into()),
+    };
+
+    // todo: tests for `with`
+    if let Some(with) = with {
+        let mut config = ModuleConfig::default();
+
+        for (key, value) in with {
+            let key = match key {
+                Value::String(s, ..) => s,
+                v => {
+                    return Err((
+                        format!("$with key: {} is not a string.", v.inspect(span)?),
+                        span,
+                    )
+                        .into())
+                }
+            };
+
+            config.insert(
+                Spanned {
+                    node: key.into(),
+                    span,
+                },
+                value.span(span),
+            )?;
+        }
+
+        let (_, stmts) = parser.load_module(&url, &mut config)?;
+
+        Ok(stmts)
+    } else {
+        parser.parse_single_import(&url, span)
+    }
+}
+
+fn module_functions(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+    args.max_args(1)?;
+
+    let module = match args.get_err(0, "module")? {
+        Value::String(s, ..) => s,
+        v => {
+            return Err((
+                format!("$module: {} is not a string.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    };
+
+    Ok(Value::Map(
+        parser.modules.get(module.into(), args.span())?.functions(),
+    ))
+}
+
+fn module_variables(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
+    args.max_args(1)?;
+
+    let module = match args.get_err(0, "module")? {
+        Value::String(s, ..) => s,
+        v => {
+            return Err((
+                format!("$module: {} is not a string.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    };
+
+    Ok(Value::Map(
+        parser.modules.get(module.into(), args.span())?.variables(),
+    ))
+}
+
+pub(crate) fn declare(f: &mut Module) {
+    f.insert_builtin("feature-exists", feature_exists);
+    f.insert_builtin("inspect", inspect);
+    f.insert_builtin("type-of", type_of);
+    f.insert_builtin("keywords", keywords);
+    f.insert_builtin("global-variable-exists", global_variable_exists);
+    f.insert_builtin("variable-exists", variable_exists);
+    f.insert_builtin("function-exists", function_exists);
+    f.insert_builtin("mixin-exists", mixin_exists);
+    f.insert_builtin("content-exists", content_exists);
+    f.insert_builtin("module-variables", module_variables);
+    f.insert_builtin("module-functions", module_functions);
+    f.insert_builtin("get-function", get_function);
+    f.insert_builtin("call", call);
+
+    f.insert_builtin_mixin("load-css", load_css);
+}
diff --git a/src/builtin/modules/mod.rs b/src/builtin/modules/mod.rs
new file mode 100644
index 0000000..f9da0c6
--- /dev/null
+++ b/src/builtin/modules/mod.rs
@@ -0,0 +1,282 @@
+use std::collections::BTreeMap;
+
+use codemap::{Span, Spanned};
+
+use crate::{
+    args::CallArgs,
+    atrule::mixin::{BuiltinMixin, Mixin},
+    builtin::Builtin,
+    common::{Identifier, QuoteKind},
+    error::SassResult,
+    parse::Parser,
+    scope::Scope,
+    value::{SassFunction, SassMap, Value},
+};
+
+mod color;
+mod list;
+mod map;
+mod math;
+mod meta;
+mod selector;
+mod string;
+
+#[derive(Debug, Default)]
+pub(crate) struct Module {
+    pub scope: Scope,
+
+    /// Whether or not this module is builtin
+    /// e.g. `"sass:math"`
+    is_builtin: bool,
+}
+
+#[derive(Debug, Default)]
+pub(crate) struct Modules(BTreeMap<Identifier, Module>);
+
+#[derive(Debug, Default)]
+pub(crate) struct ModuleConfig(BTreeMap<Identifier, Value>);
+
+impl ModuleConfig {
+    /// Removes and returns element with name
+    pub fn get(&mut self, name: Identifier) -> Option<Value> {
+        self.0.remove(&name)
+    }
+
+    /// If this structure is not empty at the end of
+    /// an `@use`, we must throw an error
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    pub fn insert(&mut self, name: Spanned<Identifier>, value: Spanned<Value>) -> SassResult<()> {
+        if self.0.insert(name.node, value.node).is_some() {
+            Err((
+                "The same variable may only be configured once.",
+                name.span.merge(value.span),
+            )
+                .into())
+        } else {
+            Ok(())
+        }
+    }
+}
+
+impl Modules {
+    pub fn insert(&mut self, name: Identifier, module: Module, span: Span) -> SassResult<()> {
+        if self.0.contains_key(&name) {
+            return Err((
+                format!("There's already a module with namespace \"{}\".", name),
+                span,
+            )
+                .into());
+        }
+
+        self.0.insert(name, module);
+
+        Ok(())
+    }
+
+    pub fn get(&self, name: Identifier, span: Span) -> SassResult<&Module> {
+        match self.0.get(&name) {
+            Some(v) => Ok(v),
+            None => Err((
+                format!(
+                    "There is no module with the namespace \"{}\".",
+                    name.as_str()
+                ),
+                span,
+            )
+                .into()),
+        }
+    }
+
+    pub fn get_mut(&mut self, name: Identifier, span: Span) -> SassResult<&mut Module> {
+        match self.0.get_mut(&name) {
+            Some(v) => Ok(v),
+            None => Err((
+                format!(
+                    "There is no module with the namespace \"{}\".",
+                    name.as_str()
+                ),
+                span,
+            )
+                .into()),
+        }
+    }
+}
+
+impl Module {
+    pub fn new_builtin() -> Self {
+        Module {
+            scope: Scope::default(),
+            is_builtin: true,
+        }
+    }
+
+    pub fn get_var(&self, name: Spanned<Identifier>) -> SassResult<&Value> {
+        if name.node.as_str().starts_with('-') {
+            return Err((
+                "Private members can't be accessed from outside their modules.",
+                name.span,
+            )
+                .into());
+        }
+
+        match self.scope.vars.get(&name.node) {
+            Some(v) => Ok(v),
+            None => Err(("Undefined variable.", name.span).into()),
+        }
+    }
+
+    pub fn update_var(&mut self, name: Spanned<Identifier>, value: Value) -> SassResult<()> {
+        if self.is_builtin {
+            return Err(("Cannot modify built-in variable.", name.span).into());
+        }
+
+        if name.node.as_str().starts_with('-') {
+            return Err((
+                "Private members can't be accessed from outside their modules.",
+                name.span,
+            )
+                .into());
+        }
+
+        if self.scope.insert_var(name.node, value).is_some() {
+            Ok(())
+        } else {
+            Err(("Undefined variable.", name.span).into())
+        }
+    }
+
+    pub fn get_mixin(&self, name: Spanned<Identifier>) -> SassResult<Mixin> {
+        if name.node.as_str().starts_with('-') {
+            return Err((
+                "Private members can't be accessed from outside their modules.",
+                name.span,
+            )
+                .into());
+        }
+
+        match self.scope.mixins.get(&name.node) {
+            Some(v) => Ok(v.clone()),
+            None => Err(("Undefined mixin.", name.span).into()),
+        }
+    }
+
+    pub fn insert_builtin_mixin(&mut self, name: &'static str, mixin: BuiltinMixin) {
+        self.scope.mixins.insert(name.into(), Mixin::Builtin(mixin));
+    }
+
+    pub fn insert_builtin_var(&mut self, name: &'static str, value: Value) {
+        self.scope.vars.insert(name.into(), value);
+    }
+
+    pub fn get_fn(&self, name: Spanned<Identifier>) -> SassResult<Option<SassFunction>> {
+        if name.node.as_str().starts_with('-') {
+            return Err((
+                "Private members can't be accessed from outside their modules.",
+                name.span,
+            )
+                .into());
+        }
+
+        Ok(self.scope.functions.get(&name.node).cloned())
+    }
+
+    pub fn var_exists(&self, name: Identifier) -> bool {
+        !name.as_str().starts_with('-') && self.scope.var_exists(name)
+    }
+
+    pub fn mixin_exists(&self, name: Identifier) -> bool {
+        !name.as_str().starts_with('-') && self.scope.mixin_exists(name)
+    }
+
+    pub fn insert_builtin(
+        &mut self,
+        name: &'static str,
+        function: fn(CallArgs, &mut Parser<'_>) -> SassResult<Value>,
+    ) {
+        let ident = name.into();
+        self.scope
+            .functions
+            .insert(ident, SassFunction::Builtin(Builtin::new(function), ident));
+    }
+
+    pub fn functions(&self) -> SassMap {
+        SassMap::new_with(
+            self.scope
+                .functions
+                .iter()
+                .filter(|(key, _)| !key.as_str().starts_with('-'))
+                .map(|(key, value)| {
+                    (
+                        Value::String(key.to_string(), QuoteKind::Quoted),
+                        Value::FunctionRef(value.clone()),
+                    )
+                })
+                .collect::<Vec<(Value, Value)>>(),
+        )
+    }
+
+    pub fn variables(&self) -> SassMap {
+        SassMap::new_with(
+            self.scope
+                .vars
+                .iter()
+                .filter(|(key, _)| !key.as_str().starts_with('-'))
+                .map(|(key, value)| {
+                    (
+                        Value::String(key.to_string(), QuoteKind::Quoted),
+                        value.clone(),
+                    )
+                })
+                .collect::<Vec<(Value, Value)>>(),
+        )
+    }
+
+    pub const fn new_from_scope(scope: Scope, is_builtin: bool) -> Self {
+        Module { scope, is_builtin }
+    }
+}
+
+pub(crate) fn declare_module_color() -> Module {
+    let mut module = Module::new_builtin();
+    color::declare(&mut module);
+    module
+}
+
+pub(crate) fn declare_module_list() -> Module {
+    let mut module = Module::new_builtin();
+    list::declare(&mut module);
+    module
+}
+
+pub(crate) fn declare_module_map() -> Module {
+    let mut module = Module::new_builtin();
+    map::declare(&mut module);
+    module
+}
+
+pub(crate) fn declare_module_math() -> Module {
+    let mut module = Module::new_builtin();
+    math::declare(&mut module);
+    module
+}
+
+pub(crate) fn declare_module_meta() -> Module {
+    let mut module = Module::new_builtin();
+    meta::declare(&mut module);
+    module
+}
+
+pub(crate) fn declare_module_selector() -> Module {
+    let mut module = Module::new_builtin();
+    selector::declare(&mut module);
+    module
+}
+
+pub(crate) fn declare_module_string() -> Module {
+    let mut module = Module::new_builtin();
+    string::declare(&mut module);
+    module
+}
diff --git a/src/builtin/modules/selector.rs b/src/builtin/modules/selector.rs
new file mode 100644
index 0000000..3181254
--- /dev/null
+++ b/src/builtin/modules/selector.rs
@@ -0,0 +1,18 @@
+use crate::{
+    builtin::modules::Module,
+    builtin::selector::{
+        is_superselector, selector_append, selector_extend, selector_nest, selector_parse,
+        selector_replace, selector_unify, simple_selectors,
+    },
+};
+
+pub(crate) fn declare(f: &mut Module) {
+    f.insert_builtin("is-superselector", is_superselector);
+    f.insert_builtin("append", selector_append);
+    f.insert_builtin("extend", selector_extend);
+    f.insert_builtin("nest", selector_nest);
+    f.insert_builtin("parse", selector_parse);
+    f.insert_builtin("replace", selector_replace);
+    f.insert_builtin("unify", selector_unify);
+    f.insert_builtin("simple-selectors", simple_selectors);
+}
diff --git a/src/builtin/modules/string.rs b/src/builtin/modules/string.rs
new file mode 100644
index 0000000..bd1b2cf
--- /dev/null
+++ b/src/builtin/modules/string.rs
@@ -0,0 +1,22 @@
+use crate::builtin::{
+    modules::Module,
+    string::{
+        quote, str_index, str_insert, str_length, str_slice, to_lower_case, to_upper_case, unquote,
+    },
+};
+
+#[cfg(feature = "random")]
+use crate::builtin::string::unique_id;
+
+pub(crate) fn declare(f: &mut Module) {
+    f.insert_builtin("quote", quote);
+    f.insert_builtin("index", str_index);
+    f.insert_builtin("insert", str_insert);
+    f.insert_builtin("length", str_length);
+    f.insert_builtin("slice", str_slice);
+    f.insert_builtin("to-lower-case", to_lower_case);
+    f.insert_builtin("to-upper-case", to_upper_case);
+    #[cfg(feature = "random")]
+    f.insert_builtin("unique-id", unique_id);
+    f.insert_builtin("unquote", unquote);
+}
diff --git a/src/color/mod.rs b/src/color/mod.rs
index ea97721..38db906 100644
--- a/src/color/mod.rs
+++ b/src/color/mod.rs
@@ -264,7 +264,7 @@ impl Color {
             return h.saturation() * Number::from(100);
         }
 
-        let red = self.red() / Number::from(255);
+        let red: Number = self.red() / Number::from(255);
         let green = self.green() / Number::from(255);
         let blue = self.blue() / Number::from(255);
 
@@ -291,7 +291,7 @@ impl Color {
             return h.luminance() * Number::from(100);
         }
 
-        let red = self.red() / Number::from(255);
+        let red: Number = self.red() / Number::from(255);
         let green = self.green() / Number::from(255);
         let blue = self.blue() / Number::from(255);
         let min = min(&red, min(&green, &blue)).clone();
diff --git a/src/lib.rs b/src/lib.rs
index 61cd1e2..4f29459 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,11 +1,11 @@
 /*! # grass
 An implementation of the Sass specification in pure rust.
 
-Spec progress as of 2020-07-24:
+Spec progress as of 2020-08-07:
 
 | Passing | Failing | Total |
 |---------|---------|-------|
-| 2935    | 2158    | 5093  |
+| 3375    | 1718    | 5093  |
 
 ## Use as library
 ```
@@ -50,6 +50,7 @@ grass input.scss
     clippy::unknown_clippy_lints,
     clippy::replace_consts,
     clippy::single_match,
+    clippy::float_arithmetic,
 
     // temporarily allowed while under heavy development.
     // eventually these allows should be refactored away
@@ -94,6 +95,7 @@ use peekmore::PeekMore;
 pub use crate::error::{SassError as Error, SassResult as Result};
 pub(crate) use crate::token::Token;
 use crate::{
+    builtin::modules::{ModuleConfig, Modules},
     lexer::Lexer,
     output::Css,
     parse::{
@@ -292,6 +294,8 @@ pub fn from_path(p: &str, options: &Options) -> Result<String> {
         extender: &mut Extender::new(empty_span),
         content_scopes: &mut Scopes::new(),
         options,
+        modules: &mut Modules::default(),
+        module_config: &mut ModuleConfig::default(),
     }
     .parse()
     .map_err(|e| raw_to_parse_error(&map, *e, options.unicode_error_messages))?;
@@ -336,6 +340,8 @@ pub fn from_string(p: String, options: &Options) -> Result<String> {
         extender: &mut Extender::new(empty_span),
         content_scopes: &mut Scopes::new(),
         options,
+        modules: &mut Modules::default(),
+        module_config: &mut ModuleConfig::default(),
     }
     .parse()
     .map_err(|e| raw_to_parse_error(&map, *e, options.unicode_error_messages))?;
@@ -371,6 +377,8 @@ pub fn from_string(p: String) -> std::result::Result<String, JsValue> {
         extender: &mut Extender::new(empty_span),
         content_scopes: &mut Scopes::new(),
         options: &Options::default(),
+        modules: &mut Modules::default(),
+        module_config: &mut ModuleConfig::default(),
     }
     .parse()
     .map_err(|e| raw_to_parse_error(&map, *e, true).to_string())?;
diff --git a/src/parse/args.rs b/src/parse/args.rs
index d8875d2..8bb3e9d 100644
--- a/src/parse/args.rs
+++ b/src/parse/args.rs
@@ -1,6 +1,6 @@
 use std::{collections::HashMap, mem};
 
-use codemap::{Span, Spanned};
+use codemap::Span;
 
 use crate::{
     args::{CallArg, CallArgs, FuncArg, FuncArgs},
@@ -72,19 +72,12 @@ impl<'a> Parser<'a> {
                     }
                 }
                 '.' => {
-                    let next = self.toks.next().ok_or(("expected \".\".", span))?;
-                    if next.kind != '.' {
-                        return Err(("expected \".\".", next.pos()).into());
-                    }
-                    let next = self.toks.next().ok_or(("expected \".\".", next.pos()))?;
-                    if next.kind != '.' {
-                        return Err(("expected \".\".", next.pos()).into());
-                    }
+                    self.expect_char('.')?;
+                    self.expect_char('.')?;
+
                     self.whitespace_or_comment();
-                    let next = self.toks.next().ok_or(("expected \")\".", next.pos()))?;
-                    if next.kind != ')' {
-                        return Err(("expected \")\".", next.pos()).into());
-                    }
+
+                    self.expect_char(')')?;
 
                     is_variadic = true;
 
@@ -119,6 +112,7 @@ impl<'a> Parser<'a> {
         }
         self.whitespace_or_comment();
         // TODO: this should NOT eat the opening curly brace
+        // todo: self.expect_char('{')?;
         match self.toks.next() {
             Some(v) if v.kind == '{' => {}
             Some(..) | None => return Err(("expected \"{\".", close_paren_span).into()),
@@ -225,11 +219,7 @@ impl<'a> Parser<'a> {
                             return Err(("expected \")\".", pos).into());
                         }
                         self.toks.next();
-                        if let Some(Token { kind: '.', .. }) = self.toks.peek() {
-                            self.toks.next();
-                        } else {
-                            return Err(("expected \".\".", pos).into());
-                        }
+                        self.expect_char('.')?;
                     } else {
                         return Err(("expected \")\".", pos).into());
                     }
@@ -323,23 +313,16 @@ impl<'a> Parser<'a> {
                             self.whitespace_or_comment();
                             continue;
                         }
-                        Some(Token { kind: '.', pos }) => {
-                            let pos = *pos;
+                        Some(Token { kind: '.', .. }) => {
                             self.toks.next();
 
-                            if let Some(Token { kind: '.', pos }) = self.toks.peek().cloned() {
-                                if !name.is_empty() {
-                                    return Err(("expected \")\".", pos).into());
-                                }
-                                self.toks.next();
-                                if let Some(Token { kind: '.', .. }) = self.toks.peek() {
-                                    self.toks.next();
-                                } else {
-                                    return Err(("expected \".\".", pos).into());
-                                }
-                            } else {
-                                return Err(("expected \")\".", pos).into());
+                            self.expect_char('.')?;
+
+                            if !name.is_empty() {
+                                return Err(("expected \")\".", self.span_before).into());
                             }
+
+                            self.expect_char('.')?;
                         }
                         Some(Token { pos, .. }) => {
                             return Err(("expected \")\".", *pos).into());
@@ -367,15 +350,8 @@ impl<'a> Parser<'a> {
         self.scopes.enter_new_scope();
         for (idx, mut arg) in fn_args.0.into_iter().enumerate() {
             if arg.is_variadic {
-                let span = args.span();
                 let arg_list = Value::ArgList(args.get_variadic()?);
-                scope.insert_var(
-                    arg.name,
-                    Spanned {
-                        node: arg_list,
-                        span,
-                    },
-                );
+                scope.insert_var(arg.name, arg_list);
                 break;
             }
             let val = match args.get(idx, arg.name) {
@@ -388,7 +364,8 @@ impl<'a> Parser<'a> {
                         )
                     }
                 },
-            }?;
+            }?
