diff --git a/src/args.rs b/src/args.rs
index 482634d..b24449a 100644
--- a/src/args.rs
+++ b/src/args.rs
@@ -60,7 +60,7 @@ impl CallArgs {
         self.0.get(&CallArg::Positional(val))
     }
 
-    pub fn get_variadic(self) -> SassResult<Value> {
+    pub fn get_variadic(self) -> SassResult<Vec<Value>> {
         let mut vals = Vec::new();
         let mut args = self
             .0
@@ -71,7 +71,7 @@ impl CallArgs {
         for arg in args {
             vals.push(arg.1);
         }
-        Ok(Value::ArgList(vals))
+        Ok(vals)
     }
 
     pub fn len(&self) -> usize {
diff --git a/src/atrule/function.rs b/src/atrule/function.rs
index 82513e2..ba2f639 100644
--- a/src/atrule/function.rs
+++ b/src/atrule/function.rs
@@ -46,7 +46,8 @@ impl Function {
     pub fn args(mut self, mut args: CallArgs) -> SassResult<Function> {
         for (idx, arg) in self.args.0.iter().enumerate() {
             if arg.is_variadic {
-                self.scope.insert_var(&arg.name, args.get_variadic()?)?;
+                self.scope
+                    .insert_var(&arg.name, Value::ArgList(args.get_variadic()?))?;
                 break;
             }
             let val = match args.remove_positional(idx) {
diff --git a/src/atrule/mixin.rs b/src/atrule/mixin.rs
index 590134e..a80cc3b 100644
--- a/src/atrule/mixin.rs
+++ b/src/atrule/mixin.rs
@@ -11,6 +11,7 @@ use crate::selector::Selector;
 use crate::utils::{
     devour_whitespace, devour_whitespace_or_comment, eat_ident, read_until_closing_curly_brace,
 };
+use crate::value::Value;
 use crate::{eat_expr, Expr, RuleSet, Stmt, Token};
 
 #[derive(Debug, Clone)]
@@ -62,7 +63,8 @@ impl Mixin {
     pub fn args(mut self, mut args: CallArgs) -> SassResult<Mixin> {
         for (idx, arg) in self.args.0.iter().enumerate() {
             if arg.is_variadic {
-                self.scope.insert_var(&arg.name, args.get_variadic()?)?;
+                self.scope
+                    .insert_var(&arg.name, Value::ArgList(args.get_variadic()?))?;
                 break;
             }
             let val = match args.remove_positional(idx) {
diff --git a/src/builtin/color/hsl.rs b/src/builtin/color/hsl.rs
index f916c6c..b5382c2 100644
--- a/src/builtin/color/hsl.rs
+++ b/src/builtin/color/hsl.rs
@@ -11,7 +11,7 @@ use crate::value::{Number, Value};
 pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     f.insert(
         "hsl".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             if args.is_empty() {
                 return Err("Missing argument $channels.".into());
             }
@@ -89,7 +89,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "hsla".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             if args.is_empty() {
                 return Err("Missing argument $channels.".into());
             }
@@ -167,7 +167,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "hue".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "color") {
                 Value::Color(c) => Ok(Value::Dimension(c.hue(), Unit::Deg)),
@@ -177,7 +177,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "saturation".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "color") {
                 Value::Color(c) => Ok(Value::Dimension(c.saturation(), Unit::Percent)),
@@ -187,7 +187,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "lightness".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "color") {
                 Value::Color(c) => Ok(Value::Dimension(c.lightness(), Unit::Percent)),
@@ -197,7 +197,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "adjust-hue".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 2);
             let color = match arg!(args, 0, "color") {
                 Value::Color(c) => c,
@@ -212,7 +212,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "lighten".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 2);
             let color = match arg!(args, 0, "color") {
                 Value::Color(c) => c,
@@ -227,7 +227,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "darken".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 2);
             let color = match arg!(args, 0, "color") {
                 Value::Color(c) => c,
@@ -242,7 +242,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "saturate".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 2);
             if args.len() == 1 {
                 return Ok(Value::Ident(
@@ -270,7 +270,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "desaturate".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 2);
             let color = match arg!(args, 0, "color") {
                 Value::Color(c) => c,
@@ -285,7 +285,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "grayscale".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             let color = match arg!(args, 0, "color") {
                 Value::Color(c) => c,
@@ -302,7 +302,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "complement".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             let color = match arg!(args, 0, "color") {
                 Value::Color(c) => c,
@@ -313,7 +313,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "invert".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 2);
