diff --git a/src/builtin/color/hsl.rs b/src/builtin/color/hsl.rs
index 98f4c09..c7accf3 100644
--- a/src/builtin/color/hsl.rs
+++ b/src/builtin/color/hsl.rs
@@ -22,7 +22,8 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
         let alpha = match arg!(args, 3, "alpha"=Value::Dimension(Number::from(1), Unit::None)) {
             Value::Dimension(n, Unit::None) => n,
             Value::Dimension(n, Unit::Percent) => n / Number::from(100),
-            _ => todo!("non-number alpha given to builtin function `rgb()`")
+            v @ Value::Dimension(..) => return Err(format!("$alpha: Expected {} to have no units or \"%\".", v).into()),
+            v => return Err(format!("$alpha: {} is not a number.", v).into()),
         };
         Ok(Value::Color(Color::from_hsla(hue, saturation, luminance, alpha)))
     });
@@ -42,7 +43,8 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
         let alpha = match arg!(args, 3, "alpha").eval() {
             Value::Dimension(n, Unit::None) => n,
             Value::Dimension(n, Unit::Percent) => n / Number::from(100),
-            _ => todo!("$alpha: Expected ____ to have no units or \"%\"."),
+            v @ Value::Dimension(..) => return Err(format!("$alpha: Expected {} to have no units or \"%\".", v).into()),
+            v => return Err(format!("$alpha: {} is not a number.", v).into()),
         };
         Ok(Value::Color(Color::from_hsla(hue, saturation, luminance, alpha)))
     });
@@ -70,9 +72,7 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             v => return Err(format!("$color: {} is not a color.", v).into()),
         };
         let degrees = match arg!(args, 1, "degrees").eval() {
-            Value::Dimension(n, Unit::None)
-            | Value::Dimension(n, Unit::Percent)
-            | Value::Dimension(n, Unit::Deg) => n,
+            Value::Dimension(n, _) => n,
             _ => todo!("expected either unitless or % number for degrees"),
         };
         Ok(Value::Color(color.adjust_hue(degrees)))
@@ -83,9 +83,8 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             v => return Err(format!("$color: {} is not a color.", v).into()),
         };
         let amount = match arg!(args, 1, "amount").eval() {
-            Value::Dimension(n, Unit::None) => n,
-            Value::Dimension(n, Unit::Percent) => n / Number::from(100),
-            _ => todo!("expected either unitless or % number for amount"),
+            Value::Dimension(n, _) => n / Number::from(100),
+            v => return Err(format!("$amount: {} is not a number.", v).into())
         };
         Ok(Value::Color(color.lighten(amount)))
     });
@@ -95,9 +94,8 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             v => return Err(format!("$color: {} is not a color.", v).into()),
         };
         let amount = match arg!(args, 1, "amount").eval() {
-            Value::Dimension(n, Unit::None) => n,
-            Value::Dimension(n, Unit::Percent) => n / Number::from(100),
-            _ => todo!("expected either unitless or % number for amount"),
+            Value::Dimension(n, _) => n / Number::from(100),
+            v => return Err(format!("$amount: {} is not a number.", v).into())
         };
         Ok(Value::Color(color.darken(amount)))
     });
@@ -119,7 +117,7 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
         };
         let amount = match arg!(args, 1, "amount").eval() {
             Value::Dimension(n, _) => n / Number::from(100),
-           v => return Err(format!("$amount: {} is not a number.", v).into())
+            v => return Err(format!("$amount: {} is not a number.", v).into())
         };
         Ok(Value::Color(color.desaturate(amount)))
     });
diff --git a/src/builtin/color/rgb.rs b/src/builtin/color/rgb.rs
index 7b14260..8d5d306 100644
--- a/src/builtin/color/rgb.rs
+++ b/src/builtin/color/rgb.rs
@@ -122,7 +122,8 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             let alpha = match arg!(args, 1, "alpha").eval() {
                 Value::Dimension(n, Unit::None) => n,
                 Value::Dimension(n, Unit::Percent) => n / Number::from(100),
-                _ => todo!("expected either unitless or % number for alpha"),
+                v @ Value::Dimension(..) => return Err(format!("$alpha: Expected {} to have no units or \"%\".", v).into()),
+                v => return Err(format!("$alpha: {} is not a number.", v).into()),
             };
             Ok(Value::Color(color.with_alpha(alpha)))
         } else {
@@ -147,7 +148,8 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             let alpha = match arg!(args, 3, "alpha"=Value::Dimension(Number::from(1), Unit::None)).eval() {
                 Value::Dimension(n, Unit::None) => n,
                 Value::Dimension(n, Unit::Percent) => n / Number::from(100),
-                _ => todo!("expected either unitless or % number for alpha")
+                v @ Value::Dimension(..) => return Err(format!("$alpha: Expected {} to have no units or \"%\".", v).into()),
+                v => return Err(format!("$alpha: {} is not a number.", v).into()),
             };
             Ok(Value::Color(Color::from_rgba(red, green, blue, alpha)))
         }
diff --git a/src/lib.rs b/src/lib.rs
index 1aa79ff..f8d9abb 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -59,8 +59,8 @@ use std::path::Path;
 use crate::atrule::{AtRule, AtRuleKind};
 use crate::common::{Keyword, Op, Pos, Scope, Symbol, Whitespace};
 use crate::css::Css;
-pub use crate::error::SassResult;
 use crate::error::SassError;
+pub use crate::error::SassResult;
 use crate::format::PrettyPrinter;
 use crate::function::Function;
 use crate::imports::import;
diff --git a/src/main.rs b/src/main.rs
index ef6cf36..1e03cde 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -69,7 +69,9 @@ fn main() {
                     eprintln!("{}", b);
                     std::process::exit(1)
                 }
-            }.print_as_css(&mut stdout).unwrap();
+            }
+            .print_as_css(&mut stdout)
+            .unwrap();
         }
     }
 }
diff --git a/src/units.rs b/src/units.rs
index 72d34fd..4314ccb 100644
--- a/src/units.rs
+++ b/src/units.rs
@@ -117,13 +117,9 @@ impl Unit {
 
