diff --git a/src/builtin/functions/math.rs b/src/builtin/functions/math.rs
index 7e6a0d7..ec17da2 100644
--- a/src/builtin/functions/math.rs
+++ b/src/builtin/functions/math.rs
@@ -2,32 +2,13 @@ use crate::{builtin::builtin_imports::*, evaluate::div};
 
 pub(crate) fn percentage(mut args: ArgumentResult, visitor: &mut Visitor) -> SassResult<Value> {
     args.max_args(1)?;
-    let num = match args.get_err(0, "number")? {
-        Value::Dimension(SassNumber {
-            num: n,
-            unit: Unit::None,
-            as_slash: _,
-        }) => n * Number::from(100),
-        v @ Value::Dimension(SassNumber { .. }) => {
-            return Err((
-                format!(
-                    "$number: Expected {} to have no units.",
-                    v.inspect(args.span())?
-                ),
-                args.span(),
-            )
-                .into())
-        }
-        v => {
-            return Err((
-                format!("$number: {} is not a number.", v.inspect(args.span())?),
-                args.span(),
-            )
-                .into())
-        }
-    };
+    let num = args
+        .get_err(0, "number")?
+        .assert_number_with_name("number", args.span)?;
+    num.assert_no_units("number", args.span)?;
+
     Ok(Value::Dimension(SassNumber {
-        num,
+        num: Number(num.num.0 * 100.0),
         unit: Unit::Percent,
         as_slash: None,
     }))
@@ -107,54 +88,26 @@ pub(crate) fn floor(mut args: ArgumentResult, visitor: &mut Visitor) -> SassResu
 
 pub(crate) fn abs(mut args: ArgumentResult, visitor: &mut Visitor) -> SassResult<Value> {
     args.max_args(1)?;
-    match args.get_err(0, "number")? {
-        Value::Dimension(SassNumber {
-            num: n,
-            unit: u,
-            as_slash: _,
-        }) => Ok(Value::Dimension(SassNumber {
-            num: (n.abs()),
-            unit: u,
-            as_slash: None,
-        })),
-        v => Err((
-            format!("$number: {} is not a number.", v.inspect(args.span())?),
-            args.span(),
-        )
-            .into()),
-    }
+    let mut num = args
+        .get_err(0, "number")?
+        .assert_number_with_name("number", args.span())?;
+
+    num.num = num.num.abs();
+
+    Ok(Value::Dimension(num))
 }
 
 pub(crate) fn comparable(mut args: ArgumentResult, visitor: &mut Visitor) -> SassResult<Value> {
     args.max_args(2)?;
-    let unit1 = match args.get_err(0, "number1")? {
-        Value::Dimension(SassNumber {
-            num: _,
-            unit: u,
-            as_slash: _,
-        }) => u,
-        v => {
-            return Err((
-                format!("$number1: {} is not a number.", v.inspect(args.span())?),
-                args.span(),
-            )
-                .into())
-        }
-    };
-    let unit2 = match args.get_err(1, "number2")? {
-        Value::Dimension(SassNumber {
-            num: _,
-            unit: u,
-            as_slash: _,
-        }) => u,
-        v => {
-            return Err((
-                format!("$number2: {} is not a number.", v.inspect(args.span())?),
-                args.span(),
-            )
-                .into())
-        }
-    };
+    let unit1 = args
+        .get_err(0, "number1")?
+        .assert_number_with_name("number1", args.span())?
+        .unit;
+
+    let unit2 = args
+        .get_err(1, "number2")?
+        .assert_number_with_name("number2", args.span())?
+        .unit;
 
     Ok(Value::bool(unit1.comparable(&unit2)))
 }
diff --git a/src/builtin/functions/meta.rs b/src/builtin/functions/meta.rs
index 4917e30..ec42481 100644
--- a/src/builtin/functions/meta.rs
+++ b/src/builtin/functions/meta.rs
@@ -295,7 +295,7 @@ pub(crate) fn get_function(mut args: ArgumentResult, visitor: &mut Visitor) -> S
     };
 
     match func {
-        Some(func) => Ok(Value::FunctionRef(func)),
+        Some(func) => Ok(Value::FunctionRef(Box::new(func))),
         None => Err((format!("Function not found: {}", name), args.span()).into()),
     }
 }
@@ -318,7 +318,7 @@ pub(crate) fn call(mut args: ArgumentResult, visitor: &mut Visitor) -> SassResul
 
     args.remove_positional(0).unwrap();
 
