diff --git a/src/abstract_tree/block.rs b/src/abstract_tree/block.rs
index bbffc1c..32beb98 100644
--- a/src/abstract_tree/block.rs
+++ b/src/abstract_tree/block.rs
@@ -23,15 +23,6 @@ use crate::{
 pub struct Block {
     is_async: bool,
     statements: Vec<Statement>,
-
-    #[serde(skip)]
-    context: Context,
-}
-
-impl Block {
-    pub fn context(&self) -> &Context {
-        &self.context
-    }
 }
 
 impl AbstractTree for Block {
@@ -62,23 +53,18 @@ impl AbstractTree for Block {
         Ok(Block {
             is_async,
             statements,
-            context: block_context,
         })
     }
 
     fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
-        self.context.inherit_from(_context)?;
-
         for statement in &self.statements {
-            statement.validate(_source, &self.context)?;
+            statement.validate(_source, _context)?;
         }
 
         Ok(())
     }
 
-    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
-        self.context.inherit_from(context)?;
-
+    fn run(&self, _source: &str, _context: &Context) -> Result<Value, RuntimeError> {
         if self.is_async {
             let statements = &self.statements;
             let final_result = RwLock::new(Ok(Value::none()));
@@ -87,7 +73,7 @@ impl AbstractTree for Block {
                 .into_par_iter()
                 .enumerate()
                 .find_map_first(|(index, statement)| {
-                    let result = statement.run(source, &self.context);
+                    let result = statement.run(_source, _context);
                     let is_last_statement = index == statements.len() - 1;
                     let is_return_statement = if let Statement::Return(_) = statement {
                         true
@@ -117,10 +103,10 @@ impl AbstractTree for Block {
 
             for statement in &self.statements {
                 if let Statement::Return(inner_statement) = statement {
-                    return inner_statement.run(source, &self.context);
+                    return inner_statement.run(_source, _context);
                 }
 
-                prev_result = Some(statement.run(source, &self.context));
+                prev_result = Some(statement.run(_source, _context));
             }
 
             prev_result.unwrap_or(Ok(Value::none()))
@@ -135,9 +121,9 @@ impl AbstractTree for Block {
                 false
             }
         }) {
-            statement.expected_type(&self.context)
+            statement.expected_type(_context)
         } else if let Some(statement) = self.statements.last() {
-            statement.expected_type(&self.context)
+            statement.expected_type(_context)
         } else {
             Ok(Type::None)
         }
diff --git a/src/abstract_tree/for.rs b/src/abstract_tree/for.rs
index 02d5b91..cdb923d 100644
--- a/src/abstract_tree/for.rs
+++ b/src/abstract_tree/for.rs
@@ -15,6 +15,9 @@ pub struct For {
     collection: Expression,
     block: Block,
     source_position: SourcePosition,
+
+    #[serde(skip)]
+    context: Context,
 }
 
 impl AbstractTree for For {
@@ -41,6 +44,8 @@ impl AbstractTree for For {
         let expression_node = node.child(3).unwrap();
         let expression = Expression::from_syntax(expression_node, source, context)?;
 
+        let loop_context = Context::with_variables_from(context)?;
+
         let item_node = node.child(4).unwrap();
         let item = Block::from_syntax(item_node, source, context)?;
 
@@ -50,6 +55,7 @@ impl AbstractTree for For {
             collection: expression,
             block: item,
             source_position: SourcePosition::from(node.range()),
+            context: loop_context,
         })
     }
 
@@ -72,8 +78,8 @@ impl AbstractTree for For {
         };
         let key = self.item_id.inner().clone();
 
-        self.block.context().set_type(key, item_type)?;
-        self.block.validate(_source, context)
+        self.context.set_type(key, item_type)?;
+        self.block.validate(_source, &self.context)
     }
 
     fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
@@ -83,17 +89,14 @@ impl AbstractTree for For {
         if let Value::Range(range) = expression_run {
             if self.is_async {
                 range.into_par_iter().try_for_each(|integer| {
-                    self.block
-                        .context()
+                    self.context
                         .set_value(key.clone(), Value::Integer(integer))?;
-                    self.block.run(source, context).map(|_value| ())
+                    self.block.run(source, &self.context).map(|_value| ())
                 })?;
             } else {
                 for i in range {
-                    self.block
-                        .context()
-                        .set_value(key.clone(), Value::Integer(i))?;
-                    self.block.run(source, context)?;
+                    self.context.set_value(key.clone(), Value::Integer(i))?;
+                    self.block.run(source, &self.context)?;
                 }
             }
 
@@ -103,20 +106,17 @@ impl AbstractTree for For {
         if let Value::List(list) = &expression_run {
             if self.is_async {
                 list.items().par_iter().try_for_each(|value| {
-                    self.block.context().set_value(key.clone(), value.clone())?;
-                    self.block.run(source, context).map(|_value| ())
+                    self.context.set_value(key.clone(), value.clone())?;
+                    self.block.run(source, &self.context).map(|_value| ())
                 })?;
             } else {
                 for value in list.items().iter() {
-                    self.block.context().set_value(key.clone(), value.clone())?;
-                    self.block.run(source, context)?;
+                    self.context.set_value(key.clone(), value.clone())?;
+                    self.block.run(source, &self.context)?;
                 }
             }
         }
 