+            .node;
             self.scopes.insert_var(arg.name, val.clone());
             scope.insert_var(arg.name, val);
         }
diff --git a/src/parse/common.rs b/src/parse/common.rs
index fa724b8..d23514a 100644
--- a/src/parse/common.rs
+++ b/src/parse/common.rs
@@ -2,7 +2,7 @@ use std::ops::{BitAnd, BitOr};
 
 use codemap::Spanned;
 
-use crate::{interner::InternedString, value::Value};
+use crate::{common::Identifier, interner::InternedString, value::Value};
 
 #[derive(Debug, Clone)]
 pub(crate) struct NeverEmptyVec<T> {
@@ -42,6 +42,7 @@ impl<T> NeverEmptyVec<T> {
 pub(super) enum SelectorOrStyle {
     Selector(String),
     Style(InternedString, Option<Box<Spanned<Value>>>),
+    ModuleVariableRedeclaration(Identifier),
 }
 
 #[derive(Debug, Copy, Clone)]
diff --git a/src/parse/control_flow.rs b/src/parse/control_flow.rs
index 4902ab2..100cb3a 100644
--- a/src/parse/control_flow.rs
+++ b/src/parse/control_flow.rs
@@ -24,14 +24,10 @@ impl<'a> Parser<'a> {
 
         let init_cond = self.parse_value(true, &|_| false)?.node;
 
-        // consume the open curly brace
-        let span_before = match self.toks.next() {
-            Some(Token { kind: '{', pos }) => pos,
-            Some(..) | None => return Err(("expected \"{\".", self.span_before).into()),
-        };
+        self.expect_char('{')?;
 
         if self.toks.peek().is_none() {
-            return Err(("expected \"}\".", span_before).into());
+            return Err(("expected \"}\".", self.span_before).into());
         }
 
         self.whitespace_or_comment();
@@ -53,6 +49,8 @@ impl<'a> Parser<'a> {
                 extender: self.extender,
                 content_scopes: self.content_scopes,
                 options: self.options,
+                modules: self.modules,
+                module_config: self.module_config,
             }
             .parse_stmt()?;
         } else {
@@ -87,12 +85,7 @@ impl<'a> Parser<'a> {
                             false
                         } else {
                             let v = self.parse_value(true, &|_| false)?.node.is_true();
-                            match self.toks.next() {
-                                Some(Token { kind: '{', .. }) => {}
-                                Some(..) | None => {
-                                    return Err(("expected \"{\".", self.span_before).into())
-                                }
-                            }
+                            self.expect_char('{')?;
                             v
                         };
                         if cond {
@@ -112,6 +105,8 @@ impl<'a> Parser<'a> {
                                 extender: self.extender,
                                 content_scopes: self.content_scopes,
                                 options: self.options,
+                                modules: self.modules,
+                                module_config: self.module_config,
                             }
                             .parse_stmt()?;
                         } else {
@@ -140,6 +135,8 @@ impl<'a> Parser<'a> {
                                 extender: self.extender,
                                 content_scopes: self.content_scopes,
                                 options: self.options,
+                                modules: self.modules,
+                                module_config: self.module_config,
                             }
                             .parse_stmt();
                         }
@@ -158,17 +155,15 @@ impl<'a> Parser<'a> {
     }
 
     pub(super) fn parse_for(&mut self) -> SassResult<Vec<Stmt>> {
+        // todo: whitespace or comment
         self.whitespace();
-        let next = self
-            .toks
-            .next()
-            .ok_or(("expected \"$\".", self.span_before))?;
-        let var: Spanned<Identifier> = match next.kind {
-            '$' => self
-                .parse_identifier_no_interpolation(false)?
-                .map_node(|i| i.into()),
-            _ => return Err(("expected \"$\".", self.span_before).into()),
-        };
+        // todo: test for error here
+        self.expect_char('$')?;
+
+        let var = self
+            .parse_identifier_no_interpolation(false)?
+            .map_node(|n| n.into());
+
         self.whitespace();
         self.span_before = match self.toks.peek() {
             Some(tok) => tok.pos,
@@ -242,10 +237,11 @@ impl<'a> Parser<'a> {
         self.whitespace();
         let from_val = self.parse_value_from_vec(from_toks, true)?;
         let from = match from_val.node {
-            Value::Dimension(n, ..) => match n.to_integer().to_isize() {
+            Value::Dimension(Some(n), ..) => match n.to_integer().to_isize() {
                 Some(v) => v,
                 None => return Err((format!("{} is not a int.", n), from_val.span).into()),
             },
+            Value::Dimension(None, ..) => todo!(),
             v => {
                 return Err((
                     format!("{} is not an integer.", v.inspect(from_val.span)?),
@@ -257,10 +253,11 @@ impl<'a> Parser<'a> {
 
         let to_val = self.parse_value(true, &|_| false)?;
         let to = match to_val.node {
-            Value::Dimension(n, ..) => match n.to_integer().to_isize() {
+            Value::Dimension(Some(n), ..) => match n.to_integer().to_isize() {
                 Some(v) => v,
                 None => return Err((format!("{} is not a int.", n), to_val.span).into()),
             },
+            Value::Dimension(None, ..) => todo!(),
             v => {
                 return Err((
                     format!("{} is not an integer.", v.to_css_string(to_val.span)?),
@@ -270,11 +267,7 @@ impl<'a> Parser<'a> {
             }
         };
 
-        // consume the open curly brace
-        match self.toks.next() {
-            Some(Token { kind: '{', pos }) => pos,
-            Some(..) | None => return Err(("expected \"{\".", to_val.span).into()),
-        };
+        self.expect_char('{')?;
 
         let body = read_until_closing_curly_brace(self.toks)?;
         self.toks.next();
@@ -299,10 +292,7 @@ impl<'a> Parser<'a> {
         for i in iter {
             self.scopes.insert_var_last(
                 var.node,
-                Spanned {
-                    node: Value::Dimension(Number::from(i), Unit::None, true),
-                    span: var.span,
-                },
+                Value::Dimension(Some(Number::from(i)), Unit::None, true),
             );
             if self.flags.in_function() {
                 let these_stmts = Parser {
@@ -320,8 +310,10 @@ impl<'a> Parser<'a> {
                     extender: self.extender,
                     content_scopes: self.content_scopes,
                     options: self.options,
+                    modules: self.modules,
+                    module_config: self.module_config,
                 }
-                .parse()?;
+                .parse_stmt()?;
                 if !these_stmts.is_empty() {
                     return Ok(these_stmts);
                 }
@@ -342,8 +334,10 @@ impl<'a> Parser<'a> {
                         extender: self.extender,
                         content_scopes: self.content_scopes,
                         options: self.options,
+                        modules: self.modules,
+                        module_config: self.module_config,
                     }
-                    .parse()?,
+                    .parse_stmt()?,
                 );
             }
         }
@@ -392,8 +386,10 @@ impl<'a> Parser<'a> {
                     extender: self.extender,
                     content_scopes: self.content_scopes,
                     options: self.options,
+                    modules: self.modules,
+                    module_config: self.module_config,
                 }
-                .parse()?;
+                .parse_stmt()?;
                 if !these_stmts.is_empty() {
                     return Ok(these_stmts);
                 }
@@ -414,8 +410,10 @@ impl<'a> Parser<'a> {
                         extender: self.extender,
                         content_scopes: self.content_scopes,
                         options: self.options,
+                        modules: self.modules,
+                        module_config: self.module_config,
                     }
-                    .parse()?,
+                    .parse_stmt()?,
                 );
             }
             val = self.parse_value_from_vec(cond.clone(), true)?;
@@ -430,15 +428,11 @@ impl<'a> Parser<'a> {
         let mut vars: Vec<Spanned<Identifier>> = Vec::new();
 
         loop {
-            let next = self
-                .toks
-                .next()
-                .ok_or(("expected \"$\".", self.span_before))?;
+            self.expect_char('$')?;
 
-            match next.kind {
-                '$' => vars.push(self.parse_identifier()?.map_node(|i| i.into())),
-                _ => return Err(("expected \"$\".", next.pos()).into()),
-            }
+            vars.push(self.parse_identifier()?.map_node(|i| i.into()));
+
+            // todo: whitespace or comment
             self.whitespace();
             if self
                 .toks
@@ -478,26 +472,14 @@ impl<'a> Parser<'a> {
 
         for row in iter {
             if vars.len() == 1 {
-                self.scopes.insert_var_last(
-                    vars[0].node,
-                    Spanned {
-                        node: row,
-                        span: vars[0].span,
-                    },
-                );
+                self.scopes.insert_var_last(vars[0].node, row);
             } else {
                 for (var, val) in vars.iter().zip(
                     row.as_list()
                         .into_iter()
                         .chain(std::iter::once(Value::Null).cycle()),
                 ) {
-                    self.scopes.insert_var_last(
-                        var.node,
-                        Spanned {
-                            node: val,
-                            span: var.span,
-                        },
-                    );
+                    self.scopes.insert_var_last(var.node, val);
                 }
             }
 
@@ -517,8 +499,10 @@ impl<'a> Parser<'a> {
                     extender: self.extender,
                     content_scopes: self.content_scopes,
                     options: self.options,
+                    modules: self.modules,
+                    module_config: self.module_config,
                 }
-                .parse()?;
+                .parse_stmt()?;
                 if !these_stmts.is_empty() {
                     return Ok(these_stmts);
                 }
@@ -539,8 +523,10 @@ impl<'a> Parser<'a> {
                         extender: self.extender,
                         content_scopes: self.content_scopes,
                         options: self.options,
+                        modules: self.modules,
+                        module_config: self.module_config,
                     }
-                    .parse()?,
+                    .parse_stmt()?,
                 );
             }
         }
diff --git a/src/parse/function.rs b/src/parse/function.rs
index 4560abe..5b02227 100644
--- a/src/parse/function.rs
+++ b/src/parse/function.rs
@@ -4,11 +4,11 @@ use peekmore::PeekMore;
 use crate::{
     args::CallArgs,
     atrule::Function,
-    common::unvendor,
+    common::{unvendor, Identifier},
     error::SassResult,
     scope::Scopes,
     utils::{read_until_closing_curly_brace, read_until_semicolon_or_closing_curly_brace},
-    value::Value,
+    value::{SassFunction, Value},
     Token,
 };
 
@@ -40,11 +40,9 @@ impl<'a> Parser<'a> {
         }
 
         self.whitespace_or_comment();
-        let args = match self.toks.next() {
-            Some(Token { kind: '(', .. }) => self.parse_func_args()?,
-            Some(Token { pos, .. }) => return Err(("expected \"(\".", pos).into()),
-            None => return Err(("expected \"(\".", span).into()),
-        };
+        self.expect_char('(')?;
+
+        let args = self.parse_func_args()?;
 
         self.whitespace();
 
@@ -57,10 +55,18 @@ impl<'a> Parser<'a> {
 
         let function = Function::new(args, body, self.at_root, span);
 
+        let name_as_ident = Identifier::from(name);
+
         if self.at_root {
-            self.global_scope.insert_fn(name, function);
+            self.global_scope.insert_fn(
+                name_as_ident,
+                SassFunction::UserDefined(Box::new(function), name_as_ident),
+            );
         } else {
-            self.scopes.insert_fn(name.into(), function);
+            self.scopes.insert_fn(
+                name_as_ident,
+                SassFunction::UserDefined(Box::new(function), name_as_ident),
+            );
         }
         Ok(())
     }
@@ -112,8 +118,10 @@ impl<'a> Parser<'a> {
             extender: self.extender,
             content_scopes: self.content_scopes,
             options: self.options,
+            modules: self.modules,
+            module_config: self.module_config,
         }
-        .parse()?;
+        .parse_stmt()?;
 
         if entered_scope {
             self.scopes.exit_scope();
diff --git a/src/parse/import.rs b/src/parse/import.rs
index ce6d918..cd17e8c 100644
--- a/src/parse/import.rs
+++ b/src/parse/import.rs
@@ -13,60 +13,13 @@ use crate::{
 
 use super::{Parser, Stmt};
 
-/// Searches the current directory of the file then searches in `load_paths` directories
-/// if the import has not yet been found.
-/// <https://sass-lang.com/documentation/at-rules/import#finding-the-file>
-/// <https://sass-lang.com/documentation/at-rules/import#load-paths>
-fn find_import(file_path: &PathBuf, name: &OsStr, load_paths: &[&Path]) -> Option<PathBuf> {
-    let paths = [
-        file_path.with_file_name(name).with_extension("scss"),
-        file_path
-            .with_file_name(format!("_{}", name.to_str().unwrap()))
-            .with_extension("scss"),
-        file_path.clone(),
-        file_path.join("index.scss"),
-        file_path.join("_index.scss"),
-    ];
-
-    for name in &paths {
-        if name.is_file() {
-            return Some(name.to_path_buf());
-        }
-    }
-
-    for path in load_paths {
-        let paths: Vec<PathBuf> = if path.is_dir() {
-            vec![
-                path.join(format!("{}.scss", name.to_str().unwrap())),
-                path.join(format!("_{}.scss", name.to_str().unwrap())),
-                path.join("index.scss"),
-                path.join("_index.scss"),
-            ]
-        } else {
-            vec![
-                path.to_path_buf(),
-                path.with_file_name(name).with_extension("scss"),
-                path.with_file_name(format!("_{}", name.to_str().unwrap()))
-                    .with_extension("scss"),
-                path.join("index.scss"),
-                path.join("_index.scss"),
-            ]
-        };
-
-        for name in paths {
-            if name.is_file() {
-                return Some(name);
-            }
-        }
-    }
-
-    None
-}
-
 impl<'a> Parser<'a> {
-    fn parse_single_import(&mut self, file_name: &str, span: Span) -> SassResult<Vec<Stmt>> {
-        let path: &Path = file_name.as_ref();
-
+    /// Searches the current directory of the file then searches in `load_paths` directories
+    /// if the import has not yet been found.
+    ///
+    /// <https://sass-lang.com/documentation/at-rules/import#finding-the-file>
+    /// <https://sass-lang.com/documentation/at-rules/import#load-paths>
+    pub(super) fn find_import(&self, path: &Path) -> Option<PathBuf> {
         let path_buf = if path.is_absolute() {
             // todo: test for absolute path imports
             path.into()
@@ -79,7 +32,59 @@ impl<'a> Parser<'a> {
 
         let name = path_buf.file_name().unwrap_or_else(|| OsStr::new(".."));
 
-        if let Some(name) = find_import(&path_buf, name, &self.options.load_paths) {
+        let paths = [
+            path_buf.with_file_name(name).with_extension("scss"),
+            path_buf
+                .with_file_name(format!("_{}", name.to_str().unwrap()))
+                .with_extension("scss"),
+            path_buf.clone(),
+            path_buf.join("index.scss"),
+            path_buf.join("_index.scss"),
+        ];
+
+        for name in &paths {
+            if name.is_file() {
+                return Some(name.to_path_buf());
+            }
+        }
+
+        for path in &self.options.load_paths {
+            let paths: Vec<PathBuf> = if path.is_dir() {
+                vec![
+                    path.join(format!("{}.scss", name.to_str().unwrap())),
+                    path.join(format!("_{}.scss", name.to_str().unwrap())),
+                    path.join("index.scss"),
+                    path.join("_index.scss"),
+                ]
+            } else {
+                vec![
+                    path.to_path_buf(),
+                    path.with_file_name(name).with_extension("scss"),
+                    path.with_file_name(format!("_{}", name.to_str().unwrap()))
+                        .with_extension("scss"),
+                    path.join("index.scss"),
+                    path.join("_index.scss"),
+                ]
+            };
+
+            for name in paths {
+                if name.is_file() {
+                    return Some(name);
+                }
+            }
+        }
+
+        None
+    }
+
+    pub(crate) fn parse_single_import(
+        &mut self,
+        file_name: &str,
+        span: Span,
+    ) -> SassResult<Vec<Stmt>> {
+        let path: &Path = file_name.as_ref();
+
+        if let Some(name) = self.find_import(path) {
             let file = self.map.add_file(
                 name.to_string_lossy().into(),
                 String::from_utf8(fs::read(&name)?)?,
@@ -102,10 +107,11 @@ impl<'a> Parser<'a> {
                 extender: self.extender,
                 content_scopes: self.content_scopes,
                 options: self.options,
+                modules: self.modules,
+                module_config: self.module_config,
             }
             .parse();
         }
-        self.whitespace();
 
         Err(("Can't find stylesheet to import.", span).into())
     }
diff --git a/src/parse/keyframes.rs b/src/parse/keyframes.rs
index c02092f..b082d03 100644
--- a/src/parse/keyframes.rs
+++ b/src/parse/keyframes.rs
@@ -63,9 +63,8 @@ impl<'a, 'b> KeyframesSelectorParser<'a, 'b> {
                         num.push_str(&eat_whole_number(self.parser.toks));
                     }
 
-                    if !matches!(self.parser.toks.next(), Some(Token { kind: '%', .. })) {
-                        return Err(("expected \"%\".", tok.pos).into());
-                    }
+                    self.parser.expect_char('%')?;
+
                     selectors.push(KeyframesSelector::Percent(num.into_boxed_str()));
                 }
                 '{' => break,
@@ -173,6 +172,8 @@ impl<'a> Parser<'a> {
                         extender: self.extender,
                         content_scopes: self.content_scopes,
                         options: self.options,
+                        modules: self.modules,
+                        module_config: self.module_config,
                     })
                     .parse_keyframes_selector()?;
 
@@ -208,6 +209,8 @@ impl<'a> Parser<'a> {
             extender: self.extender,
             content_scopes: self.content_scopes,
             options: self.options,
+            modules: self.modules,
+            module_config: self.module_config,
         }
         .parse_stmt()?;
 
diff --git a/src/parse/media.rs b/src/parse/media.rs
index 2a0c036..61f124a 100644
--- a/src/parse/media.rs
+++ b/src/parse/media.rs
@@ -25,16 +25,6 @@ impl<'a> Parser<'a> {
         Ok(false)
     }
 
-    pub fn expect_char(&mut self, c: char) -> SassResult<()> {
-        if let Some(Token { kind, .. }) = self.toks.peek() {
-            if *kind == c {
-                self.toks.next();
-                return Ok(());
-            }
-        }
-        Err((format!("expected \"{}\".", c), self.span_before).into())
-    }
-
     pub fn scan_char(&mut self, c: char) -> bool {
         if let Some(Token { kind, .. }) = self.toks.peek() {
             if *kind == c {
diff --git a/src/parse/mixin.rs b/src/parse/mixin.rs
index 7b55003..f0764b7 100644
--- a/src/parse/mixin.rs
+++ b/src/parse/mixin.rs
@@ -6,7 +6,7 @@ use peekmore::PeekMore;
 
 use crate::{
     args::{CallArgs, FuncArgs},
-    atrule::{Content, Mixin},
+    atrule::mixin::{Content, Mixin, UserDefinedMixin},
     error::SassResult,
     scope::Scopes,
     utils::read_until_closing_curly_brace,
@@ -55,7 +55,7 @@ impl<'a> Parser<'a> {
         // this is blocked on figuring out just how to check for this. presumably we could have a check
         // not when parsing initially, but rather when `@include`ing to see if an `@content` was found.
 
-        let mixin = Mixin::new(args, body, false, self.at_root);
+        let mixin = Mixin::new_user_defined(args, body, false, self.at_root);
 
         if self.at_root {
             self.global_scope.insert_mixin(name, mixin);
@@ -73,6 +73,19 @@ impl<'a> Parser<'a> {
         self.whitespace_or_comment();
         let name = self.parse_identifier()?.map_node(Into::into);
 
+        let mixin = if let Some(Token { kind: '.', .. }) = self.toks.peek() {
+            self.toks.next();
+
+            let module = name;
+            let name = self.parse_identifier()?.map_node(Into::into);
+
+            self.modules
+                .get(module.node, module.span)?
+                .get_mixin(name)?
+        } else {
+            self.scopes.get_mixin(name, self.global_scope)?
+        };
+
         self.whitespace_or_comment();
 
         let args = if let Some(Token { kind: '(', .. }) = self.toks.peek() {
@@ -91,9 +104,7 @@ impl<'a> Parser<'a> {
             ident.node.make_ascii_lowercase();
             if ident.node == "using" {
                 self.whitespace_or_comment();
-                if !matches!(self.toks.next(), Some(Token { kind: '(', .. })) {
-                    return Err(("expected \"(\".", ident.span).into());
-                }
+                self.expect_char('(')?;
 
                 Some(self.parse_func_args()?)
             } else {
@@ -125,12 +136,17 @@ impl<'a> Parser<'a> {
             self.toks.next();
         }
 
-        let Mixin {
+        let UserDefinedMixin {
             body,
             args: fn_args,
             declared_at_root,
             ..
-        } = self.scopes.get_mixin(name, self.global_scope)?;
+        } = match mixin {
+            Mixin::UserDefined(u) => u,
+            Mixin::Builtin(b) => {
+                return b(args, self);
+            }
+        };
 
         let scope = self.eval_args(fn_args, args)?;
 
@@ -164,8 +180,10 @@ impl<'a> Parser<'a> {
             extender: self.extender,
             content_scopes: self.content_scopes,
             options: self.options,
+            modules: self.modules,
+            module_config: self.module_config,
         }
-        .parse()?;
+        .parse_stmt()?;
 
         self.content.pop();
         self.scopes.exit_scope();
@@ -225,8 +243,10 @@ impl<'a> Parser<'a> {
                     extender: self.extender,
                     content_scopes: self.scopes,
                     options: self.options,
+                    modules: self.modules,
+                    module_config: self.module_config,
                 }
-                .parse()?
+                .parse_stmt()?