-    visitor.run_function_callable_with_maybe_evaled(func, MaybeEvaledArguments::Evaled(args), span)
+    visitor.run_function_callable_with_maybe_evaled(*func, MaybeEvaledArguments::Evaled(args), span)
 }
 
 #[allow(clippy::needless_pass_by_value)]
diff --git a/src/builtin/functions/string.rs b/src/builtin/functions/string.rs
index 09baead..f0daa45 100644
--- a/src/builtin/functions/string.rs
+++ b/src/builtin/functions/string.rs
@@ -100,7 +100,7 @@ pub(crate) fn str_slice(mut args: ArgumentResult, visitor: &mut Visitor) -> Sass
     } else if start > 0 {
         (start as usize).min(str_len + 1)
     } else {
-        (start + str_len as i32 + 1).max(1) as usize
+        (start + str_len as i64 + 1).max(1) as usize
     };
 
     let end = args
@@ -120,7 +120,7 @@ pub(crate) fn str_slice(mut args: ArgumentResult, visitor: &mut Visitor) -> Sass
     let mut end = end.num().assert_int(span)?;
 
     if end < 0 {
-        end += str_len as i32 + 1;
+        end += str_len as i64 + 1;
     }
 
     let end = (end.max(0) as usize).min(str_len + 1);
@@ -232,7 +232,7 @@ pub(crate) fn str_insert(mut args: ArgumentResult, visitor: &mut Visitor) -> Sas
     } else if index_int == 0 {
         insert(0, s1, &substr)
     } else {
-        let idx = (len as i32 + index_int + 1).max(0) as usize;
+        let idx = (len as i64 + index_int + 1).max(0) as usize;
         insert(idx, s1, &substr)
     };
 
diff --git a/src/builtin/modules/mod.rs b/src/builtin/modules/mod.rs
index 2a20063..fee0852 100644
--- a/src/builtin/modules/mod.rs
+++ b/src/builtin/modules/mod.rs
@@ -343,7 +343,7 @@ impl Module {
                 .map(|(key, value)| {
                     (
                         Value::String(key.to_string(), QuoteKind::Quoted).span(span),
-                        Value::FunctionRef(value),
+                        Value::FunctionRef(Box::new(value)),
                     )
                 })
                 .collect::<Vec<_>>(),
diff --git a/src/evaluate/env.rs b/src/evaluate/env.rs
index babfebc..ed4f044 100644
--- a/src/evaluate/env.rs
+++ b/src/evaluate/env.rs
@@ -134,10 +134,10 @@ impl Environment {
             Ok(v) => Ok(v),
             Err(e) => {
                 if let Some(v) = self.get_variable_from_global_modules(name.node) {
-                    return Ok(v);
+                    Ok(v)
+                } else {
+                    Err(e)
                 }
-
-                Err(e)
             }
         }
     }
diff --git a/src/evaluate/visitor.rs b/src/evaluate/visitor.rs
index eed1043..ef78df8 100644
--- a/src/evaluate/visitor.rs
+++ b/src/evaluate/visitor.rs
@@ -64,6 +64,16 @@ impl UserDefinedCallable for AstFunctionDecl {
     }
 }
 
+impl UserDefinedCallable for Arc<AstFunctionDecl> {
+    fn name(&self) -> Identifier {
+        self.name.node
+    }
+
+    fn arguments(&self) -> &ArgumentDeclaration {
+        &self.arguments
+    }
+}
+
 impl UserDefinedCallable for AstMixin {
     fn name(&self) -> Identifier {
         self.name
@@ -1112,7 +1122,7 @@ impl<'a> Visitor<'a> {
         // todo: independency
 
         let func = SassFunction::UserDefined(UserDefinedFunction {
-            function: Box::new(fn_decl),
+            function: Arc::new(fn_decl),
             name,
             env: self.env.new_closure(),
         });
@@ -1701,6 +1711,14 @@ impl<'a> Visitor<'a> {
 
         let direction = if from > to { -1 } else { 1 };
 
+        if to == i64::MAX || to == i64::MIN {
+            return Err((
+                "@for loop upper bound exceeds valid integer representation (i64::MAX)",
+                to_span,
+            )
+                .into());
+        }
+
         if !for_stmt.is_exclusive {
             to += direction;
         }
@@ -2200,23 +2218,17 @@ impl<'a> Visitor<'a> {
                 Ok(self.without_slash(val))
             }
             SassFunction::UserDefined(UserDefinedFunction { function, env, .. }) => self
-                .run_user_defined_callable(
-                    arguments,
-                    *function,
-                    &env,
-                    span,
-                    |function, visitor| {
-                        for stmt in function.children {
-                            let result = visitor.visit_stmt(stmt)?;
+                .run_user_defined_callable(arguments, function, &env, span, |function, visitor| {
+                    for stmt in function.children.clone() {
+                        let result = visitor.visit_stmt(stmt)?;
 
