diff --git a/Cargo.lock b/Cargo.lock
index e322baa..9599d91 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -356,7 +356,6 @@ dependencies = [
  "log",
  "rand",
  "rayon",
- "stanza",
 ]
 
 [[package]]
@@ -1101,12 +1100,6 @@ dependencies = [
  "winapi",
 ]
 
-[[package]]
-name = "stanza"
-version = "0.5.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5d230b987a5b524a015ded47fed54c5177598a71c90508acee1c5d0b4c955f74"
-
 [[package]]
 name = "strip-ansi-escapes"
 version = "0.2.0"
diff --git a/dust-lang/Cargo.toml b/dust-lang/Cargo.toml
index 322489b..4584cf7 100644
--- a/dust-lang/Cargo.toml
+++ b/dust-lang/Cargo.toml
@@ -17,4 +17,3 @@ env_logger = "0.11.3"
 log = "0.4.21"
 rand = "0.8.5"
 rayon = "1.9.0"
-stanza = "0.5.1"
diff --git a/dust-lang/src/abstract_tree/assignment.rs b/dust-lang/src/abstract_tree/assignment.rs
index 57dd729..0d6e2db 100644
--- a/dust-lang/src/abstract_tree/assignment.rs
+++ b/dust-lang/src/abstract_tree/assignment.rs
@@ -69,8 +69,8 @@ impl AbstractNode for Assignment {
         Ok(())
     }
 
-    fn run(self, context: &Context) -> Result<Action, RuntimeError> {
-        let action = self.statement.run(context)?;
+    fn run(self, context: &mut Context, clear_variables: bool) -> Result<Action, RuntimeError> {
+        let action = self.statement.run(context, clear_variables)?;
         let right = match action {
             Action::Return(value) => value,
             r#break => return Ok(r#break),
@@ -81,7 +81,7 @@ impl AbstractNode for Assignment {
                 context.set_value(self.identifier.node, right)?;
             }
             AssignmentOperator::AddAssign => {
-                if let Some(left) = context.get_value(&self.identifier.node)? {
+                if let Some(left) = context.use_value(&self.identifier.node)? {
                     let new_value = match (left.inner().as_ref(), right.inner().as_ref()) {
                         (ValueInner::Integer(left), ValueInner::Integer(right)) => {
                             let sum = left.saturating_add(*right);
@@ -120,7 +120,7 @@ impl AbstractNode for Assignment {
                 }
             }
             AssignmentOperator::SubAssign => {
-                if let Some(left) = context.get_value(&self.identifier.node)? {
+                if let Some(left) = context.use_value(&self.identifier.node)? {
                     let new_value = match (left.inner().as_ref(), right.inner().as_ref()) {
                         (ValueInner::Integer(left), ValueInner::Integer(right)) => {
                             let difference = left.saturating_sub(*right);
@@ -175,7 +175,7 @@ mod tests {
 
     #[test]
     fn assign_value() {
-        let context = Context::new();
+        let mut context = Context::new();
 
         Assignment::new(
             Identifier::new("foobar").with_position((0, 0)),
@@ -185,18 +185,18 @@ mod tests {
                 ValueNode::Integer(42).with_position((0, 0)),
             )),
         )
-        .run(&context)
+        .run(&mut context, true)
         .unwrap();
 
         assert_eq!(
-            context.get_value(&Identifier::new("foobar")),
+            context.use_value(&Identifier::new("foobar")),
             Ok(Some(Value::integer(42)))
         )
     }
 
     #[test]
     fn add_assign_value() {
-        let context = Context::new();
+        let mut context = Context::new();
 
         context
             .set_value(Identifier::new("foobar"), Value::integer(1))
@@ -210,18 +210,18 @@ mod tests {
                 ValueNode::Integer(41).with_position((0, 0)),
             )),
         )
-        .run(&context)
+        .run(&mut context, true)
         .unwrap();
 
         assert_eq!(
-            context.get_value(&Identifier::new("foobar")),
+            context.use_value(&Identifier::new("foobar")),
             Ok(Some(Value::integer(42)))
         )
     }
 
     #[test]
     fn subtract_assign_value() {
-        let context = Context::new();
+        let mut context = Context::new();
 
         context
             .set_value(Identifier::new("foobar"), Value::integer(43))
@@ -235,11 +235,11 @@ mod tests {
                 ValueNode::Integer(1).with_position((0, 0)),
             )),
         )
-        .run(&context)
+        .run(&mut context, true)
         .unwrap();
 
         assert_eq!(
-            context.get_value(&Identifier::new("foobar")),
+            context.use_value(&Identifier::new("foobar")),
             Ok(Some(Value::integer(42)))
         )
     }
diff --git a/dust-lang/src/abstract_tree/async_block.rs b/dust-lang/src/abstract_tree/async_block.rs
index 99516b2..8451fab 100644
--- a/dust-lang/src/abstract_tree/async_block.rs
+++ b/dust-lang/src/abstract_tree/async_block.rs
@@ -33,7 +33,7 @@ impl AbstractNode for AsyncBlock {
         Ok(())
     }
 
-    fn run(self, _context: &Context) -> Result<Action, RuntimeError> {
+    fn run(self, _context: &mut Context, _clear_variables: bool) -> Result<Action, RuntimeError> {
         let statement_count = self.statements.len();
         let final_result = RwLock::new(Ok(Action::None));
 
@@ -41,7 +41,7 @@ impl AbstractNode for AsyncBlock {
             .into_par_iter()
             .enumerate()
             .find_map_first(|(index, statement)| {
-                let result = statement.run(_context);
+                let result = statement.run(&mut _context.clone(), _clear_variables);
 
                 if index == statement_count - 1 {
                     let get_write_lock = final_result.write();
diff --git a/dust-lang/src/abstract_tree/block.rs b/dust-lang/src/abstract_tree/block.rs
index c9e0cc3..f060335 100644
--- a/dust-lang/src/abstract_tree/block.rs
+++ b/dust-lang/src/abstract_tree/block.rs
@@ -37,11 +37,11 @@ impl AbstractNode for Block {
         Ok(())
     }
 
-    fn run(self, _context: &Context) -> Result<Action, RuntimeError> {
+    fn run(self, _context: &mut Context, _clear_variables: bool) -> Result<Action, RuntimeError> {
         let mut previous = Action::None;
 
         for statement in self.statements {
-            previous = statement.run(_context)?;
+            previous = statement.run(_context, _clear_variables)?;
         }
 
         Ok(previous)
@@ -72,7 +72,7 @@ mod tests {
         ]);
 
         assert_eq!(
-            block.run(&Context::new()).unwrap(),
+            block.run(&mut Context::new(), true).unwrap(),
             Action::Return(Value::integer(42))
         )
     }
diff --git a/dust-lang/src/abstract_tree/built_in_function_call.rs b/dust-lang/src/abstract_tree/built_in_function_call.rs
index 882240b..82e25df 100644
--- a/dust-lang/src/abstract_tree/built_in_function_call.rs
+++ b/dust-lang/src/abstract_tree/built_in_function_call.rs
@@ -27,10 +27,14 @@ impl AbstractNode for BuiltInFunctionCall {
     }
 
     fn validate(&self, _context: &Context) -> Result<(), ValidationError> {
-        Ok(())
+        match self {
+            BuiltInFunctionCall::ReadLine => Ok(()),
+            BuiltInFunctionCall::Sleep(expression) => expression.validate(_context),
+            BuiltInFunctionCall::WriteLine(expression) => expression.validate(_context),
+        }
     }
 