             } else {
                 Vec::new()
             };
diff --git a/src/parse/mod.rs b/src/parse/mod.rs
index de5d5ee..13c8a5e 100644
--- a/src/parse/mod.rs
+++ b/src/parse/mod.rs
@@ -7,8 +7,10 @@ use crate::{
     atrule::{
         keyframes::{Keyframes, KeyframesRuleSet},
         media::MediaRule,
-        AtRuleKind, Content, SupportsRule, UnknownAtRule,
+        mixin::Content,
+        AtRuleKind, SupportsRule, UnknownAtRule,
     },
+    builtin::modules::{ModuleConfig, Modules},
     error::SassResult,
     scope::{Scope, Scopes},
     selector::{
@@ -25,6 +27,7 @@ use crate::{
 
 use common::{Comment, ContextFlags, NeverEmptyVec, SelectorOrStyle};
 pub(crate) use value::{HigherIntermediateValue, ValueVisitor};
+use variable::VariableValue;
 
 mod args;
 pub mod common;
@@ -35,6 +38,7 @@ mod import;
 mod keyframes;
 mod media;
 mod mixin;
+mod module;
 mod style;
 mod throw_away;
 mod value;
@@ -86,11 +90,18 @@ pub(crate) struct Parser<'a> {
     pub extender: &'a mut Extender,
 
     pub options: &'a Options<'a>,
+
+    pub modules: &'a mut Modules,
+    pub module_config: &'a mut ModuleConfig,
 }
 
 impl<'a> Parser<'a> {
     pub fn parse(&mut self) -> SassResult<Vec<Stmt>> {
         let mut stmts = Vec::new();
+
+        self.whitespace();
+        stmts.append(&mut self.load_modules()?);
+
         while self.toks.peek().is_some() {
             stmts.append(&mut self.parse_stmt()?);
             if self.flags.in_function() && !stmts.is_empty() {
@@ -101,6 +112,26 @@ impl<'a> Parser<'a> {
         Ok(stmts)
     }
 
+    pub fn expect_char(&mut self, c: char) -> SassResult<()> {
+        match self.toks.peek() {
+            Some(Token { kind, pos }) if *kind == c => {
+                self.span_before = *pos;
+                self.toks.next();
+                Ok(())
+            }
+            Some(Token { pos, .. }) => Err((format!("expected \"{}\".", c), *pos).into()),
+            None => Err((format!("expected \"{}\".", c), self.span_before).into()),
+        }
+    }
+
+    pub fn consume_char_if_exists(&mut self, c: char) {
+        if let Some(Token { kind, .. }) = self.toks.peek() {
+            if *kind == c {
+                self.toks.next();
+            }
+        }
+    }
+
     fn parse_stmt(&mut self) -> SassResult<Vec<Stmt>> {
         let mut stmts = Vec::new();
         while let Some(Token { kind, pos }) = self.toks.peek() {
@@ -212,7 +243,13 @@ impl<'a> Parser<'a> {
                         AtRuleKind::Unknown(_) => {
                             stmts.push(self.parse_unknown_at_rule(kind_string.node)?)
                         }
-                        AtRuleKind::Use => todo!("@use not yet implemented"),
+                        AtRuleKind::Use => {
+                            return Err((
+                                "@use rules must be written before any other rules.",
+                                kind_string.span,
+                            )
+                                .into())
+                        }
                         AtRuleKind::Forward => todo!("@forward not yet implemented"),
                         AtRuleKind::Extend => self.parse_extend()?,
                         AtRuleKind::Supports => stmts.push(self.parse_supports()?),
@@ -258,6 +295,9 @@ impl<'a> Parser<'a> {
                     }
                     if self.flags.in_keyframes() {
                         match self.is_selector_or_style()? {
+                            SelectorOrStyle::ModuleVariableRedeclaration(module) => {
+                                self.parse_module_variable_redeclaration(module)?
+                            }
                             SelectorOrStyle::Style(property, value) => {
                                 if let Some(value) = value {
                                     stmts.push(Stmt::Style(Style { property, value }));
@@ -285,6 +325,9 @@ impl<'a> Parser<'a> {
                     }
 
                     match self.is_selector_or_style()? {
+                        SelectorOrStyle::ModuleVariableRedeclaration(module) => {
+                            self.parse_module_variable_redeclaration(module)?
+                        }
                         SelectorOrStyle::Style(property, value) => {
                             if let Some(value) = value {
                                 stmts.push(Stmt::Style(Style { property, value }));
@@ -412,6 +455,8 @@ impl<'a> Parser<'a> {
                 extender: self.extender,
                 content_scopes: self.content_scopes,
                 options: self.options,
+                modules: self.modules,
+                module_config: self.module_config,
             },
             allows_parent,
             true,
@@ -470,10 +515,11 @@ impl<'a> Parser<'a> {
 
     pub fn parse_interpolation(&mut self) -> SassResult<Spanned<Value>> {
         let val = self.parse_value(true, &|_| false)?;
-        match self.toks.next() {
-            Some(Token { kind: '}', .. }) => {}
-            Some(..) | None => return Err(("expected \"}\".", val.span).into()),
-        }
+
+        self.span_before = val.span;
+
+        self.expect_char('}')?;
+
         Ok(val.map_node(Value::unquote))
     }
 
@@ -640,9 +686,7 @@ impl<'a> Parser<'a> {
 
         self.whitespace();
 
-        if !matches!(self.toks.next(), Some(Token { kind: '{', .. })) {
-            return Err(("expected \"{\".", self.span_before).into());
-        }
+        self.expect_char('{')?;
 
         let raw_body = self.parse_stmt()?;
 
@@ -711,8 +755,10 @@ impl<'a> Parser<'a> {
             extender: self.extender,
             content_scopes: self.content_scopes,
             options: self.options,
+            modules: self.modules,
+            module_config: self.module_config,
         }
-        .parse()?
+        .parse_stmt()?
         .into_iter()
         .filter_map(|s| match s {
             Stmt::Style(..) => {
@@ -755,14 +801,15 @@ impl<'a> Parser<'a> {
             extender: self.extender,
             content_scopes: self.content_scopes,
             options: self.options,
+            modules: self.modules,
+            module_config: self.module_config,
         }
         .parse_selector(false, true, String::new())?;
 
+        // todo: this might be superfluous
         self.whitespace();
 
-        if let Some(Token { kind: ';', .. }) = self.toks.peek() {
-            self.toks.next();
-        }
+        self.consume_char_if_exists(';');
 
         let extend_rule = ExtendRule::new(value.clone(), is_optional, self.span_before);
 
diff --git a/src/parse/module.rs b/src/parse/module.rs
new file mode 100644
index 0000000..f6a056b
--- /dev/null
+++ b/src/parse/module.rs
@@ -0,0 +1,291 @@
+use std::{convert::TryFrom, fs};
+
+use codemap::Spanned;
+use peekmore::PeekMore;
+
+use crate::{
+    atrule::AtRuleKind,
+    builtin::modules::{
+        declare_module_color, declare_module_list, declare_module_map, declare_module_math,
+        declare_module_meta, declare_module_selector, declare_module_string, Module, ModuleConfig,
+    },
+    common::Identifier,
+    error::SassResult,
+    lexer::Lexer,
+    parse::{common::Comment, Parser, Stmt, VariableValue},
+    scope::Scope,
+    utils::peek_ident_no_interpolation,
+    Token,
+};
+
+impl<'a> Parser<'a> {
+    fn parse_module_alias(&mut self) -> SassResult<Option<String>> {
+        if let Some(Token { kind: 'a', .. }) | Some(Token { kind: 'A', .. }) = self.toks.peek() {
+            let mut ident = peek_ident_no_interpolation(self.toks, false, self.span_before)?;
+            ident.node.make_ascii_lowercase();
+            if ident.node != "as" {
+                return Err(("expected \";\".", ident.span).into());
+            }
+
+            self.whitespace_or_comment();
+
+            if let Some(Token { kind: '*', .. }) = self.toks.peek() {
+                self.toks.next();
+                return Ok(Some('*'.to_string()));
+            } else {
+                let name = self.parse_identifier_no_interpolation(false)?;
+
+                return Ok(Some(name.node));
+            }
+        }
+
+        Ok(None)
+    }
+
+    fn parse_module_config(&mut self) -> SassResult<ModuleConfig> {
+        let mut config = ModuleConfig::default();
+
+        if let Some(Token { kind: 'w', .. }) | Some(Token { kind: 'W', .. }) = self.toks.peek() {
+            let mut ident = peek_ident_no_interpolation(self.toks, false, self.span_before)?;
+            ident.node.make_ascii_lowercase();
+            if ident.node != "with" {
+                return Err(("expected \";\".", ident.span).into());
+            }
+
+            self.whitespace_or_comment();
+
+            self.span_before = ident.span;
+
+            self.expect_char('(')?;
+
+            loop {
+                self.whitespace_or_comment();
+                self.expect_char('$')?;
+
+                let name = self.parse_identifier_no_interpolation(false)?;
+
+                self.whitespace_or_comment();
+                self.expect_char(':')?;
+                self.whitespace_or_comment();
+
+                let value = self.parse_value(false, &|toks| match toks.peek() {
+                    Some(Token { kind: ',', .. }) | Some(Token { kind: ')', .. }) => true,
+                    _ => false,
+                })?;
+
+                config.insert(name.map_node(|n| n.into()), value)?;
+
+                match self.toks.next() {
+                    Some(Token { kind: ',', .. }) => {
+                        continue;
+                    }
+                    Some(Token { kind: ')', .. }) => {
+                        break;
+                    }
+                    Some(..) | None => {
+                        return Err(("expected \")\".", self.span_before).into());
+                    }
+                }
+            }
+        }
+
+        Ok(config)
+    }
+
+    pub fn load_module(
+        &mut self,
+        name: &str,
+        config: &mut ModuleConfig,
+    ) -> SassResult<(Module, Vec<Stmt>)> {
+        Ok(match name {
+            "sass:color" => (declare_module_color(), Vec::new()),
+            "sass:list" => (declare_module_list(), Vec::new()),
+            "sass:map" => (declare_module_map(), Vec::new()),
+            "sass:math" => (declare_module_math(), Vec::new()),
+            "sass:meta" => (declare_module_meta(), Vec::new()),
+            "sass:selector" => (declare_module_selector(), Vec::new()),
+            "sass:string" => (declare_module_string(), Vec::new()),
+            _ => {
+                if let Some(import) = self.find_import(name.as_ref()) {
+                    let mut global_scope = Scope::new();
+
+                    let file = self
+                        .map
+                        .add_file(name.to_owned(), String::from_utf8(fs::read(&import)?)?);
+
+                    let stmts = Parser {
+                        toks: &mut Lexer::new(&file)
+                            .collect::<Vec<Token>>()
+                            .into_iter()
+                            .peekmore(),
+                        map: self.map,
+                        path: &import,
+                        scopes: self.scopes,
+                        global_scope: &mut global_scope,
+                        super_selectors: self.super_selectors,
+                        span_before: file.span.subspan(0, 0),
+                        content: self.content,
+                        flags: self.flags,
+                        at_root: self.at_root,
+                        at_root_has_selector: self.at_root_has_selector,
+                        extender: self.extender,
+                        content_scopes: self.content_scopes,
+                        options: self.options,
+                        modules: self.modules,
+                        module_config: config,
+                    }
+                    .parse()?;
+
+                    if !config.is_empty() {
+                        return Err((
+                            "This variable was not declared with !default in the @used module.",
+                            self.span_before,
+                        )
+                            .into());
+                    }
+
+                    (Module::new_from_scope(global_scope, false), stmts)
+                } else {
+                    return Err(("Can't find stylesheet to import.", self.span_before).into());
+                }
+            }
+        })
+    }
+
+    /// Returns any multiline comments that may have been found
+    /// while loading modules
+    pub(super) fn load_modules(&mut self) -> SassResult<Vec<Stmt>> {
+        let mut comments = Vec::new();
+
+        loop {
+            self.whitespace();
+            match self.toks.peek() {
+                Some(Token { kind: '@', .. }) => {
+                    self.toks.advance_cursor();
+
+                    if let Some(Token { kind, .. }) = self.toks.peek() {
+                        if !matches!(kind, 'a'..='z' | 'A'..='Z' | '\\') {
+                            break;
+                        }
+                    }
+
+                    match AtRuleKind::try_from(&peek_ident_no_interpolation(
+                        self.toks,
+                        false,
+                        self.span_before,
+                    )?)? {
+                        AtRuleKind::Use => {
+                            self.toks.truncate_iterator_to_cursor();
+                        }
+                        _ => {
+                            break;
+                        }
+                    }
+
+                    self.whitespace_or_comment();
+
+                    let quote = match self.toks.next() {
+                        Some(Token { kind: q @ '"', .. }) | Some(Token { kind: q @ '\'', .. }) => q,
+                        Some(..) | None => todo!(),
+                    };
+
+                    let Spanned { node: module, span } = self.parse_quoted_string(quote)?;
+                    let module_name = module.unquote().to_css_string(span)?;
+
+                    self.whitespace_or_comment();
+
+                    let module_alias = self.parse_module_alias()?;
+
+                    self.whitespace_or_comment();
+
+                    let mut config = self.parse_module_config()?;
+
+                    self.whitespace_or_comment();
+                    self.expect_char(';')?;
+
+                    let (module, mut stmts) =
+                        self.load_module(module_name.as_ref(), &mut config)?;
+
+                    comments.append(&mut stmts);
+
+                    // if the config isn't empty here, that means
+                    // variables were passed to a builtin module
+                    if !config.is_empty() {
+                        return Err(("Built-in modules can't be configured.", span).into());
+                    }
+
+                    let module_name = match module_alias.as_deref() {
+                        Some("*") => {
+                            self.global_scope.merge_module(module);
+                            continue;
+                        }
+                        Some(..) => module_alias.unwrap(),
+                        None => match module_name.as_ref() {
+                            "sass:color" => "color".to_owned(),
+                            "sass:list" => "list".to_owned(),
+                            "sass:map" => "map".to_owned(),
+                            "sass:math" => "math".to_owned(),
+                            "sass:meta" => "meta".to_owned(),
+                            "sass:selector" => "selector".to_owned(),
+                            "sass:string" => "string".to_owned(),
+                            _ => module_name.into_owned(),
+                        },
+                    };
+
+                    self.modules.insert(module_name.into(), module, span)?;
+                }
+                Some(Token { kind: '/', .. }) => {
+                    self.toks.next();
+                    match self.parse_comment()?.node {
+                        Comment::Silent => continue,
+                        Comment::Loud(s) => comments.push(Stmt::Comment(s)),
+                    }
+                }
+                Some(Token { kind: '$', .. }) => self.parse_variable_declaration()?,
+                Some(..) | None => break,
+            }
+        }
+
+        self.toks.reset_cursor();
+
+        Ok(comments)
+    }
+
+    pub(super) fn parse_module_variable_redeclaration(
+        &mut self,
+        module: Identifier,
+    ) -> SassResult<()> {
+        let variable = self
+            .parse_identifier_no_interpolation(false)?
+            .map_node(|n| n.into());
+
+        self.whitespace_or_comment();
+        self.expect_char(':')?;
+
+        let VariableValue {
+            val_toks,
+            global,
+            default,
+        } = self.parse_variable_value()?;
+
+        if global {
+            return Err((
+                "!global isn't allowed for variables in other modules.",
+                variable.span,
+            )
+                .into());
+        }
+
+        if default {
+            return Ok(());
+        }
+
+        let value = self.parse_value_from_vec(val_toks, true)?;
+
+        self.modules
+            .get_mut(module, variable.span)?
+            .update_var(variable, value.node)?;
+
+        Ok(())
+    }
+}
diff --git a/src/parse/style.rs b/src/parse/style.rs
index 0c40a66..0e10bd1 100644
--- a/src/parse/style.rs
+++ b/src/parse/style.rs
@@ -111,43 +111,62 @@ impl<'a> Parser<'a> {
         let mut property = self.parse_identifier()?.node;
         let whitespace_after_property = self.whitespace();
 
-        if let Some(Token { kind: ':', .. }) = self.toks.peek() {
-            self.toks.next();
-            if let Some(Token { kind, .. }) = self.toks.peek() {
-                return Ok(match kind {
-                    ':' => {
-                        if whitespace_after_property {
-                            property.push(' ');
-                        }
-                        property.push(':');
-                        SelectorOrStyle::Selector(property)
-                    }
-                    c if is_name(*c) => {
-                        if let Some(toks) = self.parse_style_value_when_no_space_after_semicolon() {
-                            let len = toks.len();
-                            if let Ok(val) = self.parse_value_from_vec(toks, false) {
-                                self.toks.take(len).for_each(drop);
-                                return Ok(SelectorOrStyle::Style(
-                                    InternedString::get_or_intern(property),
-                                    Some(Box::new(val)),
-                                ));
+        match self.toks.peek() {
+            Some(Token { kind: ':', .. }) => {
+                self.toks.next();
+                if let Some(Token { kind, .. }) = self.toks.peek() {
+                    return Ok(match kind {
+                        ':' => {
+                            if whitespace_after_property {
+                                property.push(' ');
                             }
+                            property.push(':');
+                            SelectorOrStyle::Selector(property)
                         }
+                        c if is_name(*c) => {
+                            if let Some(toks) =
+                                self.parse_style_value_when_no_space_after_semicolon()
+                            {
+                                let len = toks.len();
+                                if let Ok(val) = self.parse_value_from_vec(toks, false) {
+                                    self.toks.take(len).for_each(drop);
+                                    return Ok(SelectorOrStyle::Style(
+                                        InternedString::get_or_intern(property),
+                                        Some(Box::new(val)),
+                                    ));
+                                }
+                            }
 
-                        if whitespace_after_property {
-                            property.push(' ');
+                            if whitespace_after_property {
+                                property.push(' ');
+                            }
+                            property.push(':');
+                            return Ok(SelectorOrStyle::Selector(property));
                         }
-                        property.push(':');
-                        return Ok(SelectorOrStyle::Selector(property));
+                        _ => SelectorOrStyle::Style(InternedString::get_or_intern(property), None),
+                    });
+                }
+            }
+            Some(Token { kind: '.', .. }) => {
+                if matches!(self.toks.peek_next(), Some(Token { kind: '$', .. })) {
+                    self.toks.next();
+                    self.toks.next();
+                    return Ok(SelectorOrStyle::ModuleVariableRedeclaration(
+                        property.into(),
+                    ));
+                } else {
+                    if whitespace_after_property {
+                        property.push(' ');
                     }
-                    _ => SelectorOrStyle::Style(InternedString::get_or_intern(property), None),
-                });
+                    return Ok(SelectorOrStyle::Selector(property));
+                }
             }
-        } else {
-            if whitespace_after_property {
-                property.push(' ');
+            _ => {
+                if whitespace_after_property {
+                    property.push(' ');
+                }
+                return Ok(SelectorOrStyle::Selector(property));
             }
-            return Ok(SelectorOrStyle::Selector(property));
         }
         Err(("expected \"{\".", self.span_before).into())
     }
diff --git a/src/parse/value/eval.rs b/src/parse/value/eval.rs
index 94c5101..acb9ef3 100644
--- a/src/parse/value/eval.rs
+++ b/src/parse/value/eval.rs
@@ -3,12 +3,13 @@
 use std::cmp::Ordering;
 
 use codemap::{Span, Spanned};
+use num_traits::Zero;
 
 use crate::{
     args::CallArgs,
     common::{Op, QuoteKind},
     error::SassResult,
-    unit::{Unit, UNIT_CONVERSION_TABLE},
+    unit::Unit,
     value::{SassFunction, Value},
 };
 
@@ -119,7 +120,11 @@ impl<'a, 'b: 'a> ValueVisitor<'a, 'b> {
 
     fn unary_minus(&self, val: Value) -> SassResult<Value> {
         Ok(match val {
-            Value::Dimension(n, u, should_divide) => Value::Dimension(-n, u, should_divide),
+            Value::Dimension(Some(n), u, should_divide) => {
+                Value::Dimension(Some(-n), u, should_divide)
+            }
+            // todo: NaN test
+            Value::Dimension(None, u, should_divide) => Value::Dimension(None, u, should_divide),
             v => Value::String(format!("-{}", v.to_css_string(self.span)?), QuoteKind::None),
         })
     }
@@ -205,8 +210,10 @@ impl<'a, 'b: 'a> ValueVisitor<'a, 'b> {
                     QuoteKind::None,
                 ),
             },
-            Value::Dimension(num, unit, _) => match right {
-                Value::Dimension(num2, unit2, _) => {
+            v @ Value::Dimension(None, ..) => v,
+            Value::Dimension(Some(num), unit, _) => match right {
+                v @ Value::Dimension(None, ..) => v,
+                Value::Dimension(Some(num2), unit2, _) => {
                     if !unit.comparable(&unit2) {
                         return Err((
                             format!("Incompatible units {} and {}.", unit2, unit),
@@ -215,20 +222,13 @@ impl<'a, 'b: 'a> ValueVisitor<'a, 'b> {
                             .into());
                     }
                     if unit == unit2 {
-                        Value::Dimension(num + num2, unit, true)
+                        Value::Dimension(Some(num + num2), unit, true)
                     } else if unit == Unit::None {
-                        Value::Dimension(num + num2, unit2, true)
+                        Value::Dimension(Some(num + num2), unit2, true)
                     } else if unit2 == Unit::None {
-                        Value::Dimension(num + num2, unit, true)
+                        Value::Dimension(Some(num + num2), unit, true)
                     } else {
-                        Value::Dimension(
-                            num + num2
-                                * UNIT_CONVERSION_TABLE[unit.to_string().as_str()]
-                                    [unit2.to_string().as_str()]
-                                .clone(),
-                            unit,
-                            true,
-                        )
+                        Value::Dimension(Some(num + num2.convert(&unit2, &unit)), unit, true)
                     }
                 }
                 Value::String(s, q) => Value::String(format!("{}{}{}", num, unit, s), q),
@@ -314,8 +314,10 @@ impl<'a, 'b: 'a> ValueVisitor<'a, 'b> {
                 format!("-{}", right.to_css_string(self.span)?),
                 QuoteKind::None,
             ),
-            Value::Dimension(num, unit, _) => match right {
-                Value::Dimension(num2, unit2, _) => {
+            Value::Dimension(None, ..) => todo!(),
+            Value::Dimension(Some(num), unit, _) => match right {
+                Value::Dimension(None, ..) => todo!(),
+                Value::Dimension(Some(num2), unit2, _) => {
                     if !unit.comparable(&unit2) {
                         return Err((
                             format!("Incompatible units {} and {}.", unit2, unit),
@@ -324,20 +326,13 @@ impl<'a, 'b: 'a> ValueVisitor<'a, 'b> {
                             .into());
                     }
                     if unit == unit2 {
-                        Value::Dimension(num - num2, unit, true)
+                        Value::Dimension(Some(num - num2), unit, true)
                     } else if unit == Unit::None {
-                        Value::Dimension(num - num2, unit2, true)
+                        Value::Dimension(Some(num - num2), unit2, true)
                     } else if unit2 == Unit::None {
-                        Value::Dimension(num - num2, unit, true)
+                        Value::Dimension(Some(num - num2), unit, true)
                     } else {
-                        Value::Dimension(
-                            num - num2
-                                * UNIT_CONVERSION_TABLE[unit.to_string().as_str()]
-                                    [unit2.to_string().as_str()]
-                                .clone(),
-                            unit,
-                            true,
-                        )
+                        Value::Dimension(Some(num - num2.convert(&unit2, &unit)), unit, true)
                     }
                 }
                 Value::List(..)
