diff --git a/src/abstract_tree/assignment.rs b/src/abstract_tree/assignment.rs
index 017f60c..a08575a 100644
--- a/src/abstract_tree/assignment.rs
+++ b/src/abstract_tree/assignment.rs
@@ -3,8 +3,8 @@ use serde::{Deserialize, Serialize};
 use crate::{
     context::Context,
     error::{RuntimeError, SyntaxError, ValidationError},
-    AbstractTree, AssignmentOperator, Format, Function, Identifier, SourcePosition, Statement,
-    SyntaxNode, Type, TypeSpecification, Value,
+    AbstractTree, AssignmentOperator, Format, Identifier, SourcePosition, Statement, SyntaxNode,
+    Type, TypeSpecification, Value,
 };
 
 /// Variable assignment, including add-assign and subtract-assign operations.
diff --git a/src/abstract_tree/for.rs b/src/abstract_tree/for.rs
index cdb923d..89171d6 100644
--- a/src/abstract_tree/for.rs
+++ b/src/abstract_tree/for.rs
@@ -47,7 +47,7 @@ impl AbstractTree for For {
         let loop_context = Context::with_variables_from(context)?;
 
         let item_node = node.child(4).unwrap();
-        let item = Block::from_syntax(item_node, source, context)?;
+        let item = Block::from_syntax(item_node, source, &loop_context)?;
 
         Ok(For {
             is_async,
@@ -78,11 +78,14 @@ impl AbstractTree for For {
         };
         let key = self.item_id.inner().clone();
 
+        self.context.inherit_from(context)?;
         self.context.set_type(key, item_type)?;
         self.block.validate(_source, &self.context)
     }
 
     fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        self.context.inherit_from(context)?;
+
         let expression_run = self.collection.run(source, context)?;
         let key = self.item_id.inner();
 
diff --git a/src/abstract_tree/function_call.rs b/src/abstract_tree/function_call.rs
index 80c3052..815c627 100644
--- a/src/abstract_tree/function_call.rs
+++ b/src/abstract_tree/function_call.rs
@@ -1,6 +1,7 @@
 use serde::{Deserialize, Serialize};
 
 use crate::{
+    built_in_functions::Callable,
     error::{RuntimeError, SyntaxError, ValidationError},
     AbstractTree, Context, Expression, Format, FunctionExpression, SourcePosition, SyntaxNode,
     Type, Value,
@@ -153,19 +154,32 @@ impl AbstractTree for FunctionCall {
             FunctionExpression::Index(index) => index.run(source, context)?,
         };
         let function = value.as_function()?;
-        let parameter_expression_pairs = function
-            .parameters()
-            .unwrap()
-            .iter()
-            .zip(self.arguments.iter());
 
-        for (identifier, expression) in parameter_expression_pairs {
-            let value = expression.run(source, context)?;
+        match function {
+            crate::Function::BuiltIn(built_in_function) => {
+                let mut arguments = Vec::with_capacity(self.arguments.len());
 
-            self.context.set_value(identifier.inner().clone(), value)?;
+                for expression in &self.arguments {
+                    let value = expression.run(source, context)?;
+
+                    arguments.push(value);
+                }
+
+                built_in_function.call(&arguments, source, &self.context)
+            }
+            crate::Function::ContextDefined(function_node) => {
+                let parameter_expression_pairs =
+                    function_node.parameters().iter().zip(self.arguments.iter());
+
+                for (identifier, expression) in parameter_expression_pairs {
+                    let value = expression.run(source, context)?;
+
+                    self.context.set_value(identifier.inner().clone(), value)?;
+                }
+
+                function_node.call(source, &self.context)
+            }
         }
-
-        function.call(&[], source, &self.context)
     }
 }
 
diff --git a/src/built_in_functions/str.rs b/src/built_in_functions/str.rs
index 47184eb..af4204a 100644
--- a/src/built_in_functions/str.rs
+++ b/src/built_in_functions/str.rs
@@ -333,18 +333,20 @@ impl Callable for StrFunction {
             StrFunction::Retain => {
                 RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
-                let mut string = arguments.first().unwrap().as_string()?.clone();
-                let predicate = arguments.get(1).unwrap().as_function()?;
+                todo!();
 
-                string.retain(|char| {
-                    predicate
-                        .call(&[Value::string(char)], _source, _outer_context)
-                        .unwrap()
-                        .as_boolean()
-                        .unwrap()
-                });
+                // let mut string = arguments.first().unwrap().as_string()?.clone();
+                // let predicate = arguments.get(1).unwrap().as_function()?;
 
-                Value::String(string)
+                // string.retain(|char| {
+                //     predicate
+                //         .call(&[Value::string(char)], _source, _outer_context)
+                //         .unwrap()
+                //         .as_boolean()
+                //         .unwrap()
+                // });
+
+                // Value::String(string)
             }
             StrFunction::Split => {
                 RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
diff --git a/src/value/function.rs b/src/value/function.rs
index eb61600..37d5bc2 100644
--- a/src/value/function.rs
+++ b/src/value/function.rs
@@ -3,8 +3,7 @@ use std::fmt::{self, Display, Formatter};
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    built_in_functions::Callable, error::RuntimeError, BuiltInFunction, Context, Format,
-    FunctionNode, Identifier, Type, Value,
+    built_in_functions::Callable, BuiltInFunction, Format, FunctionNode, Identifier, Type,
 };
 
 #[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize)]
@@ -14,20 +13,6 @@ pub enum Function {
 }
 
 impl Function {
-    pub fn call(
-        &self,
-        arguments: &[Value],
-        source: &str,
-        outer_context: &Context,
-    ) -> Result<Value, RuntimeError> {
-        match self {
-            Function::BuiltIn(built_in_function) => {
-                built_in_function.call(arguments, source, outer_context)
-            }
-            Function::ContextDefined(function_node) => function_node.call(source, outer_context),
-        }
-    }
-
     pub fn r#type(&self) -> Type {
         match self {
             Function::BuiltIn(built_in_function) => built_in_function.r#type(),