-    fn run(self, context: &Context) -> Result<Action, RuntimeError> {
+    fn run(self, context: &mut Context, _clear_variables: bool) -> Result<Action, RuntimeError> {
         match self {
             BuiltInFunctionCall::ReadLine => {
                 let mut buffer = String::new();
@@ -40,7 +44,7 @@ impl AbstractNode for BuiltInFunctionCall {
                 Ok(Action::Return(Value::string(buffer)))
             }
             BuiltInFunctionCall::Sleep(expression) => {
-                let action = expression.clone().run(context)?;
+                let action = expression.clone().run(context, _clear_variables)?;
                 let value = if let Action::Return(value) = action {
                     value
                 } else {
@@ -56,7 +60,7 @@ impl AbstractNode for BuiltInFunctionCall {
                 Ok(Action::None)
             }
             BuiltInFunctionCall::WriteLine(expression) => {
-                let action = expression.clone().run(context)?;
+                let action = expression.clone().run(context, _clear_variables)?;
                 let value = if let Action::Return(value) = action {
                     value
                 } else {
diff --git a/dust-lang/src/abstract_tree/expression.rs b/dust-lang/src/abstract_tree/expression.rs
index 3048eee..1e76262 100644
--- a/dust-lang/src/abstract_tree/expression.rs
+++ b/dust-lang/src/abstract_tree/expression.rs
@@ -65,7 +65,9 @@ impl AbstractNode for Expression {
         match self {
             Expression::FunctionCall(function_call) => function_call.node.validate(context),
             Expression::Identifier(identifier) => {
-                if context.contains(&identifier.node)? {
+                let found = context.add_expected_use(&identifier.node)?;
+
+                if found {
                     Ok(())
                 } else {
                     Err(ValidationError::VariableNotFound {
@@ -85,11 +87,13 @@ impl AbstractNode for Expression {
         }
     }
 
-    fn run(self, _context: &Context) -> Result<Action, RuntimeError> {
+    fn run(self, context: &mut Context, _clear_variables: bool) -> Result<Action, RuntimeError> {
         match self {
-            Expression::FunctionCall(function_call) => function_call.node.run(_context),
+            Expression::FunctionCall(function_call) => {
+                function_call.node.run(context, _clear_variables)
+            }
             Expression::Identifier(identifier) => {
-                if let Some(value) = _context.get_value(&identifier.node)? {
+                if let Some(value) = context.use_value(&identifier.node)? {
                     Ok(Action::Return(value))
                 } else {
                     Err(RuntimeError::ValidationFailure(
@@ -100,13 +104,13 @@ impl AbstractNode for Expression {
                     ))
                 }
             }
-            Expression::MapIndex(map_index) => map_index.node.run(_context),
-            Expression::ListIndex(list_index) => list_index.node.run(_context),
-            Expression::Logic(logic) => logic.node.run(_context),
-            Expression::Math(math) => math.node.run(_context),
-            Expression::Value(value_node) => value_node.node.run(_context),
+            Expression::MapIndex(map_index) => map_index.node.run(context, _clear_variables),
+            Expression::ListIndex(list_index) => list_index.node.run(context, _clear_variables),
+            Expression::Logic(logic) => logic.node.run(context, _clear_variables),
+            Expression::Math(math) => math.node.run(context, _clear_variables),
+            Expression::Value(value_node) => value_node.node.run(context, _clear_variables),
             Expression::BuiltInFunctionCall(built_in_function_call) => {
-                built_in_function_call.node.run(_context)
+                built_in_function_call.node.run(context, _clear_variables)
             }
         }
     }
diff --git a/dust-lang/src/abstract_tree/function_call.rs b/dust-lang/src/abstract_tree/function_call.rs
index e9206f5..eb0826b 100644
--- a/dust-lang/src/abstract_tree/function_call.rs
+++ b/dust-lang/src/abstract_tree/function_call.rs
@@ -42,6 +42,8 @@ impl AbstractNode for FunctionCall {
     }
 
     fn validate(&self, context: &Context) -> Result<(), ValidationError> {
+        self.function.validate(context)?;
+
         for expression in &self.arguments {
             expression.validate(context)?;
         }
@@ -95,9 +97,9 @@ impl AbstractNode for FunctionCall {
         }
     }
 
-    fn run(self, context: &Context) -> Result<Action, RuntimeError> {
+    fn run(self, context: &mut Context, _clear_variables: bool) -> Result<Action, RuntimeError> {
         let function_position = self.function.position();
-        let action = self.function.run(context)?;
+        let action = self.function.run(context, _clear_variables)?;
         let value = if let Action::Return(value) = action {
             value
         } else {
@@ -119,7 +121,7 @@ impl AbstractNode for FunctionCall {
 
         for expression in self.arguments {
             let expression_position = expression.position();
-            let action = expression.run(context)?;
+            let action = expression.run(context, _clear_variables)?;
             let value = if let Action::Return(value) = action {
                 value
             } else {
@@ -131,7 +133,7 @@ impl AbstractNode for FunctionCall {
             arguments.push(value);
         }
 
-        let function_context = Context::new();
+        let mut function_context = Context::new();
 
         for (type_parameter, type_argument) in function
             .type_parameters()
@@ -145,6 +147,6 @@ impl AbstractNode for FunctionCall {
         }
 
         function_context.inherit_data_from(&context)?;
-        function.clone().call(arguments, function_context)
+        function.clone().call(arguments, &mut function_context)
     }
 }
diff --git a/dust-lang/src/abstract_tree/if_else.rs b/dust-lang/src/abstract_tree/if_else.rs
index 449afeb..559d8e7 100644
--- a/dust-lang/src/abstract_tree/if_else.rs
+++ b/dust-lang/src/abstract_tree/if_else.rs
@@ -89,9 +89,9 @@ impl AbstractNode for IfElse {
         Ok(())
     }
 
-    fn run(self, context: &Context) -> Result<Action, RuntimeError> {
+    fn run(self, context: &mut Context, _clear_variables: bool) -> Result<Action, RuntimeError> {
         let if_position = self.if_expression.position();
-        let action = self.if_expression.run(context)?;
+        let action = self.if_expression.run(context, _clear_variables)?;
         let value = if let Action::Return(value) = action {
             value
         } else {
@@ -102,11 +102,11 @@ impl AbstractNode for IfElse {
 
         if let ValueInner::Boolean(if_boolean) = value.inner().as_ref() {
             if *if_boolean {
-                self.if_block.node.run(context)
+                self.if_block.node.run(context, _clear_variables)
             } else {
                 for (expression, block) in self.else_ifs {
                     let expression_position = expression.position();
-                    let action = expression.run(context)?;
+                    let action = expression.run(context, _clear_variables)?;
                     let value = if let Action::Return(value) = action {
                         value
                     } else {
@@ -117,7 +117,7 @@ impl AbstractNode for IfElse {
 
                     if let ValueInner::Boolean(else_if_boolean) = value.inner().as_ref() {
                         if *else_if_boolean {
-                            return block.node.run(context);
+                            return block.node.run(context, _clear_variables);
                         }
                     } else {
                         return Err(RuntimeError::ValidationFailure(
@@ -130,7 +130,7 @@ impl AbstractNode for IfElse {
                 }
 
                 if let Some(else_statement) = self.else_block {
-                    else_statement.node.run(context)
+                    else_statement.node.run(context, _clear_variables)
                 } else {
                     Ok(Action::None)
                 }
@@ -167,7 +167,7 @@ mod tests {
                 Vec::with_capacity(0),
                 None
             )
-            .run(&Context::new())
+            .run(&mut Context::new(), true)
             .unwrap(),
             Action::Return(Value::string("foo".to_string()))
         )
diff --git a/dust-lang/src/abstract_tree/list_index.rs b/dust-lang/src/abstract_tree/list_index.rs
index 236d079..5fb7ed1 100644
--- a/dust-lang/src/abstract_tree/list_index.rs
+++ b/dust-lang/src/abstract_tree/list_index.rs
@@ -74,9 +74,9 @@ impl AbstractNode for ListIndex {
         }
     }
 
