diff --git a/src/builtin/color/opacity.rs b/src/builtin/color/opacity.rs
index 15596cb..0abc04f 100644
--- a/src/builtin/color/opacity.rs
+++ b/src/builtin/color/opacity.rs
@@ -1,162 +1,164 @@
 use super::GlobalFunctionMap;
 
 use super::Builtin;
+use crate::args::CallArgs;
 use crate::common::QuoteKind;
+use crate::error::SassResult;
+use crate::scope::Scope;
+use crate::selector::Selector;
 use crate::unit::Unit;
 use crate::value::Number;
 use crate::value::Value;
 
 pub(crate) fn register(f: &mut GlobalFunctionMap) {
-    f.insert(
-        "alpha",
-        Builtin::new(|mut args, scope, super_selector| {
-            args.max_args(1)?;
-            match arg!(args, scope, super_selector, 0, "color") {
-                Value::Color(c) => Ok(Value::Dimension(c.alpha(), Unit::None)),
-                v => Err((
+    fn alpha(mut args: CallArgs, scope: &Scope, super_selector: &Selector) -> SassResult<Value> {
+        args.max_args(1)?;
+        match arg!(args, scope, super_selector, 0, "color") {
+            Value::Color(c) => Ok(Value::Dimension(c.alpha(), Unit::None)),
+            v => Err((
+                format!("$color: {} is not a color.", v.to_css_string(args.span())?),
+                args.span(),
+            )
+                .into()),
+        }
+    }
+
+    fn opacity(mut args: CallArgs, scope: &Scope, super_selector: &Selector) -> SassResult<Value> {
+        args.max_args(1)?;
+        match arg!(args, scope, super_selector, 0, "color") {
+            Value::Color(c) => Ok(Value::Dimension(c.alpha(), Unit::None)),
+            Value::Dimension(num, unit) => Ok(Value::Ident(
+                format!("opacity({}{})", num, unit),
+                QuoteKind::None,
+            )),
+            v => Err((
+                format!("$color: {} is not a color.", v.to_css_string(args.span())?),
+                args.span(),
+            )
+                .into()),
+        }
+    }
+
+    fn opacify(mut args: CallArgs, scope: &Scope, super_selector: &Selector) -> SassResult<Value> {
+        args.max_args(2)?;
+        let color = match arg!(args, scope, super_selector, 0, "color") {
+            Value::Color(c) => c,
+            v => {
+                return Err((
                     format!("$color: {} is not a color.", v.to_css_string(args.span())?),
                     args.span(),
                 )
-                    .into()),
+                    .into())
             }
-        }),
-    );
-    f.insert(
-        "opacity",
-        Builtin::new(|mut args, scope, super_selector| {
-            args.max_args(1)?;
-            match arg!(args, scope, super_selector, 0, "color") {
-                Value::Color(c) => Ok(Value::Dimension(c.alpha(), Unit::None)),
-                Value::Dimension(num, unit) => Ok(Value::Ident(
-                    format!("opacity({}{})", num, unit),
-                    QuoteKind::None,
-                )),
-                v => Err((
+        };
+        let amount = match arg!(args, scope, super_selector, 1, "amount") {
+            Value::Dimension(n, u) => bound!(args, "amount", n, u, 0, 1),
+            v => {
+                return Err((
+                    format!(
+                        "$amount: {} is not a number.",
+                        v.to_css_string(args.span())?
+                    ),
+                    args.span(),
+                )
+                    .into())
+            }
+        };
+        Ok(Value::Color(Box::new(color.fade_in(amount))))
+    }
+
+    fn fade_in(mut args: CallArgs, scope: &Scope, super_selector: &Selector) -> SassResult<Value> {
+        args.max_args(2)?;
+        let color = match arg!(args, scope, super_selector, 0, "color") {
+            Value::Color(c) => c,
+            v => {
+                return Err((
                     format!("$color: {} is not a color.", v.to_css_string(args.span())?),
                     args.span(),
                 )
-                    .into()),
+                    .into())
             }
-        }),
-    );
-    f.insert(
-        "opacify",
-        Builtin::new(|mut args, scope, super_selector| {
-            args.max_args(2)?;
-            let color = match arg!(args, scope, super_selector, 0, "color") {
-                Value::Color(c) => c,
-                v => {
-                    return Err((
-                        format!("$color: {} is not a color.", v.to_css_string(args.span())?),
-                        args.span(),
-                    )
-                        .into())
-                }
-            };
-            let amount = match arg!(args, scope, super_selector, 1, "amount") {
-                Value::Dimension(n, u) => bound!(args, "amount", n, u, 0, 1),
-                v => {
-                    return Err((
-                        format!(
-                            "$amount: {} is not a number.",
-                            v.to_css_string(args.span())?