@@ -434,14 +429,16 @@ impl<'a, 'b: 'a> ValueVisitor<'a, 'b> {
             v => panic!("{:?}", v),
         };
         Ok(match left {
-            Value::Dimension(num, unit, _) => match right {
-                Value::Dimension(num2, unit2, _) => {
+            Value::Dimension(None, ..) => todo!(),
+            Value::Dimension(Some(num), unit, _) => match right {
+                Value::Dimension(None, ..) => todo!(),
+                Value::Dimension(Some(num2), unit2, _) => {
                     if unit == Unit::None {
-                        Value::Dimension(num * num2, unit2, true)
+                        Value::Dimension(Some(num * num2), unit2, true)
                     } else if unit2 == Unit::None {
-                        Value::Dimension(num * num2, unit, true)
+                        Value::Dimension(Some(num * num2), unit, true)
                     } else {
-                        Value::Dimension(num * num2, unit * unit2, true)
+                        Value::Dimension(Some(num * num2), unit * unit2, true)
                     }
                 }
                 _ => {
@@ -490,28 +487,31 @@ impl<'a, 'b: 'a> ValueVisitor<'a, 'b> {
                 format!("/{}", right.to_css_string(self.span)?),
                 QuoteKind::None,
             ),
-            Value::Dimension(num, unit, should_divide1) => match right {
-                Value::Dimension(num2, unit2, should_divide2) => {
+            Value::Dimension(None, ..) => todo!(),
+            Value::Dimension(Some(num), unit, should_divide1) => match right {
+                Value::Dimension(None, ..) => todo!(),
+                Value::Dimension(Some(num2), unit2, should_divide2) => {
                     if should_divide1 || should_divide2 || in_parens {
+                        if num.is_zero() && num2.is_zero() {
+                            return Ok(Value::Dimension(None, Unit::None, true));
+                        }
+
                         // `unit(1em / 1em)` => `""`
                         if unit == unit2 {
-                            Value::Dimension(num / num2, Unit::None, true)
+                            Value::Dimension(Some(num / num2), Unit::None, true)
 
                         // `unit(1 / 1em)` => `"em^-1"`
                         } else if unit == Unit::None {
-                            Value::Dimension(num / num2, Unit::None / unit2, true)
+                            Value::Dimension(Some(num / num2), Unit::None / unit2, true)
 
                         // `unit(1em / 1)` => `"em"`
                         } else if unit2 == Unit::None {
-                            Value::Dimension(num / num2, unit, true)
+                            Value::Dimension(Some(num / num2), unit, true)
 
                         // `unit(1in / 1px)` => `""`
                         } else if unit.comparable(&unit2) {
                             Value::Dimension(
-                                num / (num2
-                                    * UNIT_CONVERSION_TABLE[unit.to_string().as_str()]
-                                        [unit2.to_string().as_str()]
-                                    .clone()),
+                                Some(num / num2.convert(&unit2, &unit)),
                                 Unit::None,
                                 true,
                             )
@@ -630,28 +630,30 @@ impl<'a, 'b: 'a> ValueVisitor<'a, 'b> {
             v => panic!("{:?}", v),
         };
         Ok(match left {
-            Value::Dimension(n, u, _) => match right {
-                Value::Dimension(n2, u2, _) => {
+            Value::Dimension(None, ..) => todo!(),
+            Value::Dimension(Some(n), u, _) => match right {
+                Value::Dimension(None, ..) => todo!(),
+                Value::Dimension(Some(n2), u2, _) => {
                     if !u.comparable(&u2) {
                         return Err(
                             (format!("Incompatible units {} and {}.", u2, u), self.span).into()
                         );
                     }
                     if u == u2 {
-                        Value::Dimension(n % n2, u, true)
+                        Value::Dimension(Some(n % n2), u, true)
                     } else if u == Unit::None {
-                        Value::Dimension(n % n2, u2, true)
+                        Value::Dimension(Some(n % n2), u2, true)
                     } else if u2 == Unit::None {
-                        Value::Dimension(n % n2, u, true)
+                        Value::Dimension(Some(n % n2), u, true)
                     } else {
-                        Value::Dimension(n, u, true)
+                        Value::Dimension(Some(n), u, true)
                     }
                 }
                 _ => {
                     return Err((
                         format!(
                             "Undefined operation \"{} % {}\".",
-                            Value::Dimension(n, u, true).inspect(self.span)?,
+                            Value::Dimension(Some(n), u, true).inspect(self.span)?,
                             right.inspect(self.span)?
                         ),
                         self.span,
@@ -735,53 +737,9 @@ impl<'a, 'b: 'a> ValueVisitor<'a, 'b> {
             HigherIntermediateValue::Literal(v) => v,
             v => panic!("{:?}", v),
         };
-        let ordering = match left {
-            Value::Dimension(num, unit, _) => match &right {
-                Value::Dimension(num2, unit2, _) => {
-                    if !unit.comparable(unit2) {
-                        return Err((
-                            format!("Incompatible units {} and {}.", unit2, unit),
-                            self.span,
-                        )
-                            .into());
-                    }
-                    if &unit == unit2 || unit == Unit::None || unit2 == &Unit::None {
-                        num.cmp(num2)
-                    } else {
-                        num.cmp(
-                            &(num2.clone()
-                                * UNIT_CONVERSION_TABLE[unit.to_string().as_str()]
-                                    [unit2.to_string().as_str()]
-                                .clone()),
-                        )
-                    }
-                }
-                v => {
-                    return Err((
-                        format!(
-                            "Undefined operation \"{} {} {}\".",
-                            v.inspect(self.span)?,
-                            op,
-                            right.inspect(self.span)?
-                        ),
-                        self.span,
-                    )
-                        .into())
-                }
-            },
-            _ => {
-                return Err((
-                    format!(
-                        "Undefined operation \"{} {} {}\".",
-                        left.inspect(self.span)?,
-                        op,
-                        right.inspect(self.span)?
-                    ),
-                    self.span,
-                )
-                    .into())
-            }
-        };
+
+        let ordering = left.cmp(&right, self.span, op)?;
+
         Ok(match op {
             Op::GreaterThan => match ordering {
                 Ordering::Greater => Value::True,
diff --git a/src/parse/value/parse.rs b/src/parse/value/parse.rs
index 3c66b6e..0e59095 100644
--- a/src/parse/value/parse.rs
+++ b/src/parse/value/parse.rs
@@ -198,6 +198,8 @@ impl<'a> Parser<'a> {
             extender: self.extender,
             content_scopes: self.content_scopes,
             options: self.options,
+            modules: self.modules,
+            module_config: self.module_config,
         }
         .parse_value(in_paren, predicate)
     }
@@ -222,11 +224,56 @@ impl<'a> Parser<'a> {
             extender: self.extender,
             content_scopes: self.content_scopes,
             options: self.options,
+            modules: self.modules,
+            module_config: self.module_config,
         }
         .parse_value(in_paren, &|_| false)
     }
 
-    fn parse_ident_value(&mut self) -> SassResult<Spanned<IntermediateValue>> {
+    #[allow(clippy::eval_order_dependence)]
+    fn parse_module_item(
+        &mut self,
+        module: &str,
+        mut module_span: Span,
+    ) -> SassResult<Spanned<IntermediateValue>> {
+        Ok(IntermediateValue::Value(
+            if matches!(self.toks.peek(), Some(Token { kind: '$', .. })) {
+                self.toks.next();
+                let var = self
+                    .parse_identifier_no_interpolation(false)?
+                    .map_node(|i| i.into());
+
+                module_span = module_span.merge(var.span);
+
+                let value = self.modules.get(module.into(), module_span)?.get_var(var)?;
+                HigherIntermediateValue::Literal(value.clone())
+            } else {
+                let fn_name = self
+                    .parse_identifier_no_interpolation(false)?
+                    .map_node(|i| i.into());
+
+                let function = self
+                    .modules
+                    .get(module.into(), module_span)?
+                    .get_fn(fn_name)?
+                    .ok_or(("Undefined function.", fn_name.span))?;
+
+                if !matches!(self.toks.next(), Some(Token { kind: '(', .. })) {
+                    todo!()
+                }
+
+                let call_args = self.parse_call_args()?;
+
+                HigherIntermediateValue::Function(function, call_args)
+            },
+        )
+        .span(module_span))
+    }
+
+    fn parse_ident_value(
+        &mut self,
+        predicate: &dyn Fn(&mut PeekMoreIterator<IntoIter<Token>>) -> bool,
+    ) -> SassResult<Spanned<IntermediateValue>> {
         let Spanned { node: mut s, span } = self.parse_identifier()?;
 
         self.span_before = span;
@@ -247,68 +294,72 @@ impl<'a> Parser<'a> {
             });
         }
 
-        if let Some(Token { kind: '(', .. }) = self.toks.peek() {
-            self.toks.next();
+        match self.toks.peek() {
+            Some(Token { kind: '(', .. }) => {
+                self.toks.next();
 
-            if lower == "min" || lower == "max" {
-                match self.try_parse_min_max(&lower, true)? {
-                    Some(val) => {
-                        self.toks.truncate_iterator_to_cursor();
-                        return Ok(IntermediateValue::Value(HigherIntermediateValue::Literal(
-                            Value::String(val, QuoteKind::None),
-                        ))
-                        .span(span));
+                if lower == "min" || lower == "max" {
+                    match self.try_parse_min_max(&lower, true)? {
+                        Some(val) => {
+                            self.toks.truncate_iterator_to_cursor();
+                            return Ok(IntermediateValue::Value(HigherIntermediateValue::Literal(
+                                Value::String(val, QuoteKind::None),
+                            ))
+                            .span(span));
+                        }
+                        None => {
+                            self.toks.reset_cursor();
+                        }
                     }
+                }
+
+                let as_ident = Identifier::from(&s);
+                let func = match self.scopes.get_fn(as_ident, self.global_scope) {
+                    Some(f) => f,
                     None => {
-                        self.toks.reset_cursor();
+                        if let Some(f) = GLOBAL_FUNCTIONS.get(as_ident.as_str()) {
+                            return Ok(IntermediateValue::Value(
+                                HigherIntermediateValue::Function(
+                                    SassFunction::Builtin(f.clone(), as_ident),
+                                    self.parse_call_args()?,
+                                ),
+                            )
+                            .span(span));
+                        } else {
+                            // check for special cased CSS functions
+                            match unvendor(&lower) {
+                                "calc" | "element" | "expression" => {
+                                    s = lower;
+                                    self.parse_calc_args(&mut s)?;
+                                }
+                                "url" => match self.try_parse_url()? {
+                                    Some(val) => s = val,
+                                    None => s.push_str(&self.parse_call_args()?.to_css_string()?),
+                                },
+                                _ => s.push_str(&self.parse_call_args()?.to_css_string()?),
+                            }
+
+                            return Ok(IntermediateValue::Value(HigherIntermediateValue::Literal(
+                                Value::String(s, QuoteKind::None),
+                            ))
+                            .span(span));
+                        }
                     }
+                };
+
+                let call_args = self.parse_call_args()?;
+                return Ok(IntermediateValue::Value(HigherIntermediateValue::Function(
+                    func, call_args,
+                ))
+                .span(span));
+            }
+            Some(Token { kind: '.', .. }) => {
+                if !predicate(self.toks) {
+                    self.toks.next();
+                    return self.parse_module_item(&s, span);
                 }
             }
-
-            let as_ident = Identifier::from(&s);
-            let func = match self.scopes.get_fn(
-                Spanned {
-                    node: as_ident,
-                    span,
-                },
-                self.global_scope,
-            ) {
-                Some(f) => f,
-                None => {
-                    if let Some(f) = GLOBAL_FUNCTIONS.get(as_ident.as_str()) {
-                        return Ok(IntermediateValue::Value(HigherIntermediateValue::Function(
-                            SassFunction::Builtin(f.clone(), as_ident),
-                            self.parse_call_args()?,
-                        ))
-                        .span(span));
-                    } else {
-                        // check for special cased CSS functions
-                        match unvendor(&lower) {
-                            "calc" | "element" | "expression" => {
-                                s = lower;
-                                self.parse_calc_args(&mut s)?;
-                            }
-                            "url" => match self.try_parse_url()? {
-                                Some(val) => s = val,
-                                None => s.push_str(&self.parse_call_args()?.to_css_string()?),
-                            },
-                            _ => s.push_str(&self.parse_call_args()?.to_css_string()?),
-                        }
-
-                        return Ok(IntermediateValue::Value(HigherIntermediateValue::Literal(
-                            Value::String(s, QuoteKind::None),
-                        ))
-                        .span(span));
-                    }
-                }
-            };
-
-            let call_args = self.parse_call_args()?;
-            return Ok(IntermediateValue::Value(HigherIntermediateValue::Function(
-                SassFunction::UserDefined(Box::new(func), as_ident),
-                call_args,
-            ))
-            .span(span));
+            _ => {}
         }
 
         // check for named colors
@@ -442,7 +493,7 @@ impl<'a> Parser<'a> {
                 || (!kind.is_ascii() && !kind.is_control())
                 || (kind == '-' && self.next_is_hypen()) =>
             {
-                return Some(self.parse_ident_value());
+                return Some(self.parse_ident_value(predicate));
             }
             '0'..='9' | '.' => {
                 let Spanned {
@@ -478,7 +529,7 @@ impl<'a> Parser<'a> {
                         let n = Rational64::new_raw(parse_i64(&val.num), 1);
                         return Some(Ok(IntermediateValue::Value(
                             HigherIntermediateValue::Literal(Value::Dimension(
-                                Number::new_small(n),
+                                Some(Number::new_small(n)),
                                 unit,
                                 false,
                             )),
@@ -491,7 +542,7 @@ impl<'a> Parser<'a> {
                         let n = Rational64::new(parse_i64(&val.num), pow(10, val.dec_len));
                         return Some(Ok(IntermediateValue::Value(
                             HigherIntermediateValue::Literal(Value::Dimension(
-                                Number::new_small(n),
+                                Some(Number::new_small(n)),
                                 unit,
                                 false,
                             )),
@@ -504,7 +555,7 @@ impl<'a> Parser<'a> {
                 if val.times_ten.is_empty() {
                     return Some(Ok(IntermediateValue::Value(
                         HigherIntermediateValue::Literal(Value::Dimension(
-                            Number::new_big(n),
+                            Some(Number::new_big(n)),
                             unit,
                             false,
                         )),
@@ -533,7 +584,7 @@ impl<'a> Parser<'a> {
                 };
 
                 IntermediateValue::Value(HigherIntermediateValue::Literal(Value::Dimension(
-                    Number::new_big(n * times_ten),
+                    Some(Number::new_big(n * times_ten)),
                     unit,
                     false,
                 )))
@@ -547,6 +598,7 @@ impl<'a> Parser<'a> {
                 };
                 // todo: the above shouldn't eat the closing paren
                 if let Some(last_tok) = inner.pop() {
+                    // todo: we should remove this like we did for square braces
                     if last_tok.kind != ')' {
                         return Some(Err(("expected \")\".", span).into()));
                     }
@@ -570,7 +622,7 @@ impl<'a> Parser<'a> {
                 if let Some(Token { kind: '{', pos }) = self.toks.peek_forward(1) {
                     self.span_before = *pos;
                     self.toks.reset_cursor();
-                    return Some(self.parse_ident_value());
+                    return Some(self.parse_ident_value(predicate));
                 }
                 self.toks.reset_cursor();
                 self.toks.next();
diff --git a/src/parse/variable.rs b/src/parse/variable.rs
index fe25971..a5a77e4 100644
--- a/src/parse/variable.rs
+++ b/src/parse/variable.rs
@@ -8,10 +8,10 @@ use crate::{
 use super::Parser;
 
 #[derive(Debug)]
-struct VariableValue {
-    val_toks: Vec<Token>,
-    global: bool,
-    default: bool,
+pub(crate) struct VariableValue {
+    pub val_toks: Vec<Token>,
+    pub global: bool,
+    pub default: bool,
 }
 
 impl VariableValue {
@@ -29,9 +29,9 @@ impl<'a> Parser<'a> {