             let weight = match arg!(
                 args,
diff --git a/src/builtin/color/opacity.rs b/src/builtin/color/opacity.rs
index 5d994ab..87fc49b 100644
--- a/src/builtin/color/opacity.rs
+++ b/src/builtin/color/opacity.rs
@@ -9,7 +9,7 @@ use crate::value::Value;
 pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     f.insert(
         "alpha".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "color") {
                 Value::Color(c) => Ok(Value::Dimension(c.alpha(), Unit::None)),
@@ -19,7 +19,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "opacity".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "color") {
                 Value::Color(c) => Ok(Value::Dimension(c.alpha(), Unit::None)),
@@ -33,7 +33,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "opacify".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 2);
             let color = match arg!(args, 0, "color") {
                 Value::Color(c) => c,
@@ -48,7 +48,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "fade-in".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 2);
             let color = match arg!(args, 0, "color") {
                 Value::Color(c) => c,
@@ -63,7 +63,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "transparentize".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 2);
             let color = match arg!(args, 0, "color") {
                 Value::Color(c) => c,
@@ -78,7 +78,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "fade-out".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 2);
             let color = match arg!(args, 0, "color") {
                 Value::Color(c) => c,
diff --git a/src/builtin/color/other.rs b/src/builtin/color/other.rs
index 21c3c1c..45f376d 100644
--- a/src/builtin/color/other.rs
+++ b/src/builtin/color/other.rs
@@ -31,7 +31,7 @@ macro_rules! opt_hsl {
 }
 
 pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
-    f.insert("change-color".to_owned(), Box::new(|args, _| {
+    f.insert("change-color".to_owned(), Box::new(|mut args, _| {
         if args.get_positional(1).is_some() {
             return Err("Only one positional argument is allowed. All other arguments must be passed by name.".into());
         }
@@ -73,7 +73,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     }));
     f.insert(
         "adjust-color".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             let color = match arg!(args, 0, "color") {
                 Value::Color(c) => c,
                 v => return Err(format!("$color: {} is not a color.", v).into()),
@@ -123,7 +123,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "scale-color".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             let color = match arg!(args, 0, "color") {
                 Value::Color(c) => c,
                 v => return Err(format!("$color: {} is not a color.", v).into()),
@@ -209,7 +209,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "ie-hex-str".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             let color = match arg!(args, 0, "color") {
                 Value::Color(c) => c,
diff --git a/src/builtin/color/rgb.rs b/src/builtin/color/rgb.rs
index 46e1c38..d89dbdd 100644
--- a/src/builtin/color/rgb.rs
+++ b/src/builtin/color/rgb.rs
@@ -10,7 +10,7 @@ use crate::value::{Number, Value};
 pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     f.insert(
         "rgb".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             if args.is_empty() {
                 return Err("Missing argument $channels.".into());
             }
@@ -132,7 +132,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "rgba".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             if args.is_empty() {
                 return Err("Missing argument $channels.".into());
             }
@@ -254,7 +254,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "red".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "color") {
                 Value::Color(c) => Ok(Value::Dimension(c.red(), Unit::None)),
@@ -264,7 +264,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "green".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "color") {
                 Value::Color(c) => Ok(Value::Dimension(c.green(), Unit::None)),
@@ -274,7 +274,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "blue".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "color") {
                 Value::Color(c) => Ok(Value::Dimension(c.blue(), Unit::None)),
@@ -284,7 +284,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "mix".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 3);
             let color1 = match arg!(args, 0, "color1") {
                 Value::Color(c) => c,
diff --git a/src/builtin/list.rs b/src/builtin/list.rs
index 1019e31..635903b 100644
--- a/src/builtin/list.rs
+++ b/src/builtin/list.rs
@@ -10,7 +10,7 @@ use crate::value::{Number, Value};
 pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     f.insert(
         "length".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             let len = match arg!(args, 0, "list") {
                 Value::List(v, ..) => Number::from(v.len()),