-    fn run(self, context: &Context) -> Result<Action, RuntimeError> {
+    fn run(self, context: &mut Context, _clear_variables: bool) -> Result<Action, RuntimeError> {
         let left_position = self.left.position();
-        let left_action = self.left.run(context)?;
+        let left_action = self.left.run(context, _clear_variables)?;
         let left_value = if let Action::Return(value) = left_action {
             value
         } else {
@@ -85,7 +85,7 @@ impl AbstractNode for ListIndex {
             ));
         };
         let right_position = self.right.position();
-        let right_action = self.right.run(context)?;
+        let right_action = self.right.run(context, _clear_variables)?;
         let right_value = if let Action::Return(value) = right_action {
             value
         } else {
diff --git a/dust-lang/src/abstract_tree/logic.rs b/dust-lang/src/abstract_tree/logic.rs
index 137e284..07869f7 100644
--- a/dust-lang/src/abstract_tree/logic.rs
+++ b/dust-lang/src/abstract_tree/logic.rs
@@ -83,10 +83,10 @@ impl AbstractNode for Logic {
         }
     }
 
-    fn run(self, context: &Context) -> Result<Action, RuntimeError> {
+    fn run(self, context: &mut Context, _clear_variables: bool) -> Result<Action, RuntimeError> {
         let run_and_expect_value = |expression: Expression| -> Result<Value, RuntimeError> {
             let expression_position = expression.position();
-            let action = expression.run(context)?;
+            let action = expression.run(&mut context.clone(), _clear_variables)?;
             let value = if let Action::Return(value) = action {
                 value
             } else {
@@ -100,7 +100,7 @@ impl AbstractNode for Logic {
 
         let run_and_expect_boolean = |expression: Expression| -> Result<bool, RuntimeError> {
             let expression_position = expression.position();
-            let action = expression.run(context)?;
+            let action = expression.run(&mut context.clone(), _clear_variables)?;
             let value = if let Action::Return(value) = action {
                 value
             } else {
@@ -198,7 +198,7 @@ mod tests {
                 Expression::Value(ValueNode::Integer(42).with_position((0, 0))),
                 Expression::Value(ValueNode::Integer(42).with_position((0, 0)))
             )
-            .run(&Context::new()),
+            .run(&mut Context::new(), true),
             Ok(Action::Return(Value::boolean(true)))
         )
     }
@@ -210,7 +210,7 @@ mod tests {
                 Expression::Value(ValueNode::Integer(42).with_position((0, 0))),
                 Expression::Value(ValueNode::Integer(43).with_position((0, 0)))
             )
-            .run(&Context::new()),
+            .run(&mut Context::new(), true),
             Ok(Action::Return(Value::boolean(true)))
         )
     }
@@ -222,7 +222,7 @@ mod tests {
                 Expression::Value(ValueNode::Integer(43).with_position((0, 0))),
                 Expression::Value(ValueNode::Integer(42).with_position((0, 0)))
             )
-            .run(&Context::new()),
+            .run(&mut Context::new(), true),
             Ok(Action::Return(Value::boolean(true)))
         )
     }
@@ -234,7 +234,7 @@ mod tests {
                 Expression::Value(ValueNode::Integer(42).with_position((0, 0))),
                 Expression::Value(ValueNode::Integer(43).with_position((0, 0)))
             )
-            .run(&Context::new()),
+            .run(&mut Context::new(), true),
             Ok(Action::Return(Value::boolean(true)))
         )
     }
@@ -246,7 +246,7 @@ mod tests {
                 Expression::Value(ValueNode::Integer(42).with_position((0, 0))),
                 Expression::Value(ValueNode::Integer(41).with_position((0, 0)))
             )
-            .run(&Context::new()),
+            .run(&mut Context::new(), true),
             Ok(Action::Return(Value::boolean(true)))
         );
 
@@ -255,7 +255,7 @@ mod tests {
                 Expression::Value(ValueNode::Integer(42).with_position((0, 0))),
                 Expression::Value(ValueNode::Integer(42).with_position((0, 0))),
             )
-            .run(&Context::new()),
+            .run(&mut Context::new(), true),
             Ok(Action::Return(Value::boolean(true)))
         );
     }
@@ -267,7 +267,7 @@ mod tests {
                 Expression::Value(ValueNode::Integer(41).with_position((0, 0))),
                 Expression::Value(ValueNode::Integer(42).with_position((0, 0))),
             )
-            .run(&Context::new()),
+            .run(&mut Context::new(), true),
             Ok(Action::Return(Value::boolean(true)))
         );
 
@@ -276,7 +276,7 @@ mod tests {
                 Expression::Value(ValueNode::Integer(42).with_position((0, 0))),
                 Expression::Value(ValueNode::Integer(42).with_position((0, 0))),
             )
-            .run(&Context::new()),
+            .run(&mut Context::new(), true),
             Ok(Action::Return(Value::boolean(true)))
         );
     }
@@ -288,7 +288,7 @@ mod tests {
                 Expression::Value(ValueNode::Boolean(true).with_position((0, 0))),
                 Expression::Value(ValueNode::Boolean(true).with_position((0, 0))),
             )
-            .run(&Context::new()),
+            .run(&mut Context::new(), true),
             Ok(Action::Return(Value::boolean(true)))
         )
     }
@@ -300,7 +300,7 @@ mod tests {
                 Expression::Value(ValueNode::Boolean(true).with_position((0, 0))),
                 Expression::Value(ValueNode::Boolean(false).with_position((0, 0))),
             )
-            .run(&Context::new()),
+            .run(&mut Context::new(), true),
             Ok(Action::Return(Value::boolean(true)))
         )
     }
@@ -311,7 +311,7 @@ mod tests {
             Logic::Not(Expression::Value(
                 ValueNode::Boolean(false).with_position((0, 0))
             ))
-            .run(&Context::new()),
+            .run(&mut Context::new(), true),
             Ok(Action::Return(Value::boolean(true)))
         )
     }
diff --git a/dust-lang/src/abstract_tree/loop.rs b/dust-lang/src/abstract_tree/loop.rs
index b15139c..176a106 100644
--- a/dust-lang/src/abstract_tree/loop.rs
+++ b/dust-lang/src/abstract_tree/loop.rs
@@ -31,10 +31,10 @@ impl AbstractNode for Loop {
         Ok(())
     }
 