     pub fn kind(&self) -> UnitKind {
         match self {
-            Unit::Px
-            | Unit::Mm
-            | Unit::In
-            | Unit::Cm
-            | Unit::Q
-            | Unit::Pt
-            | Unit::Pc => UnitKind::Absolute,
+            Unit::Px | Unit::Mm | Unit::In | Unit::Cm | Unit::Q | Unit::Pt | Unit::Pc => {
+                UnitKind::Absolute
+            }
             Unit::Em
             | Unit::Rem
             | Unit::Lh
@@ -132,24 +128,13 @@ impl Unit {
             | Unit::Cap
             | Unit::Ic
             | Unit::Rlh => UnitKind::FontRelative,
-            Unit::Vw
-            | Unit::Vh
-            | Unit::Vmin
-            | Unit::Vmax
-            | Unit::Vi
-            | Unit::Vb => UnitKind::ViewportRelative,
-            Unit::Deg
-            | Unit::Grad
-            | Unit::Rad
-            | Unit::Turn => UnitKind::Angle,
-            Unit::S
-            | Unit::Ms => UnitKind::Time,
-            Unit::Hz
-            | Unit::Khz => UnitKind::Frequency,
-            Unit::Dpi
-            | Unit::Dpcm
-            | Unit::Dppx
-            | Unit::X => UnitKind::Resolution,
+            Unit::Vw | Unit::Vh | Unit::Vmin | Unit::Vmax | Unit::Vi | Unit::Vb => {
+                UnitKind::ViewportRelative
+            }
+            Unit::Deg | Unit::Grad | Unit::Rad | Unit::Turn => UnitKind::Angle,
+            Unit::S | Unit::Ms => UnitKind::Time,
+            Unit::Hz | Unit::Khz => UnitKind::Frequency,
+            Unit::Dpi | Unit::Dpcm | Unit::Dppx | Unit::X => UnitKind::Resolution,
             Unit::None => UnitKind::None,
             _ => UnitKind::Other,
         }
diff --git a/src/value/number.rs b/src/value/number.rs
index c9cdf25..410bfcf 100644
--- a/src/value/number.rs
+++ b/src/value/number.rs
@@ -1,6 +1,8 @@
 use std::convert::From;
 use std::fmt::{self, Display, Write};
-use std::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Rem, RemAssign, Sub, SubAssign, Neg};
+use std::ops::{
+    Add, AddAssign, Div, DivAssign, Mul, MulAssign, Neg, Rem, RemAssign, Sub, SubAssign,
+};
 
 use num_bigint::BigInt;
 use num_rational::BigRational;
@@ -201,8 +203,6 @@ impl Neg for Number {
     type Output = Self;
 
     fn neg(self) -> Self {
-        Number {
-            val: -self.val,
-        }
+        Number { val: -self.val }
     }
-}
\ No newline at end of file
+}
diff --git a/src/value/parse.rs b/src/value/parse.rs
index d33b0ee..9477d13 100644
--- a/src/value/parse.rs
+++ b/src/value/parse.rs
@@ -298,9 +298,7 @@ impl Value {
                 }
                 Ok(Value::Ident(s, QuoteKind::Single))
             }
-            TokenKind::Variable(ref v) => {
-                Ok(scope.get_var(v).expect("expected variable").clone())
-            }
+            TokenKind::Variable(ref v) => Ok(scope.get_var(v).expect("expected variable").clone()),
             TokenKind::Interpolation => {
                 let mut s = parse_interpolation(toks, scope)
                     .iter()