@@ -22,7 +22,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "nth".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 2);
             let list = match arg!(args, 0, "list") {
                 Value::List(v, ..) => v,
@@ -59,7 +59,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "list-separator".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             Ok(Value::Ident(
                 match arg!(args, 0, "list") {
@@ -73,7 +73,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "set-nth".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 3);
             let (mut list, sep) = match arg!(args, 0, "list") {
                 Value::List(v, sep, ..) => (v, sep),
@@ -113,7 +113,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "append".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 3);
             let (mut list, sep) = match arg!(args, 0, "list") {
                 Value::List(v, sep, ..) => (v, sep),
@@ -143,7 +143,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "join".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 3);
             let (mut list1, sep1, brackets) = match arg!(args, 0, "list") {
                 Value::List(v, sep, brackets) => (v, sep, brackets),
@@ -182,7 +182,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "is-bracketed".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             Ok(Value::bool(match arg!(args, 0, "list") {
                 Value::List(.., brackets) => match brackets {
diff --git a/src/builtin/map.rs b/src/builtin/map.rs
index b155436..474b73a 100644
--- a/src/builtin/map.rs
+++ b/src/builtin/map.rs
@@ -7,7 +7,7 @@ use crate::value::{SassMap, Value};
 pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     f.insert(
         "map-get".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 2);
             let key = arg!(args, 1, "key");
             let map = match arg!(args, 0, "map") {
@@ -20,7 +20,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "map-has-key".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 2);
             let key = arg!(args, 1, "key");
             let map = match arg!(args, 0, "map") {
@@ -33,7 +33,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "map-keys".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             let map = match arg!(args, 0, "map") {
                 Value::Map(m) => m,
@@ -49,7 +49,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "map-values".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             let map = match arg!(args, 0, "map") {
                 Value::Map(m) => m,
@@ -65,7 +65,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "map-merge".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 2);
             let mut map1 = match arg!(args, 0, "map1") {
                 Value::Map(m) => m,
diff --git a/src/builtin/math.rs b/src/builtin/math.rs
index 8eec941..24612a0 100644
--- a/src/builtin/math.rs
+++ b/src/builtin/math.rs
@@ -7,7 +7,7 @@ use crate::value::{Number, Value};
 pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     f.insert(
         "percentage".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             let num = match arg!(args, 0, "number") {
                 Value::Dimension(n, Unit::None) => n * Number::from(100),
@@ -21,7 +21,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "round".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "number") {
                 Value::Dimension(n, u) => Ok(Value::Dimension(n.round(), u)),
@@ -31,7 +31,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "ceil".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "number") {
                 Value::Dimension(n, u) => Ok(Value::Dimension(n.ceil(), u)),
@@ -41,7 +41,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "floor".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "number") {
                 Value::Dimension(n, u) => Ok(Value::Dimension(n.floor(), u)),
@@ -51,7 +51,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "abs".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "number") {
                 Value::Dimension(n, u) => Ok(Value::Dimension(n.abs(), u)),
@@ -61,7 +61,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "comparable".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 2);
             let unit1 = match arg!(args, 0, "number1") {
                 Value::Dimension(_, u) => u,
diff --git a/src/builtin/meta.rs b/src/builtin/meta.rs
index 1e9033c..8adc3a3 100644
--- a/src/builtin/meta.rs
+++ b/src/builtin/meta.rs
@@ -9,7 +9,7 @@ use crate::value::Value;
 pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     f.insert(
         "if".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 3);
             if arg!(args, 0, "condition").is_true()? {
                 Ok(arg!(args, 1, "if-true"))
@@ -20,7 +20,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "feature-exists".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "feature") {
                 Value::Ident(s, _) => match s.as_str() {
@@ -47,7 +47,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "unit".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             let unit = match arg!(args, 0, "number") {