-    fn run(self, _context: &Context) -> Result<Action, RuntimeError> {
+    fn run(self, _context: &mut Context, _clear_variables: bool) -> Result<Action, RuntimeError> {
         loop {
             for statement in &self.statements {
-                let action = statement.clone().run(_context)?;
+                let action = statement.clone().run(_context, _clear_variables)?;
 
                 match action {
                     Action::Return(_) => {}
@@ -65,81 +65,3 @@ impl Ord for Loop {
         self.statements.cmp(&other.statements)
     }
 }
-
-#[cfg(test)]
-mod tests {
-    use crate::{
-        abstract_tree::{
-            Assignment, AssignmentOperator, Block, Expression, IfElse, Logic, ValueNode, WithPos,
-        },
-        identifier::Identifier,
-        Value,
-    };
-
-    use super::*;
-
-    #[test]
-    fn basic_loop() {
-        let action = Loop::new(vec![Statement::Break(().with_position((0, 0)))])
-            .run(&Context::new())
-            .unwrap();
-
-        assert_eq!(action, Action::Break)
-    }
-
-    #[test]
-    fn complex_loop() {
-        let action = Block::new(vec![
-            Statement::Assignment(
-                Assignment::new(
-                    Identifier::new("i").with_position((0, 0)),
-                    None,
-                    AssignmentOperator::Assign,
-                    Statement::Expression(Expression::Value(
-                        ValueNode::Integer(1).with_position((0, 0)),
-                    )),
-                )
-                .with_position((0, 0)),
-            ),
-            Statement::Loop(
-                Loop::new(vec![Statement::IfElse(
-                    IfElse::new(
-                        Expression::Logic(
-                            Box::new(Logic::Greater(
-                                Expression::Identifier(Identifier::new("i").with_position((0, 0))),
-                                Expression::Value(ValueNode::Integer(2).with_position((0, 0))),
-                            ))
-                            .with_position((0, 0)),
-                        ),
-                        Block::new(vec![Statement::Break(().with_position((0, 0)))])
-                            .with_position((0, 0)),
-                        Vec::with_capacity(0),
-                        Some(
-                            Block::new(vec![Statement::Assignment(
-                                Assignment::new(
-                                    Identifier::new("i").with_position((0, 0)),
-                                    None,
-                                    AssignmentOperator::AddAssign,
-                                    Statement::Expression(Expression::Value(
-                                        ValueNode::Integer(1).with_position((0, 0)),
-                                    )),
-                                )
-                                .with_position((0, 0)),
-                            )])
-                            .with_position((0, 0)),
-                        ),
-                    )
-                    .with_position((0, 0)),
-                )])
-                .with_position((0, 0)),
-            ),
-            Statement::Expression(Expression::Identifier(
-                Identifier::new("i").with_position((0, 0)),
-            )),
-        ])
-        .run(&Context::new())
-        .unwrap();
-
-        assert_eq!(action, Action::Return(Value::integer(3)))
-    }
-}
diff --git a/dust-lang/src/abstract_tree/map_index.rs b/dust-lang/src/abstract_tree/map_index.rs
index fcc6c90..ae5989b 100644
--- a/dust-lang/src/abstract_tree/map_index.rs
+++ b/dust-lang/src/abstract_tree/map_index.rs
@@ -1,6 +1,7 @@
 use crate::{
     context::Context,
     error::{RuntimeError, ValidationError},
+    identifier::Identifier,
     value::ValueInner,
 };
 
@@ -8,25 +9,26 @@ use super::{AbstractNode, Action, Expression, Type, ValueNode, WithPosition};
 
 #[derive(Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
 pub struct MapIndex {
-    left: Expression,
-    right: Expression,
+    collection: Expression,
+    index: WithPosition<Identifier>,
 }
 
 impl MapIndex {
-    pub fn new(left: Expression, right: Expression) -> Self {
-        Self { left, right }
+    pub fn new(left: Expression, right: WithPosition<Identifier>) -> Self {
+        Self {
+            collection: left,
+            index: right,
+        }
     }
 }
 
 impl AbstractNode for MapIndex {
     fn expected_type(&self, context: &Context) -> Result<Type, ValidationError> {
-        if let (
-            Expression::Identifier(collection_identifier),
-            Expression::Identifier(index_identifier),
-        ) = (&self.left, &self.right)
+        if let (Expression::Identifier(collection_identifier), index) =
+            (&self.collection, &self.index)
         {
             let collection =
-                if let Some(collection) = context.get_value(&collection_identifier.node)? {
+                if let Some(collection) = context.use_value(&collection_identifier.node)? {
                     collection
                 } else {
                     return Err(ValidationError::VariableNotFound {
@@ -36,12 +38,12 @@ impl AbstractNode for MapIndex {
                 };
 
             if let ValueInner::Map(map) = collection.inner().as_ref() {
-                return if let Some(value) = map.get(&index_identifier.node) {
+                return if let Some(value) = map.get(&index.node) {
                     Ok(value.r#type(context)?)
                 } else {
                     Err(ValidationError::PropertyNotFound {
-                        identifier: index_identifier.node.clone(),
-                        position: self.right.position(),
+                        identifier: index.node.clone(),
+                        position: index.position,
                     })
                 };
             };
@@ -52,14 +54,14 @@ impl AbstractNode for MapIndex {
                 node: ValueNode::Map(properties),
                 ..
             }),
-            Expression::Identifier(identifier),
-        ) = (&self.left, &self.right)
+            index,
+        ) = (&self.collection, &self.index)
         {
             return if let Some(type_result) =
                 properties
                     .iter()
                     .find_map(|(property, type_option, expression)| {
-                        if property == &identifier.node {
+                        if property == &index.node {
                             if let Some(r#type) = type_option {
                                 Some(r#type.node.expected_type(context))
                             } else {
@@ -81,11 +83,11 @@ impl AbstractNode for MapIndex {
                 node: ValueNode::Structure { fields, .. },
                 ..
             }),
-            Expression::Identifier(identifier),
-        ) = (&self.left, &self.right)
+            index,
+        ) = (&self.collection, &self.index)
         {
             return if let Some(type_result) = fields.iter().find_map(|(property, expression)| {
-                if property == &identifier.node {
+                if property == &index.node {
                     Some(expression.expected_type(context))
                 } else {
                     None
@@ -97,43 +99,19 @@ impl AbstractNode for MapIndex {
             };
         }
 
-        Err(ValidationError::CannotIndexWith {
-            collection_type: self.left.expected_type(context)?,
-            collection_position: self.left.position(),
-            index_type: self.right.expected_type(context)?,
-            index_position: self.right.position(),
+        Err(ValidationError::CannotIndex {
+            r#type: self.collection.expected_type(context)?,
+            position: self.collection.position(),
         })
     }
 
     fn validate(&self, context: &Context) -> Result<(), ValidationError> {
-        let left_type = self.left.expected_type(context)?;
-
-        if let (
-            Expression::Value(WithPosition {
-                node: ValueNode::Map(_),
-                ..
-            }),
-            Expression::Identifier(_),
-        ) = (&self.left, &self.right)
-        {
-            Ok(())
-        } else if let (Expression::Identifier(_), Expression::Identifier(_)) =
-            (&self.left, &self.right)
-        {
-            Ok(())
-        } else {
-            Err(ValidationError::CannotIndexWith {
-                collection_type: left_type,
-                collection_position: self.left.position(),
-                index_type: self.right.expected_type(context)?,
-                index_position: self.right.position(),
-            })
-        }
+        self.collection.validate(context)
     }
 