         assert!(matches!(self.toks.next(), Some(Token { kind: '$', .. })));
         let ident: Identifier = self.parse_identifier_no_interpolation(false)?.node.into();
         self.whitespace();
-        if !matches!(self.toks.next(), Some(Token { kind: ':', .. })) {
-            return Err(("expected \":\".", self.span_before).into());
-        }
+
+        self.expect_char(':')?;
+
         let VariableValue {
             val_toks,
             global,
@@ -39,13 +39,24 @@ impl<'a> Parser<'a> {
         } = self.parse_variable_value()?;
 
         if default {
+            let config_val = self.module_config.get(ident);
             if self.at_root && !self.flags.in_control_flow() {
                 if !self.global_scope.var_exists(ident) {
-                    let value = self.parse_value_from_vec(val_toks, true)?;
+                    let value = if let Some(config_val) = config_val {
+                        config_val
+                    } else {
+                        self.parse_value_from_vec(val_toks, true)?.node
+                    };
+
                     self.global_scope.insert_var(ident, value);
                 }
             } else {
-                let value = self.parse_value_from_vec(val_toks, true)?;
+                let value = if let Some(config_val) = config_val {
+                    config_val
+                } else {
+                    self.parse_value_from_vec(val_toks, true)?.node
+                };
+
                 if global && !self.global_scope.var_exists(ident) {
                     self.global_scope.insert_var(ident, value.clone());
                 }
@@ -55,7 +66,7 @@ impl<'a> Parser<'a> {
             return Ok(());
         }
 
-        let value = self.parse_value_from_vec(val_toks, true)?;
+        let value = self.parse_value_from_vec(val_toks, true)?.node;
 
         if global {
             self.global_scope.insert_var(ident, value.clone());
@@ -77,7 +88,7 @@ impl<'a> Parser<'a> {
         Ok(())
     }
 
-    fn parse_variable_value(&mut self) -> SassResult<VariableValue> {
+    pub(super) fn parse_variable_value(&mut self) -> SassResult<VariableValue> {
         let mut default = false;
         let mut global = false;
 
diff --git a/src/scope.rs b/src/scope.rs
index 69fb0db..dc968e2 100644
--- a/src/scope.rs
+++ b/src/scope.rs
@@ -3,18 +3,18 @@ use std::collections::BTreeMap;
 use codemap::Spanned;
 
 use crate::{
-    atrule::{Function, Mixin},
-    builtin::GLOBAL_FUNCTIONS,
+    atrule::mixin::Mixin,
+    builtin::{modules::Module, GLOBAL_FUNCTIONS},
     common::Identifier,
     error::SassResult,
-    value::Value,
+    value::{SassFunction, Value},
 };
 
 #[derive(Debug, Default)]
 pub(crate) struct Scope {
-    vars: BTreeMap<Identifier, Spanned<Value>>,
-    mixins: BTreeMap<Identifier, Mixin>,
-    functions: BTreeMap<Identifier, Function>,
+    pub vars: BTreeMap<Identifier, Value>,
+    pub mixins: BTreeMap<Identifier, Mixin>,
+    pub functions: BTreeMap<Identifier, SassFunction>,
 }
 
 impl Scope {
@@ -31,12 +31,12 @@ impl Scope {
 
     fn get_var(&self, name: Spanned<Identifier>) -> SassResult<&Value> {
         match self.vars.get(&name.node) {
-            Some(v) => Ok(&v.node),
+            Some(v) => Ok(v),
             None => Err(("Undefined variable.", name.span).into()),
         }
     }
 
-    pub fn insert_var(&mut self, s: Identifier, v: Spanned<Value>) -> Option<Spanned<Value>> {
+    pub fn insert_var(&mut self, s: Identifier, v: Value) -> Option<Value> {
         self.vars.insert(s, v)
     }
 
@@ -55,16 +55,16 @@ impl Scope {
         self.mixins.insert(s.into(), v)
     }
 
-    fn mixin_exists(&self, name: Identifier) -> bool {
+    pub fn mixin_exists(&self, name: Identifier) -> bool {
         self.mixins.contains_key(&name)
     }
 
-    fn get_fn(&self, name: Identifier) -> Option<Function> {
+    fn get_fn(&self, name: Identifier) -> Option<SassFunction> {
         self.functions.get(&name).cloned()
     }
 
-    pub fn insert_fn<T: Into<Identifier>>(&mut self, s: T, v: Function) -> Option<Function> {
-        self.functions.insert(s.into(), v)
+    pub fn insert_fn(&mut self, s: Identifier, v: SassFunction) -> Option<SassFunction> {
+        self.functions.insert(s, v)
     }
 
     fn fn_exists(&self, name: Identifier) -> bool {
@@ -73,6 +73,16 @@ impl Scope {
         }
         self.functions.contains_key(&name)
     }
+
+    fn merge(&mut self, other: Scope) {
+        self.vars.extend(other.vars);
+        self.mixins.extend(other.mixins);
+        self.functions.extend(other.functions);
+    }
+
+    pub fn merge_module(&mut self, other: Module) {
+        self.merge(other.scope);
+    }
 }
 
 #[derive(Debug, Default)]
@@ -111,7 +121,7 @@ impl Scopes {
 
 /// Variables
 impl Scopes {
-    pub fn insert_var(&mut self, s: Identifier, v: Spanned<Value>) -> Option<Spanned<Value>> {
+    pub fn insert_var(&mut self, s: Identifier, v: Value) -> Option<Value> {
         for scope in self.0.iter_mut().rev() {
             if scope.var_exists(s) {
                 return scope.insert_var(s, v);
@@ -130,7 +140,7 @@ impl Scopes {
     /// Always insert this variable into the innermost scope
     ///
     /// Used, for example, for variables from `@each` and `@for`
-    pub fn insert_var_last(&mut self, s: Identifier, v: Spanned<Value>) -> Option<Spanned<Value>> {
+    pub fn insert_var_last(&mut self, s: Identifier, v: Value) -> Option<Value> {
         if let Some(scope) = self.0.last_mut() {
             scope.insert_var(s, v)
         } else {
@@ -141,11 +151,7 @@ impl Scopes {
         }
     }
 
-    pub fn insert_default_var(
-        &mut self,
-        s: Identifier,
-        v: Spanned<Value>,
-    ) -> Option<Spanned<Value>> {
+    pub fn insert_default_var(&mut self, s: Identifier, v: Value) -> Option<Value> {
         if let Some(scope) = self.0.last_mut() {
             if scope.var_exists(s) {
                 None
@@ -218,7 +224,7 @@ impl Scopes {
 
 /// Functions
 impl Scopes {
-    pub fn insert_fn(&mut self, s: Identifier, v: Function) -> Option<Function> {
+    pub fn insert_fn(&mut self, s: Identifier, v: SassFunction) -> Option<SassFunction> {
         if let Some(scope) = self.0.last_mut() {
             scope.insert_fn(s, v)
         } else {
@@ -229,17 +235,13 @@ impl Scopes {
         }
     }
 
-    pub fn get_fn<'a>(
-        &'a self,
-        name: Spanned<Identifier>,
-        global_scope: &'a Scope,
-    ) -> Option<Function> {
+    pub fn get_fn<'a>(&'a self, name: Identifier, global_scope: &'a Scope) -> Option<SassFunction> {
         for scope in self.0.iter().rev() {
-            if scope.fn_exists(name.node) {
-                return scope.get_fn(name.node);
+            if scope.fn_exists(name) {
+                return scope.get_fn(name);
             }
         }
-        global_scope.get_fn(name.node)
+        global_scope.get_fn(name)
     }
 
     pub fn fn_exists(&self, name: Identifier, global_scope: &Scope) -> bool {
diff --git a/src/selector/attribute.rs b/src/selector/attribute.rs
index 3fb7dc1..e1b8129 100644
--- a/src/selector/attribute.rs
+++ b/src/selector/attribute.rs
@@ -5,7 +5,9 @@ use std::{
 
 use codemap::Span;
 
-use crate::{common::QuoteKind, error::SassResult, parse::Parser, utils::is_ident, value::Value};
+use crate::{
+    common::QuoteKind, error::SassResult, parse::Parser, utils::is_ident, value::Value, Token,
+};
 
 use super::{Namespace, QualifiedName};
 
@@ -41,13 +43,8 @@ impl Hash for Attribute {
 fn attribute_name(parser: &mut Parser<'_>, start: Span) -> SassResult<QualifiedName> {
     let next = parser.toks.peek().ok_or(("Expected identifier.", start))?;
     if next.kind == '*' {
-        let pos = next.pos;
         parser.toks.next();
-        if parser.toks.peek().ok_or(("expected \"|\".", pos))?.kind != '|' {
-            return Err(("expected \"|\".", pos).into());
-        }
-
-        parser.span_before = parser.toks.next().unwrap().pos();
+        parser.expect_char('|')?;
 
         let ident = parser.parse_identifier()?.node;
         return Ok(QualifiedName {
@@ -89,19 +86,18 @@ fn attribute_name(parser: &mut Parser<'_>, start: Span) -> SassResult<QualifiedN
 }
 
 fn attribute_operator(parser: &mut Parser<'_>) -> SassResult<AttributeOp> {
-    let start = parser.span_before;
-    let op = match parser.toks.next().ok_or(("Expected \"]\".", start))?.kind {
-        '=' => return Ok(AttributeOp::Equals),
-        '~' => AttributeOp::Include,
-        '|' => AttributeOp::Dash,
-        '^' => AttributeOp::Prefix,
-        '$' => AttributeOp::Suffix,
-        '*' => AttributeOp::Contains,
-        _ => return Err(("Expected \"]\".", start).into()),
+    let op = match parser.toks.next() {
+        Some(Token { kind: '=', .. }) => return Ok(AttributeOp::Equals),
+        Some(Token { kind: '~', .. }) => AttributeOp::Include,
+        Some(Token { kind: '|', .. }) => AttributeOp::Dash,
+        Some(Token { kind: '^', .. }) => AttributeOp::Prefix,
+        Some(Token { kind: '$', .. }) => AttributeOp::Suffix,
+        Some(Token { kind: '*', .. }) => AttributeOp::Contains,
+        Some(..) | None => return Err(("Expected \"]\".", parser.span_before).into()),
     };
-    if parser.toks.next().ok_or(("expected \"=\".", start))?.kind != '=' {
-        return Err(("expected \"=\".", start).into());
-    }
+
+    parser.expect_char('=')?;
+
     Ok(op)
 }
 impl Attribute {
@@ -145,25 +141,23 @@ impl Attribute {
         };
         parser.whitespace();
 
-        let peek = parser.toks.peek().ok_or(("expected more input.", start))?;
-
-        let modifier = match peek.kind {
-            c if c.is_alphabetic() => Some(c),
+        let modifier = match parser.toks.peek().cloned() {
+            Some(Token {
+                kind: c @ 'a'..='z',
+                ..
+            })
+            | Some(Token {
+                kind: c @ 'A'..='Z',
+                ..
+            }) => {
+                parser.toks.next();
+                parser.whitespace();
+                Some(c)
+            }
             _ => None,
         };
 
-        let pos = peek.pos();
-
-        if modifier.is_some() {
-            parser.toks.next();
-            parser.whitespace();
-        }
-
-        if parser.toks.peek().ok_or(("expected \"]\".", pos))?.kind != ']' {
-            return Err(("expected \"]\".", pos).into());
-        }
-
-        parser.toks.next();
+        parser.expect_char(']')?;
 
         Ok(Attribute {
             op,
diff --git a/src/selector/parse.rs b/src/selector/parse.rs
index cc7c001..2522bce 100644
--- a/src/selector/parse.rs
+++ b/src/selector/parse.rs
@@ -317,14 +317,14 @@ impl<'a, 'b> SelectorParser<'a, 'b> {
             if SELECTOR_PSEUDO_ELEMENTS.contains(&unvendored) {
                 selector = Some(Box::new(self.parse_selector_list()?));
                 self.parser.whitespace();
-                self.expect_closing_paren()?;
+                self.parser.expect_char(')')?;
             } else {
                 argument = Some(self.declaration_value()?.into_boxed_str());
             }
         } else if SELECTOR_PSEUDO_CLASSES.contains(&unvendored) {
             selector = Some(Box::new(self.parse_selector_list()?));
             self.parser.whitespace();
-            self.expect_closing_paren()?;
+            self.parser.expect_char(')')?;
         } else if unvendored == "nth-child" || unvendored == "nth-last-child" {
             let mut this_arg = self.parse_a_n_plus_b()?;
             let found_whitespace = self.parser.whitespace();
@@ -339,7 +339,7 @@ impl<'a, 'b> SelectorParser<'a, 'b> {
                 }
                 _ => {}
             }
-            self.expect_closing_paren()?;
+            self.parser.expect_char(')')?;
             argument = Some(this_arg.into_boxed_str());
         } else {
             argument = Some(
@@ -541,14 +541,6 @@ impl<'a, 'b> SelectorParser<'a, 'b> {
             Err((format!("Expected \"{}\".", s), self.span).into())
         }
     }
-
-    fn expect_closing_paren(&mut self) -> SassResult<()> {
-        if let Some(Token { kind: ')', .. }) = self.parser.toks.next() {
-            Ok(())
-        } else {
-            Err(("expected \")\".", self.span).into())
-        }
-    }
 }
 
 /// Returns whether `c` can start a simple selector other than a type
diff --git a/src/unit/conversion.rs b/src/unit/conversion.rs
index b848d19..8862d14 100644
--- a/src/unit/conversion.rs
+++ b/src/unit/conversion.rs
@@ -7,152 +7,151 @@ use std::{collections::HashMap, f64::consts::PI};
 use num_traits::One;
 use once_cell::sync::Lazy;
 
-use crate::value::Number;
+use crate::{unit::Unit, value::Number};
 
-pub(crate) static UNIT_CONVERSION_TABLE: Lazy<
-    HashMap<&'static str, HashMap<&'static str, Number>>,
-> = Lazy::new(|| {
-    let mut from_in = HashMap::new();
-    from_in.insert("in", Number::one());
-    from_in.insert("cm", Number::one() / Number::from(2.54));
-    from_in.insert("pc", Number::small_ratio(1, 6));
-    from_in.insert("mm", Number::one() / Number::from(25.4));
-    from_in.insert("q", Number::one() / Number::from(101.6));
-    from_in.insert("pt", Number::small_ratio(1, 72));
-    from_in.insert("px", Number::small_ratio(1, 96));
+pub(crate) static UNIT_CONVERSION_TABLE: Lazy<HashMap<Unit, HashMap<Unit, Number>>> =
+    Lazy::new(|| {
+        let mut from_in = HashMap::new();
+        from_in.insert(Unit::In, Number::one());
+        from_in.insert(Unit::Cm, Number::one() / Number::from(2.54));
+        from_in.insert(Unit::Pc, Number::small_ratio(1, 6));
+        from_in.insert(Unit::Mm, Number::one() / Number::from(25.4));
+        from_in.insert(Unit::Q, Number::one() / Number::from(101.6));
+        from_in.insert(Unit::Pt, Number::small_ratio(1, 72));
+        from_in.insert(Unit::Px, Number::small_ratio(1, 96));
 
-    let mut from_cm = HashMap::new();
-    from_cm.insert("in", Number::from(2.54));
-    from_cm.insert("cm", Number::one());
-    from_cm.insert("pc", Number::from(2.54) / Number::from(6));
-    from_cm.insert("mm", Number::small_ratio(1, 10));
-    from_cm.insert("q", Number::small_ratio(1, 40));
-    from_cm.insert("pt", Number::from(2.54) / Number::from(72));
-    from_cm.insert("px", Number::from(2.54) / Number::from(96));
+        let mut from_cm = HashMap::new();
+        from_cm.insert(Unit::In, Number::from(2.54));
+        from_cm.insert(Unit::Cm, Number::one());
+        from_cm.insert(Unit::Pc, Number::from(2.54) / Number::from(6));
+        from_cm.insert(Unit::Mm, Number::small_ratio(1, 10));
+        from_cm.insert(Unit::Q, Number::small_ratio(1, 40));
+        from_cm.insert(Unit::Pt, Number::from(2.54) / Number::from(72));
+        from_cm.insert(Unit::Px, Number::from(2.54) / Number::from(96));
 
-    let mut from_pc = HashMap::new();
-    from_pc.insert("in", Number::from(6));
-    from_pc.insert("cm", Number::from(6) / Number::from(2.54));
-    from_pc.insert("pc", Number::one());
-    from_pc.insert("mm", Number::from(6) / Number::from(25.4));
-    from_pc.insert("q", Number::from(6) / Number::from(101.6));
-    from_pc.insert("pt", Number::small_ratio(1, 12));
-    from_pc.insert("px", Number::small_ratio(1, 16));
+        let mut from_pc = HashMap::new();
+        from_pc.insert(Unit::In, Number::from(6));
+        from_pc.insert(Unit::Cm, Number::from(6) / Number::from(2.54));
+        from_pc.insert(Unit::Pc, Number::one());
+        from_pc.insert(Unit::Mm, Number::from(6) / Number::from(25.4));
+        from_pc.insert(Unit::Q, Number::from(6) / Number::from(101.6));
+        from_pc.insert(Unit::Pt, Number::small_ratio(1, 12));
+        from_pc.insert(Unit::Px, Number::small_ratio(1, 16));
 
-    let mut from_mm = HashMap::new();
-    from_mm.insert("in", Number::from(25.4));
-    from_mm.insert("cm", Number::from(10));
-    from_mm.insert("pc", Number::from(25.4) / Number::from(6));
-    from_mm.insert("mm", Number::one());
-    from_mm.insert("q", Number::small_ratio(1, 4));
-    from_mm.insert("pt", Number::from(25.4) / Number::from(72));
-    from_mm.insert("px", Number::from(25.4) / Number::from(96));
+        let mut from_mm = HashMap::new();
+        from_mm.insert(Unit::In, Number::from(25.4));
+        from_mm.insert(Unit::Cm, Number::from(10));
+        from_mm.insert(Unit::Pc, Number::from(25.4) / Number::from(6));
+        from_mm.insert(Unit::Mm, Number::one());
+        from_mm.insert(Unit::Q, Number::small_ratio(1, 4));
+        from_mm.insert(Unit::Pt, Number::from(25.4) / Number::from(72));
+        from_mm.insert(Unit::Px, Number::from(25.4) / Number::from(96));
 
-    let mut from_q = HashMap::new();
-    from_q.insert("in", Number::from(101.6));
-    from_q.insert("cm", Number::from(40));
-    from_q.insert("pc", Number::from(101.6) / Number::from(6));
-    from_q.insert("mm", Number::from(4));
-    from_q.insert("q", Number::one());
-    from_q.insert("pt", Number::from(101.6) / Number::from(72));
-    from_q.insert("px", Number::from(101.6) / Number::from(96));
+        let mut from_q = HashMap::new();
+        from_q.insert(Unit::In, Number::from(101.6));
+        from_q.insert(Unit::Cm, Number::from(40));
+        from_q.insert(Unit::Pc, Number::from(101.6) / Number::from(6));
+        from_q.insert(Unit::Mm, Number::from(4));
+        from_q.insert(Unit::Q, Number::one());
+        from_q.insert(Unit::Pt, Number::from(101.6) / Number::from(72));
+        from_q.insert(Unit::Px, Number::from(101.6) / Number::from(96));
 
-    let mut from_pt = HashMap::new();
-    from_pt.insert("in", Number::from(72));
-    from_pt.insert("cm", Number::from(72) / Number::from(2.54));
-    from_pt.insert("pc", Number::from(12));
-    from_pt.insert("mm", Number::from(72) / Number::from(25.4));
-    from_pt.insert("q", Number::from(72) / Number::from(101.6));
-    from_pt.insert("pt", Number::one());
-    from_pt.insert("px", Number::small_ratio(3, 4));
+        let mut from_pt = HashMap::new();
+        from_pt.insert(Unit::In, Number::from(72));
+        from_pt.insert(Unit::Cm, Number::from(72) / Number::from(2.54));
+        from_pt.insert(Unit::Pc, Number::from(12));
+        from_pt.insert(Unit::Mm, Number::from(72) / Number::from(25.4));
+        from_pt.insert(Unit::Q, Number::from(72) / Number::from(101.6));
+        from_pt.insert(Unit::Pt, Number::one());
+        from_pt.insert(Unit::Px, Number::small_ratio(3, 4));
 
-    let mut from_px = HashMap::new();
-    from_px.insert("in", Number::from(96));
-    from_px.insert("cm", Number::from(96) / Number::from(2.54));
-    from_px.insert("pc", Number::from(16));
-    from_px.insert("mm", Number::from(96) / Number::from(25.4));
-    from_px.insert("q", Number::from(96) / Number::from(101.6));
-    from_px.insert("pt", Number::small_ratio(4, 3));
-    from_px.insert("px", Number::one());
+        let mut from_px = HashMap::new();
+        from_px.insert(Unit::In, Number::from(96));
+        from_px.insert(Unit::Cm, Number::from(96) / Number::from(2.54));
+        from_px.insert(Unit::Pc, Number::from(16));
+        from_px.insert(Unit::Mm, Number::from(96) / Number::from(25.4));
+        from_px.insert(Unit::Q, Number::from(96) / Number::from(101.6));
+        from_px.insert(Unit::Pt, Number::small_ratio(4, 3));