-                            if let Some(val) = result {
-                                return Ok(val);
-                            }
+                        if let Some(val) = result {
+                            return Ok(val);
                         }
+                    }
 
-                        Err(("Function finished without @return.", span).into())
-                    },
-                ),
+                    Err(("Function finished without @return.", span).into())
+                }),
             SassFunction::Plain { name } => {
                 let arguments = match arguments {
                     MaybeEvaledArguments::Invocation(args) => args,
diff --git a/src/value/mod.rs b/src/value/mod.rs
index 26b0263..9a4ac01 100644
--- a/src/value/mod.rs
+++ b/src/value/mod.rs
@@ -40,8 +40,7 @@ pub(crate) enum Value {
     Map(SassMap),
     ArgList(ArgList),
     /// Returned by `get-function()`
-    // todo: benchmark boxing this (function refs are infrequent)
-    FunctionRef(SassFunction),
+    FunctionRef(Box<SassFunction>),
     Calculation(SassCalculation),
 }
 
diff --git a/src/value/number.rs b/src/value/number.rs
index 1784a75..349a142 100644
--- a/src/value/number.rs
+++ b/src/value/number.rs
@@ -47,7 +47,7 @@ pub(crate) fn fuzzy_equals(a: f64, b: f64) -> bool {
     (a - b).abs() <= epsilon() && (a * inverse_epsilon()).round() == (b * inverse_epsilon()).round()
 }
 
-pub(crate) fn fuzzy_as_int(num: f64) -> Option<i32> {
+pub(crate) fn fuzzy_as_int(num: f64) -> Option<i64> {
     if !num.is_finite() {
         return None;
     }
@@ -55,7 +55,7 @@ pub(crate) fn fuzzy_as_int(num: f64) -> Option<i32> {
     let rounded = num.round();
 
     if fuzzy_equals(num, rounded) {
-        Some(rounded as i32)
+        Some(rounded as i64)
     } else {
         None
     }
@@ -110,14 +110,14 @@ impl Number {
         self.0.is_sign_negative() && !self.is_zero()
     }
 
-    pub fn assert_int(self, span: Span) -> SassResult<i32> {
+    pub fn assert_int(self, span: Span) -> SassResult<i64> {
         match fuzzy_as_int(self.0) {
             Some(i) => Ok(i),
             None => Err((format!("{} is not an int.", self.0), span).into()),
         }
     }
 
-    pub fn assert_int_with_name(self, name: &'static str, span: Span) -> SassResult<i32> {
+    pub fn assert_int_with_name(self, name: &'static str, span: Span) -> SassResult<i64> {
         match fuzzy_as_int(self.0) {
             Some(i) => Ok(i),
             None => Err((
diff --git a/src/value/sass_function.rs b/src/value/sass_function.rs
index 8da6673..e419f6b 100644
--- a/src/value/sass_function.rs
+++ b/src/value/sass_function.rs
@@ -3,24 +3,15 @@
 //! Sass functions can be either user-defined or builtin.
 //!
 //! User-defined functions are those that have been implemented in Sass
-//! using the @function rule. See the documentation of `crate::atrule::Function`
+//! using the @function rule. See the documentation of [`crate::atrule::Function`]
 //! for more information.
 //!
 //! Builtin functions are those that have been implemented in rust and are
 //! in the global scope.
 
-use std::fmt;
+use std::{fmt, sync::Arc};
 
-// use codemap::Spanned;
-
-use crate::{
-    // error::SassResult,
-    ast::AstFunctionDecl,
-    // value::Value,
-    builtin::Builtin,
-    common::Identifier,
-    evaluate::Environment,
-};
+use crate::{ast::AstFunctionDecl, builtin::Builtin, common::Identifier, evaluate::Environment};
 
 /// A Sass function
 ///
@@ -39,7 +30,7 @@ pub(crate) enum SassFunction {
 
 #[derive(Debug, Clone)]
 pub(crate) struct UserDefinedFunction {
-    pub function: Box<AstFunctionDecl>,
+    pub function: Arc<AstFunctionDecl>,
     pub name: Identifier,
     pub env: Environment,
 }