-    fn run(self, context: &Context) -> Result<Action, RuntimeError> {
-        let collection_position = self.left.position();
-        let action = self.left.run(context)?;
+    fn run(self, context: &mut Context, _clear_variables: bool) -> Result<Action, RuntimeError> {
+        let collection_position = self.collection.position();
+        let action = self.collection.run(context, _clear_variables)?;
         let collection = if let Action::Return(value) = action {
             value
         } else {
@@ -142,22 +120,18 @@ impl AbstractNode for MapIndex {
             ));
         };
 
-        if let (ValueInner::Map(map), Expression::Identifier(identifier)) =
-            (collection.inner().as_ref(), &self.right)
-        {
+        if let ValueInner::Map(map) = collection.inner().as_ref() {
             let action = map
-                .get(&identifier.node)
+                .get(&self.index.node)
                 .map(|value| Action::Return(value.clone()))
                 .unwrap_or(Action::None);
 
             Ok(action)
         } else {
             Err(RuntimeError::ValidationFailure(
-                ValidationError::CannotIndexWith {
-                    collection_type: collection.r#type(context)?,
-                    collection_position,
-                    index_type: self.right.expected_type(context)?,
-                    index_position: self.right.position(),
+                ValidationError::CannotIndex {
+                    r#type: collection.r#type(context)?,
+                    position: collection_position,
                 },
             ))
         }
diff --git a/dust-lang/src/abstract_tree/math.rs b/dust-lang/src/abstract_tree/math.rs
index b40f6a6..96223a1 100644
--- a/dust-lang/src/abstract_tree/math.rs
+++ b/dust-lang/src/abstract_tree/math.rs
@@ -88,10 +88,10 @@ impl AbstractNode for Math {
         }
     }
 
-    fn run(self, _context: &Context) -> Result<Action, RuntimeError> {
+    fn run(self, _context: &mut Context, _clear_variables: bool) -> Result<Action, RuntimeError> {
         let run_and_expect_value =
             |position: SourcePosition, expression: Expression| -> Result<Value, RuntimeError> {
-                let action = expression.run(_context)?;
+                let action = expression.run(&mut _context.clone(), _clear_variables)?;
                 let value = if let Action::Return(value) = action {
                     value
                 } else {
diff --git a/dust-lang/src/abstract_tree/mod.rs b/dust-lang/src/abstract_tree/mod.rs
index 288ab87..f98d601 100644
--- a/dust-lang/src/abstract_tree/mod.rs
+++ b/dust-lang/src/abstract_tree/mod.rs
@@ -98,13 +98,17 @@ impl AbstractTree {
         AbstractTree(statements)
     }
 
-    pub fn run(self, context: &Context) -> Result<Option<Value>, Vec<Error>> {
+    pub fn run(
+        self,
+        context: &mut Context,
+        clear_variables: bool,
+    ) -> Result<Option<Value>, Vec<Error>> {
         let valid_statements = self.validate(context)?;
         let mut previous_value = None;
 
         for statement in valid_statements {
             let position = statement.position();
-            let run = statement.run(context);
+            let run = statement.run(context, clear_variables);
 
             match run {
                 Ok(action) => match action {
@@ -124,7 +128,7 @@ impl AbstractTree {
         Ok(previous_value)
     }
 
-    fn validate(self, context: &Context) -> Result<Vec<Statement>, Vec<Error>> {
+    fn validate(self, context: &mut Context) -> Result<Vec<Statement>, Vec<Error>> {
         let mut errors = Vec::new();
         let mut valid_statements = Vec::new();
 
@@ -139,7 +143,7 @@ impl AbstractTree {
             } else if errors.is_empty() {
                 if let Statement::StructureDefinition(_) = statement {
                     let position = statement.position();
-                    let run = statement.run(context);
+                    let run = statement.run(context, true);
 
                     if let Err(runtime_error) = run {
                         errors.push(Error::Runtime {
@@ -176,5 +180,5 @@ impl Index<usize> for AbstractTree {
 pub trait AbstractNode: Sized {
     fn expected_type(&self, context: &Context) -> Result<Type, ValidationError>;
     fn validate(&self, context: &Context) -> Result<(), ValidationError>;
-    fn run(self, context: &Context) -> Result<Action, RuntimeError>;
+    fn run(self, context: &mut Context, clear_variables: bool) -> Result<Action, RuntimeError>;
 }
diff --git a/dust-lang/src/abstract_tree/statement.rs b/dust-lang/src/abstract_tree/statement.rs
index fc5c5b4..b38acf1 100644
--- a/dust-lang/src/abstract_tree/statement.rs
+++ b/dust-lang/src/abstract_tree/statement.rs
@@ -70,19 +70,25 @@ impl AbstractNode for Statement {
         }
     }
 
-    fn run(self, _context: &Context) -> Result<Action, RuntimeError> {
-        match self {
-            Statement::Assignment(assignment) => assignment.node.run(_context),
-            Statement::AsyncBlock(async_block) => async_block.node.run(_context),
-            Statement::Block(block) => block.node.run(_context),
+    fn run(self, context: &mut Context, clear_variables: bool) -> Result<Action, RuntimeError> {
+        let result = match self {
+            Statement::Assignment(assignment) => assignment.node.run(context, clear_variables),
+            Statement::AsyncBlock(async_block) => async_block.node.run(context, clear_variables),
+            Statement::Block(block) => block.node.run(context, clear_variables),
             Statement::Break(_) => Ok(Action::Break),
-            Statement::Expression(expression) => expression.run(_context),
-            Statement::IfElse(if_else) => if_else.node.run(_context),
-            Statement::Loop(r#loop) => r#loop.node.run(_context),
-            Statement::While(r#while) => r#while.node.run(_context),
+            Statement::Expression(expression) => expression.run(context, clear_variables),
+            Statement::IfElse(if_else) => if_else.node.run(context, clear_variables),
+            Statement::Loop(r#loop) => r#loop.node.run(context, clear_variables),
+            Statement::While(r#while) => r#while.node.run(context, clear_variables),
             Statement::StructureDefinition(structure_definition) => {
-                structure_definition.node.run(_context)
+                structure_definition.node.run(context, clear_variables)
             }
+        };
+
+        if clear_variables {
+            context.clean()?;
         }
+
+        result
     }
 }
diff --git a/dust-lang/src/abstract_tree/structure_definition.rs b/dust-lang/src/abstract_tree/structure_definition.rs
index 91f0a25..18fb543 100644
--- a/dust-lang/src/abstract_tree/structure_definition.rs
+++ b/dust-lang/src/abstract_tree/structure_definition.rs
@@ -27,7 +27,7 @@ impl AbstractNode for StructureDefinition {
         Ok(())
     }
 