+        from_px.insert(Unit::Px, Number::one());
 
-    let mut from_deg = HashMap::new();
-    from_deg.insert("deg", Number::one());
-    from_deg.insert("grad", Number::small_ratio(9, 10));
-    from_deg.insert("rad", Number::from(180) / Number::from(PI));
-    from_deg.insert("turn", Number::from(360));
+        let mut from_deg = HashMap::new();
+        from_deg.insert(Unit::Deg, Number::one());
+        from_deg.insert(Unit::Grad, Number::small_ratio(9, 10));
+        from_deg.insert(Unit::Rad, Number::from(180) / Number::from(PI));
+        from_deg.insert(Unit::Turn, Number::from(360));
 
-    let mut from_grad = HashMap::new();
-    from_grad.insert("deg", Number::small_ratio(10, 9));
-    from_grad.insert("grad", Number::one());
-    from_grad.insert("rad", Number::from(200) / Number::from(PI));
-    from_grad.insert("turn", Number::from(400));
+        let mut from_grad = HashMap::new();
+        from_grad.insert(Unit::Deg, Number::small_ratio(10, 9));
+        from_grad.insert(Unit::Grad, Number::one());
+        from_grad.insert(Unit::Rad, Number::from(200) / Number::from(PI));
+        from_grad.insert(Unit::Turn, Number::from(400));
 
-    let mut from_rad = HashMap::new();
-    from_rad.insert("deg", Number::from(PI) / Number::from(180));
-    from_rad.insert("grad", Number::from(PI) / Number::from(200));
-    from_rad.insert("rad", Number::one());
-    from_rad.insert("turn", Number::from(2.0 * PI));
+        let mut from_rad = HashMap::new();
+        from_rad.insert(Unit::Deg, Number::from(PI) / Number::from(180));
+        from_rad.insert(Unit::Grad, Number::from(PI) / Number::from(200));
+        from_rad.insert(Unit::Rad, Number::one());
+        from_rad.insert(Unit::Turn, Number::from(2.0 * PI));
 
-    let mut from_turn = HashMap::new();
-    from_turn.insert("deg", Number::small_ratio(1, 360));
-    from_turn.insert("grad", Number::small_ratio(1, 400));
-    from_turn.insert("rad", Number::one() / Number::from(2.0 * PI));
-    from_turn.insert("turn", Number::one());
+        let mut from_turn = HashMap::new();
+        from_turn.insert(Unit::Deg, Number::small_ratio(1, 360));
+        from_turn.insert(Unit::Grad, Number::small_ratio(1, 400));
+        from_turn.insert(Unit::Rad, Number::one() / Number::from(2.0 * PI));
+        from_turn.insert(Unit::Turn, Number::one());
 
-    let mut from_s = HashMap::new();
-    from_s.insert("s", Number::one());
-    from_s.insert("ms", Number::small_ratio(1, 1000));
+        let mut from_s = HashMap::new();
+        from_s.insert(Unit::S, Number::one());
+        from_s.insert(Unit::Ms, Number::small_ratio(1, 1000));
 
-    let mut from_ms = HashMap::new();
-    from_ms.insert("s", Number::from(1000));
-    from_ms.insert("ms", Number::one());
+        let mut from_ms = HashMap::new();
+        from_ms.insert(Unit::S, Number::from(1000));
+        from_ms.insert(Unit::Ms, Number::one());
 
-    let mut from_hz = HashMap::new();
-    from_hz.insert("Hz", Number::one());
-    from_hz.insert("kHz", Number::from(1000));
+        let mut from_hz = HashMap::new();
+        from_hz.insert(Unit::Hz, Number::one());
+        from_hz.insert(Unit::Khz, Number::from(1000));
 
-    let mut from_khz = HashMap::new();
-    from_khz.insert("Hz", Number::small_ratio(1, 1000));
-    from_khz.insert("kHz", Number::one());
+        let mut from_khz = HashMap::new();
+        from_khz.insert(Unit::Hz, Number::small_ratio(1, 1000));
+        from_khz.insert(Unit::Khz, Number::one());
 
-    let mut from_dpi = HashMap::new();
-    from_dpi.insert("dpi", Number::one());
-    from_dpi.insert("dpcm", Number::from(2.54));
-    from_dpi.insert("dppx", Number::from(96));
+        let mut from_dpi = HashMap::new();
+        from_dpi.insert(Unit::Dpi, Number::one());
+        from_dpi.insert(Unit::Dpcm, Number::from(2.54));
+        from_dpi.insert(Unit::Dppx, Number::from(96));
 
-    let mut from_dpcm = HashMap::new();
-    from_dpcm.insert("dpi", Number::one() / Number::from(2.54));
-    from_dpcm.insert("dpcm", Number::one());
-    from_dpcm.insert("dppx", Number::from(96) / Number::from(2.54));
+        let mut from_dpcm = HashMap::new();
+        from_dpcm.insert(Unit::Dpi, Number::one() / Number::from(2.54));
+        from_dpcm.insert(Unit::Dpcm, Number::one());
+        from_dpcm.insert(Unit::Dppx, Number::from(96) / Number::from(2.54));
 
-    let mut from_dppx = HashMap::new();
-    from_dppx.insert("dpi", Number::small_ratio(1, 96));
-    from_dppx.insert("dpcm", Number::from(2.54) / Number::from(96));
-    from_dppx.insert("dppx", Number::one());
+        let mut from_dppx = HashMap::new();
+        from_dppx.insert(Unit::Dpi, Number::small_ratio(1, 96));
+        from_dppx.insert(Unit::Dpcm, Number::from(2.54) / Number::from(96));
+        from_dppx.insert(Unit::Dppx, Number::one());
 
-    let mut m = HashMap::new();
-    m.insert("in", from_in);
-    m.insert("cm", from_cm);
-    m.insert("pc", from_pc);
-    m.insert("mm", from_mm);
-    m.insert("q", from_q);
-    m.insert("pt", from_pt);
-    m.insert("px", from_px);
+        let mut m = HashMap::new();
+        m.insert(Unit::In, from_in);
+        m.insert(Unit::Cm, from_cm);
+        m.insert(Unit::Pc, from_pc);
+        m.insert(Unit::Mm, from_mm);
+        m.insert(Unit::Q, from_q);
+        m.insert(Unit::Pt, from_pt);
+        m.insert(Unit::Px, from_px);
 
-    m.insert("deg", from_deg);
-    m.insert("grad", from_grad);
-    m.insert("rad", from_rad);
-    m.insert("turn", from_turn);
+        m.insert(Unit::Deg, from_deg);
+        m.insert(Unit::Grad, from_grad);
+        m.insert(Unit::Rad, from_rad);
+        m.insert(Unit::Turn, from_turn);
 
-    m.insert("s", from_s);
-    m.insert("ms", from_ms);
+        m.insert(Unit::S, from_s);
+        m.insert(Unit::Ms, from_ms);
 
-    m.insert("Hz", from_hz);
-    m.insert("kHz", from_khz);
+        m.insert(Unit::Hz, from_hz);
+        m.insert(Unit::Khz, from_khz);
 
-    m.insert("dpi", from_dpi);
-    m.insert("dpcm", from_dpcm);
-    m.insert("dppx", from_dppx);
+        m.insert(Unit::Dpi, from_dpi);
+        m.insert(Unit::Dpcm, from_dpcm);
+        m.insert(Unit::Dppx, from_dppx);
 
-    m
-});
+        m
+    });
diff --git a/src/value/map.rs b/src/value/map.rs
index e5071c6..fc2c3f9 100644
--- a/src/value/map.rs
+++ b/src/value/map.rs
@@ -34,6 +34,10 @@ impl SassMap {
         SassMap(Vec::new())
     }
 
+    pub const fn new_with(elements: Vec<(Value, Value)>) -> SassMap {
+        SassMap(elements)
+    }
+
     /// We take by value here (consuming the map) in order to
     /// save a clone of the value, since the only place this
     /// should be called is in a builtin function, which throws
diff --git a/src/value/mod.rs b/src/value/mod.rs
index 8d0bbad..7e7291a 100644
--- a/src/value/mod.rs
+++ b/src/value/mod.rs
@@ -1,14 +1,16 @@
+use std::cmp::Ordering;
+
 use peekmore::PeekMore;
 
 use codemap::{Span, Spanned};
 
 use crate::{
     color::Color,
-    common::{Brackets, ListSeparator, QuoteKind},
+    common::{Brackets, ListSeparator, Op, QuoteKind},
     error::SassResult,
     parse::Parser,
     selector::Selector,
-    unit::{Unit, UNIT_CONVERSION_TABLE},
+    unit::Unit,
     utils::hex_char_for,
     {Cow, Token},
 };
@@ -29,7 +31,8 @@ pub(crate) enum Value {
     True,
     False,
     Null,
-    Dimension(Number, Unit, bool),
+    /// A `None` value for `Number` indicates a `NaN` value
+    Dimension(Option<Number>, Unit, bool),
     List(Vec<Value>, ListSeparator, Brackets),
     Color(Box<Color>),
     String(String, QuoteKind),
@@ -46,8 +49,8 @@ impl PartialEq for Value {
                 Value::String(s2, ..) => s1 == s2,
                 _ => false,
             },
-            Value::Dimension(n, unit, _) => match other {
-                Value::Dimension(n2, unit2, _) => {
+            Value::Dimension(Some(n), unit, _) => match other {
+                Value::Dimension(Some(n2), unit2, _) => {
                     if !unit.comparable(unit2) {
                         false
                     } else if unit == unit2 {
@@ -55,14 +58,12 @@ impl PartialEq for Value {
                     } else if unit == &Unit::None || unit2 == &Unit::None {
                         false
                     } else {
-                        n == &(n2.clone()
-                            * UNIT_CONVERSION_TABLE[unit.to_string().as_str()]
-                                [unit2.to_string().as_str()]
-                            .clone())
+                        n == &n2.clone().convert(unit2, unit)
                     }
                 }
                 _ => false,
             },
+            Value::Dimension(None, ..) => false,
             Value::List(list1, sep1, brackets1) => match other {
                 Value::List(list2, sep2, brackets2) => {
                     if sep1 != sep2 || brackets1 != brackets2 || list1.len() != list2.len() {
@@ -200,9 +201,21 @@ impl Value {
             Value::Important => Cow::const_str("!important"),
             Value::Dimension(num, unit, _) => match unit {
                 Unit::Mul(..) | Unit::Div(..) => {
-                    return Err((format!("{}{} isn't a valid CSS value.", num, unit), span).into());
+                    if let Some(num) = num {
+                        return Err(
+                            (format!("{}{} isn't a valid CSS value.", num, unit), span).into()
+                        );
+                    } else {
+                        return Err((format!("NaN{} isn't a valid CSS value.", unit), span).into());
+                    }
+                }
+                _ => {
+                    if let Some(num) = num {
+                        Cow::owned(format!("{}{}", num, unit))
+                    } else {
+                        Cow::owned(format!("NaN{}", unit))
+                    }
                 }
-                _ => Cow::owned(format!("{}{}", num, unit)),
             },
             Value::Map(..) | Value::FunctionRef(..) => {
                 return Err((
@@ -322,14 +335,68 @@ impl Value {
         }
     }
 
+    pub fn cmp(&self, other: &Self, span: Span, op: Op) -> SassResult<Ordering> {
+        Ok(match self {
+            Value::Dimension(None, ..) => todo!(),
+            Value::Dimension(Some(num), unit, _) => match &other {
+                Value::Dimension(None, ..) => todo!(),
+                Value::Dimension(Some(num2), unit2, _) => {
+                    if !unit.comparable(unit2) {
+                        return Err(
+                            (format!("Incompatible units {} and {}.", unit2, unit), span).into(),
+                        );
+                    }
+                    if unit == unit2 || unit == &Unit::None || unit2 == &Unit::None {
+                        num.cmp(num2)
+                    } else {
+                        num.cmp(&num2.clone().convert(unit2, unit))
+                    }
+                }
+                v => {
+                    return Err((
+                        format!(
+                            "Undefined operation \"{} {} {}\".",
+                            v.inspect(span)?,
+                            op,
+                            other.inspect(span)?
+                        ),
+                        span,
+                    )
+                        .into())
+                }
+            },
+            _ => {
+                return Err((
+                    format!(
+                        "Undefined operation \"{} {} {}\".",
+                        self.inspect(span)?,
+                        op,
+                        other.inspect(span)?
+                    ),
+                    span,
+                )
+                    .into())
+            }
+        })
+    }
+
+    pub fn unitless(&self) -> bool {
+        #[allow(clippy::match_same_arms)]
+        match self {
+            Value::Dimension(_, Unit::None, _) => true,
+            Value::Dimension(..) => false,
+            _ => true,
+        }
+    }
+
     pub fn not_equals(&self, other: &Self) -> bool {
         match self {
             Value::String(s1, ..) => match other {
                 Value::String(s2, ..) => s1 != s2,
                 _ => true,
             },
-            Value::Dimension(n, unit, _) => match other {
-                Value::Dimension(n2, unit2, _) => {
+            Value::Dimension(Some(n), unit, _) => match other {
+                Value::Dimension(Some(n2), unit2, _) => {
                     if !unit.comparable(unit2) {
                         true
                     } else if unit == unit2 {
@@ -337,10 +404,7 @@ impl Value {
                     } else if unit == &Unit::None || unit2 == &Unit::None {
                         true
                     } else {
-                        n != &(n2.clone()
-                            * UNIT_CONVERSION_TABLE[unit.to_string().as_str()]
-                                [unit2.to_string().as_str()]
-                            .clone())
+                        n != &n2.clone().convert(unit2, unit)
                     }
                 }
                 _ => true,
@@ -405,7 +469,8 @@ impl Value {
                     .collect::<SassResult<Vec<String>>>()?
                     .join(", ")
             )),
-            Value::Dimension(num, unit, _) => Cow::owned(format!("{}{}", num, unit)),
+            Value::Dimension(Some(num), unit, _) => Cow::owned(format!("{}{}", num, unit)),
+            Value::Dimension(None, unit, ..) => Cow::owned(format!("NaN{}", unit)),
             Value::ArgList(args) if args.is_empty() => Cow::const_str("()"),
             Value::ArgList(args) if args.len() == 1 => Cow::owned(format!(
                 "({},)",
@@ -477,6 +542,8 @@ impl Value {
             extender: parser.extender,
             content_scopes: parser.content_scopes,
             options: parser.options,
+            modules: parser.modules,
+            module_config: parser.module_config,
         }
         .parse_selector(allows_parent, true, String::new())?
         .0)
diff --git a/src/value/number/mod.rs b/src/value/number/mod.rs
index 00152b5..522f915 100644
--- a/src/value/number/mod.rs
+++ b/src/value/number/mod.rs
@@ -8,7 +8,11 @@ use std::{
 
 use num_bigint::BigInt;
 use num_rational::{BigRational, Rational64};
-use num_traits::{CheckedAdd, CheckedDiv, CheckedMul, CheckedSub, Num, One, Signed, Zero};
+use num_traits::{
+    CheckedAdd, CheckedDiv, CheckedMul, CheckedSub, Num, One, Signed, ToPrimitive, Zero,
+};
+
+use crate::unit::{Unit, UNIT_CONVERSION_TABLE};
 
 use integer::Integer;
 
@@ -16,7 +20,7 @@ mod integer;
 
 const PRECISION: usize = 10;
 
-#[derive(Clone, Eq, PartialEq, Ord)]
+#[derive(Clone, Eq, PartialEq)]
 pub(crate) enum Number {
     Small(Rational64),
     Big(Box<BigRational>),
@@ -106,6 +110,84 @@ impl Number {
 
         self
     }
+
+    #[allow(clippy::cast_precision_loss)]
+    fn as_float(self) -> Option<f64> {
+        Some(match self {
+            Number::Small(n) => ((*n.numer() as f64) / (*n.denom() as f64)),
+            Number::Big(n) => ((n.numer().to_f64()?) / (n.denom().to_f64()?)),
+        })
+    }
+
+    pub fn sqrt(self) -> Option<Self> {
+        Some(Number::Big(Box::new(BigRational::from_float(
+            self.as_float()?.sqrt(),
+        )?)))
+    }
+
+    pub fn ln(self) -> Option<Self> {
+        Some(Number::Big(Box::new(BigRational::from_float(
+            self.as_float()?.ln(),
+        )?)))
+    }
+
+    pub fn pow(self, exponent: Self) -> Option<Self> {
+        Some(Number::Big(Box::new(BigRational::from_float(
+            self.as_float()?.powf(exponent.as_float()?),
+        )?)))
+    }
+
+    pub fn pi() -> Self {
+        Number::from(std::f64::consts::PI)
+    }
+
+    pub fn atan2(self, other: Self) -> Option<Self> {
+        Some(Number::Big(Box::new(BigRational::from_float(
+            self.as_float()?.atan2(other.as_float()?),
+        )?)))
+    }
+
+    /// Invariants: `from.comparable(&to)` must be true
+    pub fn convert(self, from: &Unit, to: &Unit) -> Self {
+        self * UNIT_CONVERSION_TABLE[to][from].clone()
+    }
+}
+
+macro_rules! trig_fn(
+    ($name:ident, $name_deg:ident) => {
+        pub fn $name(self) -> Option<Self> {
+            Some(Number::Big(Box::new(BigRational::from_float(
+                self.as_float()?.$name(),
+            )?)))
+        }
+
+        pub fn $name_deg(self) -> Option<Self> {
+            Some(Number::Big(Box::new(BigRational::from_float(
+                self.as_float()?.to_radians().$name(),
+            )?)))
+        }
+    }
+);
+
+macro_rules! inverse_trig_fn(
+    ($name:ident) => {
+        pub fn $name(self) -> Option<Self> {
+            Some(Number::Big(Box::new(BigRational::from_float(
+                self.as_float()?.$name().to_degrees(),
+            )?)))
+        }
+    }
+);
+
+/// Trigonometry methods
+impl Number {
+    trig_fn!(cos, cos_deg);
+    trig_fn!(sin, sin_deg);
+    trig_fn!(tan, tan_deg);
+
+    inverse_trig_fn!(acos);
+    inverse_trig_fn!(asin);
+    inverse_trig_fn!(atan);
 }
 
 impl Default for Number {
@@ -321,6 +403,30 @@ impl PartialOrd for Number {
     }
 }
 
+impl Ord for Number {
+    fn cmp(&self, other: &Self) -> Ordering {
+        match self {
+            Self::Small(val1) => match other {
+                Self::Small(val2) => val1.cmp(val2),
+                Self::Big(val2) => {
+                    let tuple: (i64, i64) = (*val1).into();
+                    BigRational::new_raw(BigInt::from(tuple.0), BigInt::from(tuple.1)).cmp(val2)
+                }
+            },
+            Self::Big(val1) => match other {
+                Self::Small(val2) => {
+                    let tuple: (i64, i64) = (*val2).into();
+                    (**val1).cmp(&BigRational::new_raw(
+                        BigInt::from(tuple.0),
+                        BigInt::from(tuple.1),
+                    ))
+                }
+                Self::Big(val2) => val1.cmp(val2),
+            },
+        }
+    }
+}
+
 impl Add for Number {
     type Output = Self;
 
diff --git a/tests/division.rs b/tests/division.rs
index 1c02dce..ad7bcfb 100644
--- a/tests/division.rs
+++ b/tests/division.rs
@@ -169,3 +169,8 @@ test!(
     "a {\n  color: 1 + 3 / 4;\n}\n",
     "a {\n  color: 1.75;\n}\n"
 );
+test!(
+    zero_div_zero_is_nan,
+    "a {\n  color: (0 / 0);\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
diff --git a/tests/get-function.rs b/tests/get-function.rs
index 31583f6..2ec4e9f 100644
--- a/tests/get-function.rs
+++ b/tests/get-function.rs
@@ -121,3 +121,8 @@ test!(
     "a {\n  color: call(call(get-function(get-function), darken), red, 10%);\n}\n",
     "a {\n  color: #cc0000;\n}\n"
 );
+test!(
+    get_function_of_module,
+    "@use 'sass:math';\na {\n  color: call(get-function(cos, $module: math), 2);\n}\n",
+    "a {\n  color: -0.4161468365;\n}\n"
+);
diff --git a/tests/imports.rs b/tests/imports.rs
index f076028..9210585 100644
--- a/tests/imports.rs
+++ b/tests/imports.rs
@@ -1,43 +1,10 @@
 #![cfg(test)]
+
 use std::io::Write;
-use tempfile::Builder;
 
 #[macro_use]
 mod macros;
 
-/// Create a temporary file with the given name
-/// and contents.