-        self.block.context().unset(&key)?;
-        context.inherit_from(self.block.context())?;
-
         Ok(Value::none())
     }
 }
diff --git a/src/abstract_tree/function_node.rs b/src/abstract_tree/function_node.rs
index 0b579c5..b468dc1 100644
--- a/src/abstract_tree/function_node.rs
+++ b/src/abstract_tree/function_node.rs
@@ -14,6 +14,9 @@ pub struct FunctionNode {
     body: Block,
     r#type: Type,
     syntax_position: SourcePosition,
+
+    #[serde(skip)]
+    context: Context,
 }
 
 impl FunctionNode {
@@ -28,6 +31,7 @@ impl FunctionNode {
             body,
             r#type,
             syntax_position,
+            context: Context::new(),
         }
     }
 
@@ -61,28 +65,24 @@ impl FunctionNode {
         &self,
         arguments: &[Value],
         source: &str,
-        outer_context: &Context,
+        _context: &Context,
     ) -> Result<Value, RuntimeError> {
         let parameter_argument_pairs = self.parameters.iter().zip(arguments.iter());
 
         for (identifier, value) in parameter_argument_pairs {
             let key = identifier.inner().clone();
 
-            self.body.context().set_value(key, value.clone())?;
+            self.context.set_value(key, value.clone())?;
         }
 
-        let return_value = self.body.run(source, outer_context)?;
+        let return_value = self.body.run(source, &self.context)?;
 
         Ok(return_value)
     }
 }
 
 impl AbstractTree for FunctionNode {
-    fn from_syntax(
-        node: SyntaxNode,
-        source: &str,
-        outer_context: &Context,
-    ) -> Result<Self, SyntaxError> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
         SyntaxError::expect_syntax_node(source, "function", node)?;
 
         let child_count = node.child_count();
@@ -93,28 +93,28 @@ impl AbstractTree for FunctionNode {
             let child = node.child(index).unwrap();
 
             if child.kind() == "identifier" {
-                let identifier = Identifier::from_syntax(child, source, outer_context)?;
+                let identifier = Identifier::from_syntax(child, source, context)?;
 
                 parameters.push(identifier);
             }
 
             if child.kind() == "type_specification" {
-                let type_specification =
-                    TypeSpecification::from_syntax(child, source, outer_context)?;
+                let type_specification = TypeSpecification::from_syntax(child, source, context)?;
 
                 parameter_types.push(type_specification.take_inner());
             }
         }
 
         let return_type_node = node.child(child_count - 2).unwrap();
-        let return_type = TypeSpecification::from_syntax(return_type_node, source, outer_context)?;
+        let return_type = TypeSpecification::from_syntax(return_type_node, source, context)?;
+
+        let function_context = Context::with_variables_from(context)?;
 
         let body_node = node.child(child_count - 1).unwrap();
-        let body = Block::from_syntax(body_node, source, &outer_context)?;
+        let body = Block::from_syntax(body_node, source, &context)?;
 
         for (parameter, parameter_type) in parameters.iter().zip(parameter_types.iter()) {
-            body.context()
-                .set_type(parameter.inner().clone(), parameter_type.clone())?;
+            function_context.set_type(parameter.inner().clone(), parameter_type.clone())?;
         }
 
         let r#type = Type::function(parameter_types, return_type.take_inner());
@@ -125,6 +125,7 @@ impl AbstractTree for FunctionNode {
             body,
             r#type,
             syntax_position,
+            context: function_context,
         })
     }
 
diff --git a/src/context.rs b/src/context.rs
index b3db8cf..25418d0 100644
--- a/src/context.rs
+++ b/src/context.rs
@@ -38,7 +38,13 @@ impl Context {
         let mut self_variables = self.inner.write()?;
 
         for (identifier, value_data) in other.inner.read()?.iter() {
-            self_variables.insert(identifier.clone(), value_data.clone());
+            let existing_data = self_variables.get(identifier);
+
+            if let Some(ValueData::Value { .. }) = existing_data {
+                continue;
+            } else {
+                self_variables.insert(identifier.clone(), value_data.clone());
+            }
         }
 
         Ok(())
diff --git a/tests/for_loop.rs b/tests/for_loop.rs
index 221d5ea..110fb4d 100644
--- a/tests/for_loop.rs
+++ b/tests/for_loop.rs
@@ -74,7 +74,7 @@ fn modify_map() {
 }
 
 #[test]
-fn modify_list_values() {
+fn do_not_modify_list_values() {
     let result = interpret(
         "
         list = [1 2 3]
@@ -85,39 +85,14 @@ fn modify_list_values() {
 
     assert_eq!(
         Ok(Value::List(List::with_items(vec![
+            Value::Integer(1),
             Value::Integer(2),
             Value::Integer(3),
-            Value::Integer(4),
         ]))),
         result
     );
 }
 
-#[test]
-fn modify_map_values() {
-    let result = interpret(
-        "
-        map = {
-            x = 0
-            y = 1
-        }
-        
-        for [key, value] in map {
-            value += 1
-        }
-        
-        map
-        ",
-    );
-
-    let map = Map::new();
-
-    map.set("x".to_string(), Value::Integer(1)).unwrap();
-    map.set("y".to_string(), Value::Integer(2)).unwrap();
-
-    assert_eq!(Ok(Value::Map(map)), result);
-}
-
 #[test]
 fn r#break() {
     let result = interpret(