-    fn run(self, context: &Context) -> Result<Action, RuntimeError> {
+    fn run(self, context: &mut Context, _clear_variables: bool) -> Result<Action, RuntimeError> {
         let struct_type = Type::Structure {
             name: self.name.clone(),
             fields: self.fields,
diff --git a/dust-lang/src/abstract_tree/type.rs b/dust-lang/src/abstract_tree/type.rs
index 5cb0b77..fccc719 100644
--- a/dust-lang/src/abstract_tree/type.rs
+++ b/dust-lang/src/abstract_tree/type.rs
@@ -152,7 +152,7 @@ impl AbstractNode for Type {
         Ok(())
     }
 
-    fn run(self, _: &Context) -> Result<Action, RuntimeError> {
+    fn run(self, _context: &mut Context, _clear_variables: bool) -> Result<Action, RuntimeError> {
         Ok(Action::None)
     }
 }
diff --git a/dust-lang/src/abstract_tree/value_node.rs b/dust-lang/src/abstract_tree/value_node.rs
index 3308ad8..f855ebf 100644
--- a/dust-lang/src/abstract_tree/value_node.rs
+++ b/dust-lang/src/abstract_tree/value_node.rs
@@ -181,7 +181,7 @@ impl AbstractNode for ValueNode {
         Ok(())
     }
 
-    fn run(self, _context: &Context) -> Result<Action, RuntimeError> {
+    fn run(self, _context: &mut Context, _clear_variables: bool) -> Result<Action, RuntimeError> {
         let value = match self {
             ValueNode::Boolean(boolean) => Value::boolean(boolean),
             ValueNode::Float(float) => Value::float(float),
@@ -191,7 +191,7 @@ impl AbstractNode for ValueNode {
 
                 for expression in expression_list {
                     let expression_position = expression.position();
-                    let action = expression.run(_context)?;
+                    let action = expression.run(_context, _clear_variables)?;
                     let value = if let Action::Return(value) = action {
                         WithPosition {
                             node: value,
@@ -213,7 +213,7 @@ impl AbstractNode for ValueNode {
 
                 for (identifier, _type, expression) in property_list {
                     let expression_position = expression.position();
-                    let action = expression.run(_context)?;
+                    let action = expression.run(_context, _clear_variables)?;
                     let value = if let Action::Return(value) = action {
                         value
                     } else {
@@ -243,7 +243,7 @@ impl AbstractNode for ValueNode {
 
                 for (identifier, expression) in expressions {
                     let expression_position = expression.position();
-                    let action = expression.run(_context)?;
+                    let action = expression.run(_context, _clear_variables)?;
                     let value = if let Action::Return(value) = action {
                         value
                     } else {
diff --git a/dust-lang/src/abstract_tree/while.rs b/dust-lang/src/abstract_tree/while.rs
index 602bc8e..472eb7c 100644
--- a/dust-lang/src/abstract_tree/while.rs
+++ b/dust-lang/src/abstract_tree/while.rs
@@ -37,10 +37,13 @@ impl AbstractNode for While {
         Ok(())
     }
 
-    fn run(self, _context: &Context) -> Result<Action, RuntimeError> {
+    fn run(self, _context: &mut Context, _clear_variables: bool) -> Result<Action, RuntimeError> {
         let get_boolean = || -> Result<Value, RuntimeError> {
             let expression_position = self.expression.position();
-            let action = self.expression.clone().run(_context)?;
+            let action = self
+                .expression
+                .clone()
+                .run(&mut _context.clone(), _clear_variables)?;
 
             if let Action::Return(value) = action {
                 Ok(value)
@@ -53,7 +56,9 @@ impl AbstractNode for While {
 
         while let ValueInner::Boolean(true) = get_boolean()?.inner().as_ref() {
             for statement in &self.statements {
-                let action = statement.clone().run(_context)?;
+                let action = statement
+                    .clone()
+                    .run(&mut _context.clone(), _clear_variables)?;
 
                 match action {
                     Action::Return(_) => {}
@@ -66,63 +71,3 @@ impl AbstractNode for While {
         Ok(Action::None)
     }
 }
-
-#[cfg(test)]
-mod tests {
-    use crate::{
-        abstract_tree::{Assignment, AssignmentOperator, Block, Logic, ValueNode, WithPos},
-        identifier::Identifier,
-    };
-
-    use super::*;
-
-    #[test]
-    fn simple_while_loop() {
-        let action = Statement::Block(
-            Block::new(vec![
-                Statement::Assignment(
-                    Assignment::new(
-                        Identifier::new("i").with_position((0, 0)),
-                        None,
-                        AssignmentOperator::Assign,
-                        Statement::Expression(Expression::Value(
-                            ValueNode::Integer(3).with_position((0, 0)),
-                        )),
-                    )
-                    .with_position((0, 0)),
-                ),
-                Statement::While(
-                    While::new(
-                        Expression::Logic(
-                            Box::new(Logic::Less(
-                                Expression::Identifier(Identifier::new("i").with_position((0, 0))),
-                                Expression::Value(ValueNode::Integer(3).with_position((0, 0))),
-                            ))
-                            .with_position((0, 0)),
-                        ),
-                        vec![Statement::Assignment(
-                            Assignment::new(
-                                Identifier::new("i").with_position((0, 0)),
-                                None,
-                                AssignmentOperator::AddAssign,
-                                Statement::Expression(Expression::Value(
-                                    ValueNode::Integer(1).with_position((0, 0)),
-                                )),
-                            )
-                            .with_position((0, 0)),
-                        )],
-                    )
-                    .with_position((0, 0)),
-                ),
-                Statement::Expression(Expression::Identifier(
-                    Identifier::new("i").with_position((0, 0)),
-                )),
-            ])
-            .with_position((0, 0)),
-        )
-        .run(&Context::new())
-        .unwrap();
-
-        assert_eq!(action, Action::Return(Value::integer(3)))
-    }
-}
diff --git a/dust-lang/src/context.rs b/dust-lang/src/context.rs
index c3d01bd..af095bf 100644
--- a/dust-lang/src/context.rs
+++ b/dust-lang/src/context.rs
@@ -3,6 +3,8 @@ use std::{
     sync::{Arc, RwLock, RwLockReadGuard},
 };
 
+use rayon::iter::{IntoParallelIterator, ParallelIterator};
+
 use crate::{
     abstract_tree::Type,
     error::{RwLockPoisonError, ValidationError},
@@ -12,7 +14,7 @@ use crate::{
 
 #[derive(Clone, Debug)]
 pub struct Context {
-    inner: Arc<RwLock<BTreeMap<Identifier, ValueData>>>,
+    inner: Arc<RwLock<BTreeMap<Identifier, (ValueData, UsageData)>>>,
 }
 
 impl Context {
@@ -22,7 +24,7 @@ impl Context {
         }
     }
 
-    pub fn with_data(data: BTreeMap<Identifier, ValueData>) -> Self {
+    pub fn with_data(data: BTreeMap<Identifier, (ValueData, UsageData)>) -> Self {
         Self {
             inner: Arc::new(RwLock::new(data)),
         }
@@ -30,16 +32,17 @@ impl Context {
 
     pub fn inner(
         &self,
-    ) -> Result<RwLockReadGuard<BTreeMap<Identifier, ValueData>>, RwLockPoisonError> {
+    ) -> Result<RwLockReadGuard<BTreeMap<Identifier, (ValueData, UsageData)>>, RwLockPoisonError>
+    {
         Ok(self.inner.read()?)
     }
 
     pub fn inherit_types_from(&self, other: &Context) -> Result<(), RwLockPoisonError> {
         let mut self_data = self.inner.write()?;
 