-///
-/// This must be a macro rather than a function
-/// because the tempfile will be deleted when it
-/// exits scope
-macro_rules! tempfile {
-    ($name:literal, $content:literal) => {
-        let mut f = Builder::new()
-            .rand_bytes(0)
-            .prefix("")
-            .suffix($name)
-            .tempfile_in("")
-            .unwrap();
-        write!(f, "{}", $content).unwrap();
-    };
-    ($name:literal, $content:literal, dir=$dir:literal) => {
-        let _d = Builder::new()
-            .rand_bytes(0)
-            .prefix("")
-            .suffix($dir)
-            .tempdir_in("")
-            .unwrap();
-        let mut f = Builder::new()
-            .rand_bytes(0)
-            .prefix("")
-            .suffix($name)
-            .tempfile_in($dir)
-            .unwrap();
-        write!(f, "{}", $content).unwrap();
-    };
-}
-
 #[test]
 fn imports_variable() {
     let input = "@import \"imports_variable\";\na {\n color: $a;\n}";
@@ -59,17 +26,8 @@ fn import_no_semicolon() {
 fn import_no_quotes() {
     let input = "@import import_no_quotes";
     tempfile!("import_no_quotes", "$a: red;");
-    match grass::from_string(input.to_string(), &grass::Options::default()) {
-        Ok(..) => panic!("did not fail"),
-        Err(e) => assert_eq!(
-            "Error: Expected string.",
-            e.to_string()
-                .chars()
-                .take_while(|c| *c != '\n')
-                .collect::<String>()
-                .as_str()
-        ),
-    }
+
+    assert_err!("Error: Expected string.", input);
 }
 
 #[test]
diff --git a/tests/macros.rs b/tests/macros.rs
index c6c1c7c..9e4d3c2 100644
--- a/tests/macros.rs
+++ b/tests/macros.rs
@@ -51,3 +51,54 @@ macro_rules! error {
         }
     };
 }
+
+/// Create a temporary file with the given name
+/// and contents.
+///
+/// This must be a macro rather than a function
+/// because the tempfile will be deleted when it
+/// exits scope
+#[macro_export]
+macro_rules! tempfile {
+    ($name:literal, $content:literal) => {
+        let mut f = tempfile::Builder::new()
+            .rand_bytes(0)
+            .prefix("")
+            .suffix($name)
+            .tempfile_in("")
+            .unwrap();
+        write!(f, "{}", $content).unwrap();
+    };
+    ($name:literal, $content:literal, dir=$dir:literal) => {
+        let _d = tempfile::Builder::new()
+            .rand_bytes(0)
+            .prefix("")
+            .suffix($dir)
+            .tempdir_in("")
+            .unwrap();
+        let mut f = tempfile::Builder::new()
+            .rand_bytes(0)
+            .prefix("")
+            .suffix($name)
+            .tempfile_in($dir)
+            .unwrap();
+        write!(f, "{}", $content).unwrap();
+    };
+}
+
+#[macro_export]
+macro_rules! assert_err {
+    ($err:literal, $input:expr) => {
+        match grass::from_string($input.to_string(), &grass::Options::default()) {
+            Ok(..) => panic!("did not fail"),
+            Err(e) => assert_eq!(
+                $err,
+                e.to_string()
+                    .chars()
+                    .take_while(|c| *c != '\n')
+                    .collect::<String>()
+                    .as_str()
+            ),
+        }
+    };
+}
diff --git a/tests/math-module.rs b/tests/math-module.rs
new file mode 100644
index 0000000..4f83a4d
--- /dev/null
+++ b/tests/math-module.rs
@@ -0,0 +1,592 @@
+#![cfg(test)]
+
+#[macro_use]
+mod macros;
+
+test!(
+    clamp_in_the_middle,
+    "@use 'sass:math';\na {\n  color: math.clamp(0, 1, 2);\n}\n",
+    "a {\n  color: 1;\n}\n"
+);
+test!(
+    clamp_first_is_bigger,
+    "@use 'sass:math';\na {\n  color: math.clamp(2, 1, 0);\n}\n",
+    "a {\n  color: 2;\n}\n"
+);
+test!(
+    clamp_all_same_unit,
+    "@use 'sass:math';\na {\n  color: math.clamp(0px, 1px, 2px);\n}\n",
+    "a {\n  color: 1px;\n}\n"
+);
+test!(
+    clamp_all_different_but_compatible_unit,
+    "@use 'sass:math';\na {\n  color: math.clamp(0mm, 1cm, 2in);\n}\n",
+    "a {\n  color: 1cm;\n}\n"
+);
+error!(
+    clamp_only_min_has_no_unit,
+    "@use 'sass:math';\na {\n  color: math.clamp(0, 1cm, 2in);\n}\n",
+    "Error: $min is unitless but $number has unit cm. Arguments must all have units or all be unitless."
+);
+error!(
+    clamp_only_number_has_no_unit,
+    "@use 'sass:math';\na {\n  color: math.clamp(0mm, 1, 2in);\n}\n",
+    "Error: $min has unit mm but $number is unitless. Arguments must all have units or all be unitless."
+);
+error!(
+    clamp_only_max_has_no_unit,
+    "@use 'sass:math';\na {\n  color: math.clamp(0mm, 1cm, 2);\n}\n",
+    "Error: $min has unit mm but $max is unitless. Arguments must all have units or all be unitless."
+);
+test!(
+    sqrt_zero,
+    "@use 'sass:math';\na {\n  color: math.sqrt(0);\n}\n",
+    "a {\n  color: 0;\n}\n"
+);
+test!(
+    sqrt_small_positive,
+    "@use 'sass:math';\na {\n  color: math.sqrt(99);\n}\n",
+    "a {\n  color: 9.9498743711;\n}\n"
+);
+test!(
+    sqrt_small_negative,
+    "@use 'sass:math';\na {\n  color: math.sqrt(-99);\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+test!(
+    sqrt_big_positive,
+    "@use 'sass:math';\na {\n  color: math.sqrt(9999999999999999999999999999999999999999999999999);\n}\n",
+    "a {\n  color: 3162277660168379038695424;\n}\n"
+);
+test!(
+    sqrt_big_negative,
+    "@use 'sass:math';\na {\n  color: math.sqrt(-9999999999999999999999999999999999999999999999999);\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+test!(
+    sqrt_irrational,
+    "@use 'sass:math';\na {\n  color: math.sqrt(2);\n}\n",
+    "a {\n  color: 1.4142135624;\n}\n"
+);
+test!(
+    sqrt_of_nan,
+    "@use 'sass:math';\na {\n  color: math.sqrt((0 / 0));\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+error!(
+    sqrt_with_units,
+    "@use 'sass:math';\na {\n  color: math.sqrt(1px);\n}\n",
+    "Error: $number: Expected 1px to have no units."
+);
+error!(
+    cos_non_angle,
+    "@use 'sass:math';\na {\n  color: math.cos(1px);\n}\n",
+    "Error: $number: Expected 1px to be an angle."
+);
+test!(
+    cos_small_degree,
+    "@use 'sass:math';\na {\n  color: math.cos(1deg);\n}\n",
+    "a {\n  color: 0.9998476952;\n}\n"
+);
+test!(
+    cos_small_radian,
+    "@use 'sass:math';\na {\n  color: math.cos(1rad);\n}\n",
+    "a {\n  color: 0.5403023059;\n}\n"
+);
+test!(
+    cos_small_no_unit,
+    "@use 'sass:math';\na {\n  color: math.cos(1);\n}\n",
+    "a {\n  color: 0.5403023059;\n}\n"
+);
+test!(
+    cos_small_negative_degree,
+    "@use 'sass:math';\na {\n  color: math.cos(-1deg);\n}\n",
+    "a {\n  color: 0.9998476952;\n}\n"
+);
+test!(
+    cos_small_negative_radian,
+    "@use 'sass:math';\na {\n  color: math.cos(-1rad);\n}\n",
+    "a {\n  color: 0.5403023059;\n}\n"
+);
+test!(
+    cos_small_negative_no_unit,
+    "@use 'sass:math';\na {\n  color: math.cos(-1);\n}\n",
+    "a {\n  color: 0.5403023059;\n}\n"
+);
+test!(
+    cos_pi,
+    "@use 'sass:math';\na {\n  color: math.cos(math.$pi);\n}\n",
+    "a {\n  color: -1;\n}\n"
+);
+test!(
+    cos_two_pi,
+    "@use 'sass:math';\na {\n  color: math.cos(2 * math.$pi);\n}\n",
+    "a {\n  color: 1;\n}\n"
+);
+error!(
+    sin_non_angle,
+    "@use 'sass:math';\na {\n  color: math.sin(1px);\n}\n",
+    "Error: $number: Expected 1px to be an angle."
+);
+test!(
+    sin_small_degree,
+    "@use 'sass:math';\na {\n  color: math.sin(1deg);\n}\n",
+    "a {\n  color: 0.0174524064;\n}\n"
+);
+test!(
+    sin_small_radian,
+    "@use 'sass:math';\na {\n  color: math.sin(1rad);\n}\n",
+    "a {\n  color: 0.8414709848;\n}\n"
+);
+test!(
+    sin_small_no_unit,
+    "@use 'sass:math';\na {\n  color: math.sin(1);\n}\n",
+    "a {\n  color: 0.8414709848;\n}\n"
+);
+test!(
+    sin_small_negative_degree,
+    "@use 'sass:math';\na {\n  color: math.sin(-1deg);\n}\n",
+    "a {\n  color: -0.0174524064;\n}\n"
+);
+test!(
+    sin_small_negative_radian,
+    "@use 'sass:math';\na {\n  color: math.sin(-1rad);\n}\n",
+    "a {\n  color: -0.8414709848;\n}\n"
+);
+test!(
+    sin_small_negative_no_unit,
+    "@use 'sass:math';\na {\n  color: math.sin(-1);\n}\n",
+    "a {\n  color: -0.8414709848;\n}\n"
+);
+test!(
+    sin_pi,
+    "@use 'sass:math';\na {\n  color: math.sin(math.$pi);\n}\n",
+    "a {\n  color: 0;\n}\n"
+);
+test!(
+    sin_two_pi,
+    "@use 'sass:math';\na {\n  color: math.sin(2 * math.$pi);\n}\n",
+    "a {\n  color: 0;\n}\n"
+);
+error!(
+    tan_non_angle,
+    "@use 'sass:math';\na {\n  color: math.tan(1px);\n}\n",
+    "Error: $number: Expected 1px to be an angle."
+);
+test!(
+    tan_small_degree,
+    "@use 'sass:math';\na {\n  color: math.tan(1deg);\n}\n",
+    "a {\n  color: 0.0174550649;\n}\n"
+);
+test!(
+    tan_small_radian,
+    "@use 'sass:math';\na {\n  color: math.tan(1rad);\n}\n",
+    "a {\n  color: 1.5574077247;\n}\n"
+);
+test!(
+    tan_small_no_unit,
+    "@use 'sass:math';\na {\n  color: math.tan(1);\n}\n",
+    "a {\n  color: 1.5574077247;\n}\n"
+);
+test!(
+    tan_small_negative_degree,
+    "@use 'sass:math';\na {\n  color: math.tan(-1deg);\n}\n",
+    "a {\n  color: -0.0174550649;\n}\n"
+);
+test!(
+    tan_small_negative_radian,
+    "@use 'sass:math';\na {\n  color: math.tan(-1rad);\n}\n",
+    "a {\n  color: -1.5574077247;\n}\n"
+);
+test!(
+    tan_small_negative_no_unit,
+    "@use 'sass:math';\na {\n  color: math.tan(-1);\n}\n",
+    "a {\n  color: -1.5574077247;\n}\n"
+);
+test!(
+    tan_pi,
+    "@use 'sass:math';\na {\n  color: math.tan(math.$pi);\n}\n",
+    "a {\n  color: 0;\n}\n"
+);
+test!(
+    tan_two_pi,
+    "@use 'sass:math';\na {\n  color: math.tan(2 * math.$pi);\n}\n",
+    "a {\n  color: 0;\n}\n"
+);
+test!(
+    acos_above_one,
+    "@use 'sass:math';\na {\n  color: math.acos(2);\n}\n",
+    "a {\n  color: NaNdeg;\n}\n"
+);
+test!(
+    acos_below_negative_one,
+    "@use 'sass:math';\na {\n  color: math.acos(-2);\n}\n",
+    "a {\n  color: NaNdeg;\n}\n"
+);
+test!(
+    acos_one,
+    "@use 'sass:math';\na {\n  color: math.acos(1);\n}\n",
+    "a {\n  color: 0deg;\n}\n"
+);
+test!(
+    acos_negative_one,
+    "@use 'sass:math';\na {\n  color: math.acos(-1);\n}\n",
+    "a {\n  color: 180deg;\n}\n"
+);
+test!(
+    acos_zero,
+    "@use 'sass:math';\na {\n  color: math.acos(0);\n}\n",
+    "a {\n  color: 90deg;\n}\n"
+);
+test!(
+    acos_point_five,
+    "@use 'sass:math';\na {\n  color: math.acos(.5);\n}\n",
+    "a {\n  color: 60deg;\n}\n"
+);
+test!(
+    acos_nan,
+    "@use 'sass:math';\na {\n  color: math.acos((0 / 0));\n}\n",
+    "a {\n  color: NaNdeg;\n}\n"
+);
+test!(
+    asin_above_one,
+    "@use 'sass:math';\na {\n  color: math.asin(2);\n}\n",
+    "a {\n  color: NaNdeg;\n}\n"
+);
+test!(
+    asin_below_negative_one,
+    "@use 'sass:math';\na {\n  color: math.asin(-2);\n}\n",
+    "a {\n  color: NaNdeg;\n}\n"
+);
+test!(
+    asin_one,
+    "@use 'sass:math';\na {\n  color: math.asin(1);\n}\n",
+    "a {\n  color: 90deg;\n}\n"
+);
+test!(
+    asin_negative_one,
+    "@use 'sass:math';\na {\n  color: math.asin(-1);\n}\n",
+    "a {\n  color: -90deg;\n}\n"
+);
+test!(
+    asin_zero,
+    "@use 'sass:math';\na {\n  color: math.asin(0);\n}\n",
+    "a {\n  color: 0deg;\n}\n"
+);
+test!(
+    asin_point_five,
+    "@use 'sass:math';\na {\n  color: math.asin(.5);\n}\n",
+    "a {\n  color: 30deg;\n}\n"
+);
+test!(
+    asin_nan,
+    "@use 'sass:math';\na {\n  color: math.asin((0 / 0));\n}\n",
+    "a {\n  color: NaNdeg;\n}\n"
+);
+test!(
+    atan_above_one,
+    "@use 'sass:math';\na {\n  color: math.atan(2);\n}\n",
+    "a {\n  color: 63.4349488229deg;\n}\n"
+);
+test!(
+    atan_below_negative_one,
+    "@use 'sass:math';\na {\n  color: math.atan(-2);\n}\n",
+    "a {\n  color: -63.4349488229deg;\n}\n"
+);
+test!(
+    atan_one,
+    "@use 'sass:math';\na {\n  color: math.atan(1);\n}\n",
+    "a {\n  color: 45deg;\n}\n"
+);
+test!(
+    atan_negative_one,
+    "@use 'sass:math';\na {\n  color: math.atan(-1);\n}\n",
+    "a {\n  color: -45deg;\n}\n"
+);
+test!(
+    atan_zero,
+    "@use 'sass:math';\na {\n  color: math.atan(0);\n}\n",
+    "a {\n  color: 0deg;\n}\n"
+);
+test!(
+    atan_point_five,
+    "@use 'sass:math';\na {\n  color: math.atan(.5);\n}\n",
+    "a {\n  color: 26.5650511771deg;\n}\n"
+);
+test!(
+    atan_nan,
+    "@use 'sass:math';\na {\n  color: math.atan((0 / 0));\n}\n",
+    "a {\n  color: NaNdeg;\n}\n"
+);
+test!(
+    log_above_one,
+    "@use 'sass:math';\na {\n  color: math.log(2);\n}\n",
+    "a {\n  color: 0.6931471806;\n}\n"
+);
+test!(
+    log_below_negative_one,
+    "@use 'sass:math';\na {\n  color: math.log(-2);\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+test!(
+    log_one,
+    "@use 'sass:math';\na {\n  color: math.log(1);\n}\n",
+    "a {\n  color: 0;\n}\n"
+);
+test!(
+    log_negative_one,
+    "@use 'sass:math';\na {\n  color: math.log(-1);\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+test!(
+    #[ignore = "we do not support Infinity"]
+    log_zero,
+    "@use 'sass:math';\na {\n  color: math.log(0);\n}\n",
+    "a {\n  color: -Infinity;\n}\n"
+);
+test!(
+    log_point_five,
+    "@use 'sass:math';\na {\n  color: math.log(.5);\n}\n",
+    "a {\n  color: -0.6931471806;\n}\n"
+);
+test!(
+    log_nan,
+    "@use 'sass:math';\na {\n  color: math.log((0 / 0));\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+test!(
+    log_base_nan,
+    "@use 'sass:math';\na {\n  color: math.log(1, (0 / 0));\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+test!(
+    log_base_above_one,
+    "@use 'sass:math';\na {\n  color: math.log(2, 2);\n}\n",
+    "a {\n  color: 1;\n}\n"
+);
+test!(
+    log_base_below_negative_one,
+    "@use 'sass:math';\na {\n  color: math.log(2, -2);\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+test!(
+    #[ignore = "we do not support Infinity"]
+    log_base_one,
+    "@use 'sass:math';\na {\n  color: math.log(2, 1);\n}\n",
+    "a {\n  color: Infinity;\n}\n"
+);
+test!(
+    log_base_negative_one,
+    "@use 'sass:math';\na {\n  color: math.log(2, -1);\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+test!(
+    log_base_zero,
+    "@use 'sass:math';\na {\n  color: math.log(2, 0);\n}\n",
+    "a {\n  color: 0;\n}\n"
+);
+test!(
+    log_base_point_five,
+    "@use 'sass:math';\na {\n  color: math.log(2, .5);\n}\n",
+    "a {\n  color: -1;\n}\n"
+);
+
+test!(
+    pow_exponent_and_base_one,
+    "@use 'sass:math';\na {\n  color: math.pow(1, 1);\n}\n",
+    "a {\n  color: 1;\n}\n"
+);
+test!(
+    pow_exponent_and_base_ten,
+    "@use 'sass:math';\na {\n  color: math.pow(10, 10);\n}\n",
+    "a {\n  color: 10000000000;\n}\n"
+);
+test!(
+    pow_base_negative_exponent_positive,
+    "@use 'sass:math';\na {\n  color: math.pow(-2, 3);\n}\n",
+    "a {\n  color: -8;\n}\n"
+);
+test!(
+    pow_base_positive_exponent_negative,
+    "@use 'sass:math';\na {\n  color: math.pow(2, -3);\n}\n",
+    "a {\n  color: 0.125;\n}\n"
+);
+test!(
+    pow_base_negative_exponent_negative,
+    "@use 'sass:math';\na {\n  color: math.pow(-2, -3);\n}\n",
+    "a {\n  color: -0.125;\n}\n"
+);
+test!(
+    pow_base_decimal,
+    "@use 'sass:math';\na {\n  color: math.pow(2.4, 3);\n}\n",
+    "a {\n  color: 13.824;\n}\n"
+);
+test!(
+    pow_exponent_decimal,
+    "@use 'sass:math';\na {\n  color: math.pow(2, 3.5);\n}\n",
+    "a {\n  color: 11.313708499;\n}\n"
+);
+test!(
+    pow_base_nan,
+    "@use 'sass:math';\na {\n  color: math.pow((0 / 0), 3);\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+test!(
+    pow_exponent_nan,
+    "@use 'sass:math';\na {\n  color: math.pow(2, (0 / 0));\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+test!(
+    pow_base_and_exponent_nan,
+    "@use 'sass:math';\na {\n  color: math.pow((0 / 0), (0 / 0));\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+test!(
+    pow_exponent_zero,
+    "@use 'sass:math';\na {\n  color: math.pow(2, 0);\n}\n",
+    "a {\n  color: 1;\n}\n"
+);
+test!(
+    hypot_all_same_unit,
+    "@use 'sass:math';\na {\n  color: math.hypot(1px, 2px, 3px, 4px, 5px);\n}\n",
+    "a {\n  color: 7.4161984871px;\n}\n"
+);
+test!(
+    hypot_negative,
+    "@use 'sass:math';\na {\n  color: math.hypot(1px, 2px, 3px, 4px, 5px, -20px);\n}\n",
+    "a {\n  color: 21.3307290077px;\n}\n"
+);
+test!(
+    hypot_all_different_but_comparable_unit,
+    "@use 'sass:math';\na {\n  color: math.hypot(1in, 2cm, 3mm, 4pt, 5pc);\n}\n",
+    "a {\n  color: 1.5269191636in;\n}\n"
+);
+test!(
+    hypot_all_no_unit,
+    "@use 'sass:math';\na {\n  color: math.hypot(1, 2, 3);\n}\n",
+    "a {\n  color: 3.7416573868;\n}\n"
+);
+test!(
+    hypot_nan_has_comparable_unit,
+    "@use 'sass:math';\na {\n  color: math.hypot(1deg, 2deg, math.acos(2));\n}\n",
+    "a {\n  color: NaNdeg;\n}\n"
+);
+error!(
+    hypot_no_args,
+    "@use 'sass:math';\na {\n  color: math.hypot();\n}\n",
+    "Error: At least one argument must be passed."
