diff --git a/dust-lang/src/abstract_tree/built_in_function.rs b/dust-lang/src/abstract_tree/built_in_function.rs
index d6d6a37..49ef105 100644
--- a/dust-lang/src/abstract_tree/built_in_function.rs
+++ b/dust-lang/src/abstract_tree/built_in_function.rs
@@ -155,7 +155,24 @@ impl FunctionLogic for WriteLine {
     }
 
     fn call(context: &Context, manage_memory: bool) -> Result<Option<Value>, RuntimeError> {
-        todo!()
+        let value = if let Some(value) = context.get_value(&Identifier::new("output"))? {
+            value
+        } else {
+            return Err(RuntimeError::ValidationFailure(
+                ValidationError::BuiltInFunctionFailure("output does not exist"),
+            ));
+        };
+        let input = if let ValueInner::String(string) = value.inner().as_ref() {
+            string
+        } else {
+            return Err(RuntimeError::ValidationFailure(
+                ValidationError::BuiltInFunctionFailure("output is not a string"),
+            ));
+        };
+
+        println!("{input}");
+
+        Ok(None)
     }
 }
 
diff --git a/dust-lang/src/abstract_tree/function_call.rs b/dust-lang/src/abstract_tree/function_call.rs
index 5d91caf..d8a7e02 100644
--- a/dust-lang/src/abstract_tree/function_call.rs
+++ b/dust-lang/src/abstract_tree/function_call.rs
@@ -104,6 +104,16 @@ impl AbstractNode for FunctionCall {
         context: &Context,
         manage_memory: bool,
     ) -> Result<Option<Evaluation>, RuntimeError> {
+        if let Expression::Value(WithPosition {
+            node: ValueNode::BuiltInFunction(function),
+            ..
+        }) = *self.function_expression
+        {
+            return function
+                .call(context, manage_memory)
+                .map(|value_option| value_option.map(|value| Evaluation::Return(value)));
+        }
+
         let function_position = self.function_expression.position();
         let evaluation = self.function_expression.evaluate(context, manage_memory)?;
         let value = if let Some(Evaluation::Return(value)) = evaluation {
diff --git a/dust-lang/src/abstract_tree/value_node.rs b/dust-lang/src/abstract_tree/value_node.rs
index 1a93570..50591bd 100644
--- a/dust-lang/src/abstract_tree/value_node.rs
+++ b/dust-lang/src/abstract_tree/value_node.rs
@@ -173,8 +173,6 @@ impl AbstractNode for ValueNode {
         {
             body.node.validate(&context_template, _manage_memory)?;
 
-            println!("beep");
-
             let ((expected_return, expected_position), actual_return) =
                 match (return_type, body.node.expected_type(&context_template)?) {
                     (Some(constructor), Some(r#type)) => (
@@ -373,11 +371,7 @@ impl AbstractNode for ValueNode {
 
                 Value::structure(name, fields)
             }
-            ValueNode::BuiltInFunction(built_in_function) => {
-                return built_in_function
-                    .call(context, manage_memory)
-                    .map(|value_option| value_option.map(|value| Evaluation::Return(value)));
-            }
+            ValueNode::BuiltInFunction(function) => Value::built_in_function(function),
         };
 
         Ok(Some(Evaluation::Return(value)))
diff --git a/dust-lang/src/lib.rs b/dust-lang/src/lib.rs
index d431b85..a1089da 100644
--- a/dust-lang/src/lib.rs
+++ b/dust-lang/src/lib.rs
@@ -13,7 +13,7 @@ use std::{
 };
 
 use abstract_tree::{AbstractTree, Type};
-use ariadne::{Color, Config, Fmt, Label, Report, ReportKind};
+use ariadne::{Color, Fmt, Label, Report, ReportKind};
 use chumsky::prelude::*;
 use context::Context;
 use error::{DustError, RuntimeError, TypeConflict, ValidationError};
@@ -307,13 +307,12 @@ impl InterpreterError {
                             .with_label(
                                 Label::new((self.source_id.clone(), position.0..position.1)).with_message("Error occured here.")
                             ),
-
-                            if let RuntimeError::ValidationFailure(validation_error) = error {
-                                Some(validation_error)
-                            } else {
-                                None
-                            },
-                        )
+                        if let RuntimeError::ValidationFailure(validation_error) = error {
+                            Some(validation_error)
+                        } else {
+                            None
+                        },
+                    )
                 }
             };
 
@@ -432,7 +431,14 @@ impl InterpreterError {
                         Label::new((self.source_id.clone(), position.0..position.1))
                             .with_message("Expected a statement that ends in an expression."),
                     ),
-                    ValidationError::ExpectedFunction { .. } => todo!(),
+                    ValidationError::ExpectedFunction { actual, position } => builder.add_label(
+                        Label::new((self.source_id.clone(), position.0..position.1)).with_message(
+                            format!(
+                                "Expected a function value but got {}.",
+                                actual.fg(type_color)
+                            ),
+                        ),
+                    ),
                     ValidationError::ExpectedValue(_) => todo!(),
                     ValidationError::FieldNotFound {
                         identifier,
@@ -481,12 +487,11 @@ impl InterpreterError {
                     }
                     ValidationError::EnumVariantNotFound { .. } => todo!(),
                     ValidationError::ExpectedList { .. } => todo!(),
-                    ValidationError::BuiltInFunctionFailure(_) => todo!(),
+                    ValidationError::BuiltInFunctionFailure(reason) => builder
+                        .add_label(Label::new((self.source_id.clone(), 0..0)).with_message(reason)),
                 }
             }
 
-            builder = builder.with_config(Config::default().with_multiline_arrows(false));
-
             let report = builder.finish();
 
             reports.push(report);
diff --git a/dust-lang/src/value.rs b/dust-lang/src/value.rs
index e37d393..d35d936 100644
--- a/dust-lang/src/value.rs
+++ b/dust-lang/src/value.rs
@@ -32,6 +32,10 @@ impl Value {
         Value(Arc::new(ValueInner::Boolean(boolean)))
     }
 
+    pub fn built_in_function(function: BuiltInFunction) -> Self {
+        Value(Arc::new(ValueInner::BuiltInFunction(function)))
+    }
+
     pub fn enum_instance(
         type_name: Identifier,
         variant: Identifier,
@@ -631,7 +635,7 @@ impl ValueInner {
                     });
                 }
             }
-            ValueInner::BuiltInFunction(_) => todo!(),
+            ValueInner::BuiltInFunction(function) => function.r#type(),
         };
 
         Ok(r#type)