-        for (identifier, value_data) in other.inner.read()?.iter() {
+        for (identifier, (value_data, usage_data)) in other.inner.read()?.iter() {
             if let ValueData::Type(Type::Function { .. }) = value_data {
-                self_data.insert(identifier.clone(), value_data.clone());
+                self_data.insert(identifier.clone(), (value_data.clone(), usage_data.clone()));
             }
         }
 
@@ -49,8 +52,8 @@ impl Context {
     pub fn inherit_data_from(&self, other: &Context) -> Result<(), RwLockPoisonError> {
         let mut self_data = self.inner.write()?;
 
-        for (identifier, value_data) in other.inner.read()?.iter() {
-            self_data.insert(identifier.clone(), value_data.clone());
+        for (identifier, (value_data, usage_data)) in other.inner.read()?.iter() {
+            self_data.insert(identifier.clone(), (value_data.clone(), usage_data.clone()));
         }
 
         Ok(())
@@ -61,7 +64,9 @@ impl Context {
     }
 
     pub fn get_type(&self, identifier: &Identifier) -> Result<Option<Type>, ValidationError> {
-        if let Some(value_data) = self.inner.read()?.get(identifier) {
+        if let Some((value_data, _)) = self.inner.read()?.get(identifier) {
+            log::trace!("Using {identifier}'s type.");
+
             let r#type = match value_data {
                 ValueData::Type(r#type) => r#type.clone(),
                 ValueData::Value(value) => value.r#type(self)?,
@@ -73,8 +78,13 @@ impl Context {
         }
     }
 
-    pub fn get_value(&self, identifier: &Identifier) -> Result<Option<Value>, RwLockPoisonError> {
-        if let Some(ValueData::Value(value)) = self.inner.read()?.get(identifier) {
+    pub fn use_value(&self, identifier: &Identifier) -> Result<Option<Value>, RwLockPoisonError> {
+        if let Some((ValueData::Value(value), usage_data)) = self.inner.write()?.get_mut(identifier)
+        {
+            log::trace!("Using {identifier}'s value.");
+
+            usage_data.actual += 1;
+
             Ok(Some(value.clone()))
         } else {
             Ok(None)
@@ -82,26 +92,75 @@ impl Context {
     }
 
     pub fn set_type(&self, identifier: Identifier, r#type: Type) -> Result<(), RwLockPoisonError> {
+        log::info!("Setting {identifier} to type {}.", r#type);
+
         self.inner
             .write()?
-            .insert(identifier, ValueData::Type(r#type));
+            .insert(identifier, (ValueData::Type(r#type), UsageData::new()));
 
         Ok(())
     }
 
     pub fn set_value(&self, identifier: Identifier, value: Value) -> Result<(), RwLockPoisonError> {
-        self.inner
-            .write()?
-            .insert(identifier, ValueData::Value(value));
+        log::info!("Setting {identifier} to value {value}.");
+
+        let mut inner = self.inner.write()?;
+        let old_usage_data = inner.remove(&identifier).map(|(_, usage_data)| usage_data);
+
+        if let Some(usage_data) = old_usage_data {
+            inner.insert(identifier, (ValueData::Value(value), usage_data));
+        } else {
+            inner.insert(identifier, (ValueData::Value(value), UsageData::new()));
+        }
 
         Ok(())
     }
 
     pub fn remove(&self, identifier: &Identifier) -> Result<Option<ValueData>, RwLockPoisonError> {
-        let removed = self.inner.write()?.remove(identifier);
+        let removed = self
+            .inner
+            .write()?
+            .remove(identifier)
+            .map(|(value_data, _)| value_data);
 
         Ok(removed)
     }
+
+    pub fn clean(&mut self) -> Result<(), RwLockPoisonError> {
+        let clean_variables = self
+            .inner
+            .write()?
+            .clone()
+            .into_par_iter()
+            .filter(|(identifier, (_, usage_data))| {
+                if usage_data.actual < usage_data.expected {
+                    true
+                } else {
+                    log::debug!("Removing variable {identifier}.");
+
+                    false
+                }
+            })
+            .collect();
+
+        self.inner = Arc::new(RwLock::new(clean_variables));
+
+        Ok(())
+    }
+
+    pub fn add_expected_use(&self, identifier: &Identifier) -> Result<bool, RwLockPoisonError> {
+        let mut inner = self.inner.write()?;
+
+        if let Some((_, usage_data)) = inner.get_mut(identifier) {
+            log::trace!("Adding expected use for variable {identifier}.");
+
+            usage_data.expected += 1;
+
+            Ok(true)
+        } else {
+            Ok(false)
+        }
+    }
 }
 
 #[derive(Clone, Debug, PartialEq)]
@@ -109,3 +168,18 @@ pub enum ValueData {
     Type(Type),
     Value(Value),
 }
+
+#[derive(Clone, Debug)]
+pub struct UsageData {
+    pub actual: u32,
+    pub expected: u32,
+}
+
+impl UsageData {
+    pub fn new() -> Self {
+        Self {
+            actual: 0,
+            expected: 0,
+        }
+    }
+}
diff --git a/dust-lang/src/lib.rs b/dust-lang/src/lib.rs
index c6d10f3..ded5f90 100644
--- a/dust-lang/src/lib.rs
+++ b/dust-lang/src/lib.rs
@@ -33,7 +33,7 @@ pub fn interpret_without_std(
     source_id: &str,
     source: &str,
 ) -> Result<Option<Value>, InterpreterError> {
-    let interpreter = Interpreter::new(Context::new());
+    let mut interpreter = Interpreter::new(Context::new());
 
     interpreter.run(Arc::from(source_id.to_string()), Arc::from(source))
 }
@@ -52,7 +52,7 @@ impl Interpreter {
     }
 
     pub fn run(
-        &self,
+        &mut self,
         source_id: Arc<str>,
         source: Arc<str>,
     ) -> Result<Option<Value>, InterpreterError> {
@@ -70,7 +70,7 @@ impl Interpreter {
             errors,
         })?;
         let value_option = abstract_tree
-            .run(&self.context)
+            .run(&mut self.context, true)
             .map_err(|errors| InterpreterError { source_id, errors })?;
 
         Ok(value_option)
@@ -134,7 +134,7 @@ impl Interpreter {
                 };
 
                 abstract_tree
-                    .run(&self.context)
+                    .run(&mut self.context.clone(), false)
                     .map_err(|errors| InterpreterError { source_id, errors })
                     .err()
             });
diff --git a/dust-lang/src/parser.rs b/dust-lang/src/parser.rs
index fbfbbaf..ad1d593 100644
--- a/dust-lang/src/parser.rs
+++ b/dust-lang/src/parser.rs
@@ -322,13 +322,29 @@ pub fn parser<'src>(
                     just(Token::Control(Control::DoubleColon)),
                 );
 