+);
+error!(
+    hypot_first_has_no_unit_third_has_unit,
+    "@use 'sass:math';\na {\n  color: math.hypot(1, 2, 3px);\n}\n",
+    "Error: Argument 1 is unitless but argument 3 has unit px. Arguments must all have units or all be unitless."
+);
+error!(
+    hypot_non_numeric_argument,
+    "@use 'sass:math';\na {\n  color: math.hypot(1, red, 3);\n}\n", "Error: red is not a number."
+);
+error!(
+    hypot_units_not_comparable,
+    "@use 'sass:math';\na {\n  color: math.hypot(1px, 2in, 3rem);\n}\n",
+    "Error: Incompatible units px and rem."
+);
+error!(
+    hypot_nan_has_no_unit_but_first_has_unit,
+    "@use 'sass:math';\na {\n  color: math.hypot(1deg, 2deg, (0 / 0));\n}\n",
+    "Error: Argument 1 has unit deg but argument 3 is unitless. Arguments must all have units or all be unitless."
+);
+test!(
+    atan2_both_positive,
+    "@use 'sass:math';\na {\n  color: math.atan2(3, 4);\n}\n",
+    "a {\n  color: 36.8698976458deg;\n}\n"
+);
+test!(
+    atan2_first_negative,
+    "@use 'sass:math';\na {\n  color: math.atan2(-3, 4);\n}\n",
+    "a {\n  color: -36.8698976458deg;\n}\n"
+);
+test!(
+    atan2_second_negative,
+    "@use 'sass:math';\na {\n  color: math.atan2(3, -4);\n}\n",
+    "a {\n  color: 143.1301023542deg;\n}\n"
+);
+test!(
+    atan2_both_negative,
+    "@use 'sass:math';\na {\n  color: math.atan2(-3, -4);\n}\n",
+    "a {\n  color: -143.1301023542deg;\n}\n"
+);
+test!(
+    atan2_first_positive_second_zero,
+    "@use 'sass:math';\na {\n  color: math.atan2(3, 0);\n}\n",
+    "a {\n  color: 90deg;\n}\n"
+);
+test!(
+    atan2_first_negative_second_zero,
+    "@use 'sass:math';\na {\n  color: math.atan2(-3, 0);\n}\n",
+    "a {\n  color: -90deg;\n}\n"
+);
+test!(
+    atan2_first_zero_second_positive,
+    "@use 'sass:math';\na {\n  color: math.atan2(0, 4);\n}\n",
+    "a {\n  color: 0deg;\n}\n"
+);
+test!(
+    atan2_first_zero_second_negative,
+    "@use 'sass:math';\na {\n  color: math.atan2(0, -4);\n}\n",
+    "a {\n  color: 180deg;\n}\n"
+);
+test!(
+    atan2_both_zero,
+    "@use 'sass:math';\na {\n  color: math.atan2(0, 0);\n}\n",
+    "a {\n  color: 0deg;\n}\n"
+);
+test!(
+    atan2_both_same_unit,
+    "@use 'sass:math';\na {\n  color: math.atan2(3px, 4px);\n}\n",
+    "a {\n  color: 36.8698976458deg;\n}\n"
+);
+test!(
+    atan2_both_different_but_comparable_unit,
+    "@use 'sass:math';\na {\n  color: math.atan2(3px, 4in);\n}\n",
+    "a {\n  color: 0.4476141709deg;\n}\n"
+);
+error!(
+    atan2_first_unitless_second_unit,
+    "@use 'sass:math';\na {\n  color: math.atan2(3, 4rem);\n}\n",
+    "Error: $y is unitless but $x has unit rem. Arguments must all have units or all be unitless."
+);
+error!(
+    atan2_first_unit_second_unitless,
+    "@use 'sass:math';\na {\n  color: math.atan2(3px, 4);\n}\n",
+    "Error: $y has unit px but $x is unitless. Arguments must all have units or all be unitless."
+);
+error!(
+    atan2_incompatible_units,
+    "@use 'sass:math';\na {\n  color: math.atan2(3px, 4rem);\n}\n",
+    "Error: Incompatible units px and rem."
+);
+error!(
+    atan2_nan_incompatible_units,
+    "@use 'sass:math';\na {\n  color: math.atan2(math.acos(2), 3);\n}\n",
+    "Error: $y has unit deg but $x is unitless. Arguments must all have units or all be unitless."
+);
+test!(
+    atan2_first_nan,
+    "@use 'sass:math';\na {\n  color: math.atan2((0/0), 0);\n}\n",
+    "a {\n  color: NaNdeg;\n}\n"
+);
+test!(
+    atan2_second_nan,
+    "@use 'sass:math';\na {\n  color: math.atan2(0, (0/0));\n}\n",
+    "a {\n  color: NaNdeg;\n}\n"
+);
+test!(
+    atan2_both_nan,
+    "@use 'sass:math';\na {\n  color: math.atan2((0/0), (0/0));\n}\n",
+    "a {\n  color: NaNdeg;\n}\n"
+);
+test!(
+    atan2_nan_with_same_units,
+    "@use 'sass:math';\na {\n  color: math.atan2(math.acos(2), 3deg);\n}\n",
+    "a {\n  color: NaNdeg;\n}\n"
+);
diff --git a/tests/meta-module.rs b/tests/meta-module.rs
new file mode 100644
index 0000000..e6ee9ba
--- /dev/null
+++ b/tests/meta-module.rs
@@ -0,0 +1,65 @@
+#![cfg(test)]
+
+use std::io::Write;
+
+#[macro_use]
+mod macros;
+
+test!(
+    module_functions_builtin,
+    "@use 'sass:meta';\na {\n  color: inspect(meta.module-functions(meta));\n}\n",
+    "a {\n  color: (\"feature-exists\": get-function(\"feature-exists\"), \"inspect\": get-function(\"inspect\"), \"type-of\": get-function(\"type-of\"), \"keywords\": get-function(\"keywords\"), \"global-variable-exists\": get-function(\"global-variable-exists\"), \"variable-exists\": get-function(\"variable-exists\"), \"function-exists\": get-function(\"function-exists\"), \"mixin-exists\": get-function(\"mixin-exists\"), \"content-exists\": get-function(\"content-exists\"), \"module-variables\": get-function(\"module-variables\"), \"module-functions\": get-function(\"module-functions\"), \"get-function\": get-function(\"get-function\"), \"call\": get-function(\"call\"));\n}\n"
+);
+test!(
+    module_variables_builtin,
+    "@use 'sass:meta';\n@use 'sass:math';\na {\n  color: inspect(meta.module-variables(math));\n}\n",
+    "a {\n  color: (\"e\": 2.7182818285, \"pi\": 3.1415926536);\n}\n"
+);
+test!(
+    global_var_exists_module,
+    "@use 'sass:math';\na {\n  color: global-variable-exists(pi, $module: math);\n}\n",
+    "a {\n  color: true;\n}\n"
+);
+
+#[test]
+fn mixin_exists_module() {
+    let input = "@use \"mixin_exists_module\" as module;\na {\n color: mixin-exists(foo, $module: module);\n}";
+    tempfile!("mixin_exists_module.scss", "@mixin foo {}");
+    assert_eq!(
+        "a {\n  color: true;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default()).expect(input)
+    );
+}
+
+#[test]
+fn load_css_simple() {
+    let input = "@use \"sass:meta\";\na {\n @include meta.load-css(load_css_simple);\n}";
+    tempfile!("load_css_simple.scss", "a { color: red; }");
+    assert_eq!(
+        "a a {\n  color: red;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default()).expect(input)
+    );
+}
+
+#[test]
+fn load_css_explicit_args() {
+    let input = "@use \"sass:meta\";\na {\n @include meta.load-css($module: load_css_explicit_args, $with: null);\n}";
+    tempfile!("load_css_explicit_args.scss", "a { color: red; }");
+    assert_eq!(
+        "a a {\n  color: red;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default()).expect(input)
+    );
+}
+
+#[test]
+fn load_css_non_string_url() {
+    let input = "@use \"sass:meta\";\na {\n @include meta.load-css(2);\n}";
+    tempfile!("load_css_non_string_url.scss", "a { color: red; }");
+    assert_err!("Error: $module: 2 is not a string.", input);
+}
+
+#[test]
+fn load_css_non_map_with() {
+    let input = "@use \"sass:meta\";\na {\n @include meta.load-css(foo, 2);\n}";
+    assert_err!("Error: $with: 2 is not a map.", input);
+}
diff --git a/tests/meta.rs b/tests/meta.rs
index 6690635..56f4d4b 100644
--- a/tests/meta.rs
+++ b/tests/meta.rs
@@ -195,6 +195,11 @@ test!(
     "a {\n  color: type-of(- 2)\n}\n",
     "a {\n  color: number;\n}\n"
 );
+test!(
+    type_of_nan,
+    "a {\n  color: type-of((0 / 0))\n}\n",
+    "a {\n  color: number;\n}\n"
+);
 test!(
     type_of_arglist,
     "@mixin foo($a...) {color: type-of($a);}\na {@include foo(1, 2, 3, 4, 5);}",
diff --git a/tests/ordering.rs b/tests/ordering.rs
index ae00b88..5a2b644 100644
--- a/tests/ordering.rs
+++ b/tests/ordering.rs
@@ -63,3 +63,8 @@ test!(
     "a {\n  color: 0 < 1;\n}\n",
     "a {\n  color: true;\n}\n"
 );
+test!(
+    ord_the_same_as_partial_ord,
+    "a {\n  color: 2in > 1cm;\n}\n",
+    "a {\n  color: true;\n}\n"
+);
diff --git a/tests/use.rs b/tests/use.rs
new file mode 100644
index 0000000..0dca5c4
--- /dev/null
+++ b/tests/use.rs
@@ -0,0 +1,333 @@
+#![cfg(test)]
+
+use std::io::Write;
+
+#[macro_use]
+mod macros;
+
+error!(
+    after_style,
+    "a {}
+    @use \"foo\";
+    ",
+    "Error: @use rules must be written before any other rules."
+);
+error!(
+    interpolation_in_as_identifier,
+    "@use \"sass:math\" as m#{a}th;", "Error: expected \";\"."
+);
+error!(
+    use_as_quoted_string,
+    "@use \"sass:math\" as \"math\";", "Error: Expected identifier."
+);
+error!(
+    use_as_missing_s,
+    "@use \"sass:math\" a math;", "Error: expected \";\"."
+);
+error!(
+    unknown_module_get_variable,
+    "a { color: foo.$bar; }", "Error: There is no module with the namespace \"foo\"."
+);
+error!(
+    unknown_module_get_function,
+    "a { color: foo.bar(); }", "Error: There is no module with the namespace \"foo\"."
+);
+error!(
+    unknown_function,
+    "@use \"sass:math\";\na { color: math.bar(); }", "Error: Undefined function."
+);
+test!(
+    use_as,
+    "@use \"sass:math\" as foo;
+    a {
+        color: foo.clamp(0, 1, 2);
+    }",
+    "a {\n  color: 1;\n}\n"
+);
+test!(
+    use_as_uppercase,
+    "@use \"sass:math\" AS foo;
+    a {
+        color: foo.clamp(0, 1, 2);
+    }",
+    "a {\n  color: 1;\n}\n"
+);
+test!(
+    use_as_universal,
+    "@use \"sass:math\" as *;
+    a {
+        color: cos(2);
+    }",
+    "a {\n  color: -0.4161468365;\n}\n"
+);
+
+#[test]
+fn use_user_defined_same_directory() {
+    let input = "@use \"use_user_defined_same_directory\";\na {\n color: use_user_defined_same_directory.$a;\n}";
+    tempfile!(
+        "use_user_defined_same_directory.scss",
+        "$a: red; a { color: $a; }"
+    );
+    assert_eq!(
+        "a {\n  color: red;\n}\n\na {\n  color: red;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default()).expect(input)
+    );
+}
+
+#[test]
+fn private_variable_begins_with_underscore() {
+    let input = "@use \"private_variable_begins_with_underscore\" as module;\na {\n color: module.$_foo;\n}";
+    tempfile!(
+        "private_variable_begins_with_underscore.scss",
+        "$_foo: red; a { color: $_foo; }"
+    );
+
+    assert_err!(
+        "Error: Private members can't be accessed from outside their modules.",
+        input
+    );
+}
+
+#[test]
+fn private_variable_begins_with_hyphen() {
+    let input =
+        "@use \"private_variable_begins_with_hyphen\" as module;\na {\n color: module.$-foo;\n}";
+    tempfile!(
+        "private_variable_begins_with_hyphen.scss",
+        "$-foo: red; a { color: $-foo; }"
+    );
+
+    assert_err!(
+        "Error: Private members can't be accessed from outside their modules.",
+        input
+    );
+}
+
+#[test]
+fn private_function() {
+    let input = "@use \"private_function\" as module;\na {\n color: module._foo(green);\n}";
+    tempfile!(
+        "private_function.scss",
+        "@function _foo($a) { @return $a; } a { color: _foo(red); }"
+    );
+
+    assert_err!(
+        "Error: Private members can't be accessed from outside their modules.",
+        input
+    );
+}
+
+#[test]
+fn global_variable_exists_private() {
+    let input = r#"
+        @use "global_variable_exists_private" as module;
+        a {
+            color: global-variable-exists($name: foo, $module: module);
+            color: global-variable-exists($name: _foo, $module: module);
+        }"#;
+    tempfile!(
+        "global_variable_exists_private.scss",
+        "$foo: red;\n$_foo: red;\n"
+    );
+
+    assert_eq!(
+        "a {\n  color: true;\n  color: false;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default()).expect(input)
+    );
+}
+
+#[test]
+fn use_user_defined_as() {
+    let input = "@use \"use_user_defined_as\" as module;\na {\n color: module.$a;\n}";
+    tempfile!("use_user_defined_as.scss", "$a: red; a { color: $a; }");
+    assert_eq!(
+        "a {\n  color: red;\n}\n\na {\n  color: red;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default()).expect(input)
+    );
+}
+
+#[test]
+fn use_user_defined_function() {
+    let input = "@use \"use_user_defined_function\" as module;\na {\n color: module.foo(red);\n}";
+    tempfile!(
+        "use_user_defined_function.scss",
+        "@function foo($a) { @return $a; }"
+    );
+    assert_eq!(
+        "a {\n  color: red;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default()).expect(input)
+    );
+}
+
+#[test]
+fn use_idempotent_no_alias() {
+    let input = "@use \"use_idempotent_no_alias\";\n@use \"use_idempotent_no_alias\";\n";
+    tempfile!("use_idempotent_no_alias.scss", "");
+
+    assert_err!(
+        "Error: There's already a module with namespace \"use-idempotent-no-alias\".",
+        input
+    );
+}
+
+#[test]
+fn use_idempotent_with_alias() {
+    let input = "@use \"use_idempotent_with_alias__a\" as foo;\n@use \"use_idempotent_with_alias__b\" as foo;\n";
+    tempfile!("use_idempotent_with_alias__a.scss", "");
+    tempfile!("use_idempotent_with_alias__b.scss", "");
+
+    assert_err!(
+        "Error: There's already a module with namespace \"foo\".",
+        input
+    );
+}
+
+#[test]
+fn use_idempotent_builtin() {
+    let input = "@use \"sass:math\";\n@use \"sass:math\";\n";
+
+    assert_err!(
+        "Error: There's already a module with namespace \"math\".",
+        input
+    );
+}
+
+#[test]
+fn use_with_simple() {
+    let input = "@use \"use_with_simple\" with ($a: red);\na {\n color: use_with_simple.$a;\n}";
+    tempfile!("use_with_simple.scss", "$a: green !default;");
+    assert_eq!(
+        "a {\n  color: red;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default()).expect(input)
+    );
+}
+
+#[test]
+fn use_as_with() {
+    let input = "@use \"use_as_with\" as module with ($a: red);\na {\n color: module.$a;\n}";
+    tempfile!("use_as_with.scss", "$a: green !default;");
+    assert_eq!(
+        "a {\n  color: red;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default()).expect(input)
+    );
+}
+
+#[test]
+fn use_whitespace_and_comments() {
+    let input = "@use  /**/  \"use_whitespace_and_comments\"  /**/  as  /**/  foo  /**/  with  /**/  (  /**/  $a  /**/  :  /**/  red  /**/  )  /**/  ;";
+    tempfile!(
+        "use_whitespace_and_comments.scss",
+        "$a: green !default; a { color: $a }"
+    );
+    assert_eq!(
+        "a {\n  color: red;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default()).expect(input)
+    );
+}
+
+#[test]
+fn use_with_builtin_module() {
+    let input = "@use \"sass:math\" with ($e: 2.7);";
+
+    assert_err!("Error: Built-in modules can't be configured.", input);
+}
+
+#[test]
+fn use_with_variable_never_used() {
+    let input = "@use \"use_with_variable_never_used\" with ($a: red);";
+    tempfile!("use_with_variable_never_used.scss", "");
+
+    assert_err!(
+        "Error: This variable was not declared with !default in the @used module.",
+        input
+    );
+}
+
+#[test]
+fn use_with_same_variable_multiple_times() {
+    let input = "@use \"use_with_same_variable_multiple_times\" as foo with ($a: b, $a: c);";
+    tempfile!("use_with_same_variable_multiple_times.scss", "");
+
+    assert_err!(
+        "Error: The same variable may only be configured once.",
+        input
+    );
+}
+
+#[test]
+fn use_variable_redeclaration_var_dne() {
+    let input = "@use \"use_variable_redeclaration_var_dne\" as mod;\nmod.$a: red;";
+    tempfile!("use_variable_redeclaration_var_dne.scss", "");
+
+    assert_err!("Error: Undefined variable.", input);
+}
+
+#[test]
+fn use_variable_redeclaration_global() {
+    let input = "@use \"use_variable_redeclaration_global\" as mod;\nmod.$a: red !global;";
+    tempfile!("use_variable_redeclaration_global.scss", "$a: green;");
+
+    assert_err!(
+        "Error: !global isn't allowed for variables in other modules.",
+        input
+    );
+}
+
+#[test]
+fn use_variable_redeclaration_simple() {
+    let input =
+        "@use \"use_variable_redeclaration_simple\" as mod;\nmod.$a: red; a { color: mod.$a; }";
+    tempfile!("use_variable_redeclaration_simple.scss", "$a: green;");
+
+    assert_eq!(
+        "a {\n  color: red;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default()).expect(input)
+    );
+}
+
+#[test]
+fn use_variable_redeclaration_default() {
+    let input = "@use \"use_variable_redeclaration_default\" as mod;\nmod.$a: 1 % red !default; a { color: mod.$a; }";
+    tempfile!("use_variable_redeclaration_default.scss", "$a: green;");
+
+    assert_eq!(
+        "a {\n  color: green;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default()).expect(input)
+    );
+}
+
+#[test]
+fn use_variable_redeclaration_private() {
+    let input = "@use \"use_variable_redeclaration_private\" as mod;\nmod.$-a: red;";
+    tempfile!("use_variable_redeclaration_private.scss", "$a: green;");
+
+    assert_err!(
+        "Error: Private members can't be accessed from outside their modules.",
+        input
+    );
+}
+
+#[test]
+fn use_variable_redeclaration_builtin() {
+    let input = "@use \"sass:math\";\nmath.$e: red;";
+
+    assert_err!("Error: Cannot modify built-in variable.", input);
+}
+
+#[test]
+fn use_variable_declaration_between_use() {
+    let input = r#"
+        $a: red;
+        $b: green;
+        @use "sass:math";
+        $b: red;
+        @use "sass:meta";
+        a {
+            color: $a $b;
+        }"#;
+
+    assert_eq!(
+        "a {\n  color: red red;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default()).expect(input)
+    );
+}