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 79a09d9..882240b 100644
--- a/dust-lang/src/abstract_tree/built_in_function_call.rs
+++ b/dust-lang/src/abstract_tree/built_in_function_call.rs
@@ -40,8 +40,8 @@ impl AbstractNode for BuiltInFunctionCall {
                 Ok(Action::Return(Value::string(buffer)))
             }
             BuiltInFunctionCall::Sleep(expression) => {
-                let expression_run = expression.clone().run(context)?;
-                let expression_value = if let Action::Return(value) = expression_run {
+                let action = expression.clone().run(context)?;
+                let value = if let Action::Return(value) = action {
                     value
                 } else {
                     return Err(RuntimeError::ValidationFailure(
@@ -49,15 +49,28 @@ impl AbstractNode for BuiltInFunctionCall {
                     ));
                 };
 
-                if let ValueInner::Integer(milliseconds) = expression_value.inner().as_ref() {
+                if let ValueInner::Integer(milliseconds) = value.inner().as_ref() {
                     thread::sleep(Duration::from_millis(*milliseconds as u64));
-
-                    Ok(Action::None)
-                } else {
-                    panic!("Expected an integer.");
                 }
+
+                Ok(Action::None)
+            }
+            BuiltInFunctionCall::WriteLine(expression) => {
+                let action = expression.clone().run(context)?;
+                let value = if let Action::Return(value) = action {
+                    value
+                } else {
+                    return Err(RuntimeError::ValidationFailure(
+                        ValidationError::InterpreterExpectedReturn(expression.position()),
+                    ));
+                };
+
+                if let ValueInner::String(output) = value.inner().as_ref() {
+                    println!("{output}");
+                }
+
+                Ok(Action::None)
             }
-            BuiltInFunctionCall::WriteLine(_) => todo!(),
         }
     }
 }
diff --git a/dust-lang/src/abstract_tree/mod.rs b/dust-lang/src/abstract_tree/mod.rs
index 8c78271..9579325 100644
--- a/dust-lang/src/abstract_tree/mod.rs
+++ b/dust-lang/src/abstract_tree/mod.rs
@@ -109,6 +109,7 @@ impl AbstractTree {
             match run {
                 Ok(action) => match action {
                     Action::Return(value) => previous_value = Some(value),
+                    Action::None => previous_value = None,
                     _ => {}
                 },
                 Err(runtime_error) => {
diff --git a/dust-lang/src/value.rs b/dust-lang/src/value.rs
index 4df9962..dd3ce88 100644
--- a/dust-lang/src/value.rs
+++ b/dust-lang/src/value.rs
@@ -160,9 +160,6 @@ impl Display for Value {
 
                 write!(f, "): {} {:?}", return_type.node, body.node)
             }
-            ValueInner::Function(function) => {
-                write!(f, "{function}")
-            }
             ValueInner::Structure { name, fields } => {
                 let mut table = create_table();
 
@@ -329,9 +326,3 @@ impl Function {
         self.body.node.run(&context)
     }
 }
-
-impl Display for Function {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        todo!()
-    }
-}