-            let atom = choice((
-                range.clone(),
+            let map_atom = choice((
+                map.clone(),
                 structure_instance.clone(),
+                identifier_expression.clone(),
+            ));
+
+            let map_index = map_atom
+                .then_ignore(just(Token::Control(Control::Dot)))
+                .then(positioned_identifier.clone())
+                .map_with(|(expression, identifier), state| {
+                    Expression::MapIndex(
+                        Box::new(MapIndex::new(expression, identifier)).with_position(state.span()),
+                    )
+                });
+
+            let atom = choice((
+                map_index.clone(),
+                range.clone(),
                 parsed_function.clone(),
                 list.clone(),
-                map.clone(),
                 basic_value.clone(),
+                map.clone(),
+                structure_instance.clone(),
                 identifier_expression.clone(),
                 expression.clone().delimited_by(
                     just(Token::Control(Control::ParenOpen)),
@@ -384,15 +400,6 @@ pub fn parser<'src>(
                         )
                     },
                 ),
-                infix(
-                    left(4),
-                    just(Token::Control(Control::Dot)),
-                    |left, _, right, span| {
-                        Expression::MapIndex(
-                            Box::new(MapIndex::new(left, right)).with_position(span),
-                        )
-                    },
-                ),
                 infix(
                     left(1),
                     just(Token::Operator(Operator::Equal)),
@@ -493,8 +500,9 @@ pub fn parser<'src>(
             ));
 
             choice((
-                built_in_function_call,
                 logic_math_indexes_and_function_calls,
+                map_index,
+                built_in_function_call,
                 range,
                 structure_instance,
                 parsed_function,
@@ -765,7 +773,7 @@ mod tests {
                         )])
                         .with_position((0, 10))
                     ),
-                    Expression::Identifier(Identifier::new("x").with_position((11, 12)))
+                    Identifier::new("x").with_position((11, 12))
                 ))
                 .with_position((0, 12))
             ))
@@ -775,7 +783,7 @@ mod tests {
             Statement::Expression(Expression::MapIndex(
                 Box::new(MapIndex::new(
                     Expression::Identifier(Identifier::new("foo").with_position((0, 3))),
-                    Expression::Identifier(Identifier::new("x").with_position((4, 5)))
+                    Identifier::new("x").with_position((4, 5))
                 ))
                 .with_position((0, 5))
             ))
@@ -924,22 +932,14 @@ mod tests {
     #[test]
     fn function_call() {
         assert_eq!(
-            parse(&lex("io.read_line()").unwrap()).unwrap()[0],
+            parse(&lex("foobar()").unwrap()).unwrap()[0],
             Statement::Expression(Expression::FunctionCall(
                 FunctionCall::new(
-                    Expression::MapIndex(
-                        Box::new(MapIndex::new(
-                            Expression::Identifier(Identifier::new("io").with_position((0, 2))),
-                            Expression::Identifier(
-                                Identifier::new("read_line").with_position((3, 12))
-                            )
-                        ))
-                        .with_position((0, 12))
-                    ),
+                    Expression::Identifier(Identifier::new("foobar").with_position((0, 6))),
                     Vec::with_capacity(0),
                     Vec::with_capacity(0),
                 )
-                .with_position((0, 14))
+                .with_position((0, 8))
             ))
         )
     }
diff --git a/dust-lang/src/value.rs b/dust-lang/src/value.rs
index dd3ce88..d46bece 100644
--- a/dust-lang/src/value.rs
+++ b/dust-lang/src/value.rs
@@ -6,12 +6,6 @@ use std::{
     sync::Arc,
 };
 
-use stanza::{
-    renderer::{console::Console, Renderer},
-    style::{HAlign, MinWidth, Styles},
-    table::Table,
-};
-
 use crate::{
     abstract_tree::{AbstractNode, Action, Block, Type, WithPos, WithPosition},
     context::Context,
@@ -104,31 +98,31 @@ impl Value {
 
 impl Display for Value {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        fn create_table() -> Table {
-            Table::with_styles(Styles::default().with(HAlign::Centred).with(MinWidth(3)))
-        }
-
         match self.inner().as_ref() {
             ValueInner::Boolean(boolean) => write!(f, "{boolean}"),
             ValueInner::Float(float) => write!(f, "{float}"),
             ValueInner::Integer(integer) => write!(f, "{integer}"),
             ValueInner::List(list) => {
-                let mut table = create_table();
+                write!(f, "[")?;
 
-                for value in list {
-                    table = table.with_row([value.node.to_string()]);
+                for (index, value) in list.into_iter().enumerate() {
+                    if index == list.len() - 1 {
+                        write!(f, "{}", value.node)?;
+                    } else {
+                        write!(f, "{}, ", value.node)?;
+                    }
                 }
 
-                write!(f, "{}", Console::default().render(&table))
+                write!(f, "]")
             }
             ValueInner::Map(map) => {
-                let mut table = create_table();
+                write!(f, "[")?;
 
-                for (identifier, value) in map {
-                    table = table.with_row([identifier.as_str(), &value.to_string()]);
+                for (key, value) in map {
+                    writeln!(f, "{key} = {value},")?;
                 }
 
-                write!(f, "{}", Console::default().render(&table))
+                write!(f, "]")
             }
             ValueInner::Range(_) => todo!(),
             ValueInner::String(string) => write!(f, "{string}"),
@@ -161,13 +155,13 @@ impl Display for Value {
                 write!(f, "): {} {:?}", return_type.node, body.node)
             }
             ValueInner::Structure { name, fields } => {
-                let mut table = create_table();
+                write!(f, "{}\n{{", name.node)?;
 
-                for (identifier, value) in fields {
-                    table = table.with_row([identifier.as_str(), &value.to_string()]);
+                for (key, value) in fields {
+                    writeln!(f, "{key} = {value},")?;
                 }
 
-                write!(f, "{}\n{}", name.node, Console::default().render(&table))
+                write!(f, "}}")
             }
         }
     }
@@ -318,11 +312,15 @@ impl Function {
         &self.type_parameters
     }
 
-    pub fn call(self, arguments: Vec<Value>, context: Context) -> Result<Action, RuntimeError> {
+    pub fn call(
+        self,
+        arguments: Vec<Value>,
+        context: &mut Context,
+    ) -> Result<Action, RuntimeError> {
         for ((identifier, _), value) in self.parameters.into_iter().zip(arguments.into_iter()) {
             context.set_value(identifier.clone(), value)?;
         }
 
-        self.body.node.run(&context)
+        self.body.node.run(context, true)
     }
 }
diff --git a/dust-shell/src/cli.rs b/dust-shell/src/cli.rs
index c7e8aef..5b9a647 100644
--- a/dust-shell/src/cli.rs
+++ b/dust-shell/src/cli.rs
@@ -19,7 +19,7 @@ use reedline::{
 };
 
 pub fn run_shell(context: Context) -> Result<(), io::Error> {
-    let interpreter = Interpreter::new(context.clone());
+    let mut interpreter = Interpreter::new(context.clone());
     let mut keybindings = default_emacs_keybindings();
 
     keybindings.add_binding(
@@ -222,7 +222,7 @@ impl Completer for DustCompleter {
             }
         }
 
-        for (key, value_data) in self.context.inner().unwrap().iter() {
+        for (key, (value_data, _)) in self.context.inner().unwrap().iter() {
             let description = match value_data {
                 ValueData::Value(value) => value.to_string(),
                 ValueData::Type(r#type) => r#type.to_string(),