-                        ),
-                        args.span(),
-                    )
-                        .into())
-                }
-            };
-            Ok(Value::Color(Box::new(color.fade_in(amount))))
-        }),
-    );
-    f.insert(
-        "fade-in",
-        Builtin::new(|mut args, scope, super_selector| {
-            args.max_args(2)?;
-            let color = match arg!(args, scope, super_selector, 0, "color") {
-                Value::Color(c) => c,
-                v => {
-                    return Err((
-                        format!("$color: {} is not a color.", v.to_css_string(args.span())?),
-                        args.span(),
-                    )
-                        .into())
-                }
-            };
-            let amount = match arg!(args, scope, super_selector, 1, "amount") {
-                Value::Dimension(n, u) => bound!(args, "amount", n, u, 0, 1),
-                v => {
-                    return Err((
-                        format!(
-                            "$amount: {} is not a number.",
-                            v.to_css_string(args.span())?
-                        ),
-                        args.span(),
-                    )
-                        .into())
-                }
-            };
-            Ok(Value::Color(Box::new(color.fade_in(amount))))
-        }),
-    );
-    f.insert(
-        "transparentize",
-        Builtin::new(|mut args, scope, super_selector| {
-            args.max_args(2)?;
-            let color = match arg!(args, scope, super_selector, 0, "color") {
-                Value::Color(c) => c,
-                v => {
-                    return Err((
-                        format!("$color: {} is not a color.", v.to_css_string(args.span())?),
-                        args.span(),
-                    )
-                        .into())
-                }
-            };
-            let amount = match arg!(args, scope, super_selector, 1, "amount") {
-                Value::Dimension(n, u) => bound!(args, "amount", n, u, 0, 1),
-                v => {
-                    return Err((
-                        format!(
-                            "$amount: {} is not a number.",
-                            v.to_css_string(args.span())?
-                        ),
-                        args.span(),
-                    )
-                        .into())
-                }
-            };
-            Ok(Value::Color(Box::new(color.fade_out(amount))))
-        }),
-    );
-    f.insert(
-        "fade-out",
-        Builtin::new(|mut args, scope, super_selector| {
-            args.max_args(2)?;
-            let color = match arg!(args, scope, super_selector, 0, "color") {
-                Value::Color(c) => c,
-                v => {
-                    return Err((
-                        format!("$color: {} is not a color.", v.to_css_string(args.span())?),
-                        args.span(),
-                    )
-                        .into())
-                }
-            };
-            let amount = match arg!(args, scope, super_selector, 1, "amount") {
-                Value::Dimension(n, u) => bound!(args, "amount", n, u, 0, 1),
-                v => {
-                    return Err((
-                        format!(
-                            "$amount: {} is not a number.",
-                            v.to_css_string(args.span())?
-                        ),
-                        args.span(),
-                    )
-                        .into())
-                }
-            };
-            Ok(Value::Color(Box::new(color.fade_out(amount))))
-        }),
-    );
+        };
+        let amount = match arg!(args, scope, super_selector, 1, "amount") {
+            Value::Dimension(n, u) => bound!(args, "amount", n, u, 0, 1),
+            v => {
+                return Err((
+                    format!(
+                        "$amount: {} is not a number.",
+                        v.to_css_string(args.span())?
+                    ),
+                    args.span(),
+                )
+                    .into())
+            }
+        };
+        Ok(Value::Color(Box::new(color.fade_in(amount))))
+    }
+
+    fn transparentize(
+        mut args: CallArgs,
+        scope: &Scope,
+        super_selector: &Selector,
+    ) -> SassResult<Value> {
+        args.max_args(2)?;
+        let color = match arg!(args, scope, super_selector, 0, "color") {
+            Value::Color(c) => c,
+            v => {
+                return Err((
+                    format!("$color: {} is not a color.", v.to_css_string(args.span())?),
+                    args.span(),
+                )
+                    .into())
+            }
+        };
+        let amount = match arg!(args, scope, super_selector, 1, "amount") {
+            Value::Dimension(n, u) => bound!(args, "amount", n, u, 0, 1),
+            v => {
+                return Err((
+                    format!(
+                        "$amount: {} is not a number.",
+                        v.to_css_string(args.span())?
+                    ),
+                    args.span(),
+                )
+                    .into())
+            }
+        };
+        Ok(Value::Color(Box::new(color.fade_out(amount))))
+    }
+
+    fn fade_out(mut args: CallArgs, scope: &Scope, super_selector: &Selector) -> SassResult<Value> {
+        args.max_args(2)?;
+        let color = match arg!(args, scope, super_selector, 0, "color") {
+            Value::Color(c) => c,
+            v => {
+                return Err((
+                    format!("$color: {} is not a color.", v.to_css_string(args.span())?),
+                    args.span(),
+                )
+                    .into())
+            }
+        };
+        let amount = match arg!(args, scope, super_selector, 1, "amount") {
+            Value::Dimension(n, u) => bound!(args, "amount", n, u, 0, 1),
+            v => {
+                return Err((
+                    format!(
+                        "$amount: {} is not a number.",
+                        v.to_css_string(args.span())?
+                    ),
+                    args.span(),
+                )
+                    .into())
+            }
+        };
+        Ok(Value::Color(Box::new(color.fade_out(amount))))
+    }
+
+    f.insert("alpha", Builtin::new(alpha));
+    f.insert("opacity", Builtin::new(opacity));
+    f.insert("opacify", Builtin::new(opacify));
+    f.insert("fade-in", Builtin::new(fade_in));
+    f.insert("transparentize", Builtin::new(transparentize));
+    f.insert("fade-out", Builtin::new(fade_out));
 }