                 Value::Dimension(_, u) => u.to_string(),
@@ -58,7 +58,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "type-of".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             let value = arg!(args, 0, "value");
             Ok(Value::Ident(value.kind()?.to_owned(), QuoteKind::None))
@@ -66,7 +66,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "unitless".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "number") {
                 Value::Dimension(_, Unit::None) => Ok(Value::True),
@@ -77,7 +77,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "inspect".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             Ok(Value::Ident(
                 match arg!(args, 0, "value") {
@@ -90,7 +90,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "variable-exists".to_owned(),
-        Box::new(|args, scope| {
+        Box::new(|mut args, scope| {
             max_args!(args, 1);
             match arg!(args, 0, "name") {
                 Value::Ident(s, _) => Ok(Value::bool(scope.var_exists(&s))),
@@ -100,7 +100,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "global-variable-exists".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "name") {
                 Value::Ident(s, _) => Ok(Value::bool(global_var_exists(&s))),
@@ -110,7 +110,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "mixin-exists".to_owned(),
-        Box::new(|args, scope| {
+        Box::new(|mut args, scope| {
             max_args!(args, 2);
             match arg!(args, 0, "name") {
                 Value::Ident(s, _) => Ok(Value::bool(scope.mixin_exists(&s))),
@@ -120,7 +120,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "function-exists".to_owned(),
-        Box::new(|args, scope| {
+        Box::new(|mut args, scope| {
             max_args!(args, 2);
             match arg!(args, 0, "name") {
                 Value::Ident(s, _) => Ok(Value::bool(
diff --git a/src/builtin/mod.rs b/src/builtin/mod.rs
index 570c16f..3c0548d 100644
--- a/src/builtin/mod.rs
+++ b/src/builtin/mod.rs
@@ -17,7 +17,7 @@ mod meta;
 mod selector;
 mod string;
 
-pub(crate) type Builtin = Box<dyn Fn(&mut CallArgs, &Scope) -> SassResult<Value> + Send + Sync>;
+pub(crate) type Builtin = Box<dyn Fn(CallArgs, &Scope) -> SassResult<Value> + Send + Sync>;
 
 pub(crate) static GLOBAL_FUNCTIONS: Lazy<HashMap<String, Builtin>> = Lazy::new(|| {
     let mut m = HashMap::new();
diff --git a/src/builtin/string.rs b/src/builtin/string.rs
index c0da316..b488b31 100644
--- a/src/builtin/string.rs
+++ b/src/builtin/string.rs
@@ -11,7 +11,7 @@ use crate::value::{Number, Value};
 pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     f.insert(
         "to-upper-case".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "string") {
                 Value::Ident(i, q) => Ok(Value::Ident(i.to_ascii_uppercase(), q)),
@@ -21,7 +21,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "to-lower-case".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "string") {
                 Value::Ident(i, q) => Ok(Value::Ident(i.to_ascii_lowercase(), q)),
@@ -31,7 +31,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "str-length".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "string") {
                 Value::Ident(i, _) => Ok(Value::Dimension(
@@ -44,7 +44,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "quote".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "string") {
                 Value::Ident(i, _) => Ok(Value::Ident(i, QuoteKind::Double)),
@@ -54,7 +54,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "unquote".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "string") {
                 i @ Value::Ident(..) => Ok(i.unquote()),
@@ -64,7 +64,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "str-slice".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 3);
             let (string, quotes) = match arg!(args, 0, "string") {
                 Value::Ident(s, q) => (s, q),
@@ -127,7 +127,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "str-index".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 2);
             let s1 = match arg!(args, 0, "string") {
                 Value::Ident(i, _) => i,
@@ -147,7 +147,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     );
     f.insert(
         "str-insert".to_owned(),
-        Box::new(|args, _| {
+        Box::new(|mut args, _| {
             max_args!(args, 3);
             let (s1, quotes) = match arg!(args, 0, "string") {
                 Value::Ident(i, q) => (i, q.normalize()),
diff --git a/src/value/parse.rs b/src/value/parse.rs
index bb754a1..c604dc8 100644
--- a/src/value/parse.rs
+++ b/src/value/parse.rs
@@ -367,7 +367,7 @@ impl Value {
                     Err(_) => match GLOBAL_FUNCTIONS.get(&s) {
                         Some(f) => {
                             return Ok(IntermediateValue::Value(f(
-                                &mut eat_call_args(toks, scope, super_selector)?,
+                                eat_call_args(toks, scope, super_selector)?,
                                 scope,
                             )?))
                         }