diff --git a/dust-lang/src/abstract_tree/built_in_function.rs b/dust-lang/src/abstract_tree/built_in_function.rs
index 49ef105..4616c41 100644
--- a/dust-lang/src/abstract_tree/built_in_function.rs
+++ b/dust-lang/src/abstract_tree/built_in_function.rs
@@ -1,4 +1,4 @@
-use std::{fs::read_to_string, io::stdin};
+use std::{fs::read_to_string, io::stdin, thread::sleep, time::Duration};
 
 use serde::{Deserialize, Serialize};
 use serde_json::from_str;
@@ -11,7 +11,7 @@ use crate::{
     Value,
 };
 
-use super::{AbstractNode, Evaluation, Expression, Type, TypeConstructor};
+use super::Type;
 
 #[derive(Clone, Debug, Eq, PartialEq, PartialOrd, Ord, Serialize, Deserialize)]
 pub enum BuiltInFunction {
@@ -98,13 +98,29 @@ impl FunctionLogic for ReadFile {
     fn r#type() -> Type {
         Type::Function {
             type_parameters: None,
-            value_parameters: None,
+            value_parameters: Some(vec![(Identifier::new("path"), Type::String)]),
             return_type: Some(Box::new(Type::String)),
         }
     }
 
     fn call(context: &Context, manage_memory: bool) -> Result<Option<Value>, RuntimeError> {
-        todo!()
+        let value = if let Some(value) = context.get_value(&Identifier::new("path"))? {
+            value
+        } else {
+            return Err(RuntimeError::ValidationFailure(
+                ValidationError::BuiltInFunctionFailure("path does not exist"),
+            ));
+        };
+        let path = if let ValueInner::String(string) = value.inner().as_ref() {
+            string
+        } else {
+            return Err(RuntimeError::ValidationFailure(
+                ValidationError::BuiltInFunctionFailure("path is not a string"),
+            ));
+        };
+        let file_content = read_to_string(path)?;
+
+        Ok(Some(Value::string(file_content)))
     }
 }
 
@@ -121,7 +137,11 @@ impl FunctionLogic for ReadLine {
     }
 
     fn call(context: &Context, manage_memory: bool) -> Result<Option<Value>, RuntimeError> {
-        todo!()
+        let mut user_input = String::new();
+
+        stdin().read_line(&mut user_input)?;
+
+        Ok(Some(Value::string(user_input)))
     }
 }
 
@@ -132,13 +152,30 @@ impl FunctionLogic for Sleep {
     fn r#type() -> Type {
         Type::Function {
             type_parameters: None,
-            value_parameters: None,
+            value_parameters: Some(vec![(Identifier::new("milliseconds"), Type::Integer)]),
             return_type: None,
         }
     }
 
     fn call(context: &Context, manage_memory: bool) -> Result<Option<Value>, RuntimeError> {
-        todo!()
+        let value = if let Some(value) = context.get_value(&Identifier::new("milliseconds"))? {
+            value
+        } else {
+            return Err(RuntimeError::ValidationFailure(
+                ValidationError::BuiltInFunctionFailure("milliseconds does not exist"),
+            ));
+        };
+        let milliseconds = if let ValueInner::Integer(integer) = value.inner().as_ref() {
+            integer
+        } else {
+            return Err(RuntimeError::ValidationFailure(
+                ValidationError::BuiltInFunctionFailure("milliseconds is not an integer"),
+            ));
+        };
+
+        sleep(Duration::from_millis(*milliseconds as u64));
+
+        Ok(None)
     }
 }
 
@@ -149,7 +186,7 @@ impl FunctionLogic for WriteLine {
     fn r#type() -> Type {
         Type::Function {
             type_parameters: None,
-            value_parameters: None,
+            value_parameters: Some(vec![(Identifier::new("output"), Type::String)]),
             return_type: None,
         }
     }
@@ -162,7 +199,7 @@ impl FunctionLogic for WriteLine {
                 ValidationError::BuiltInFunctionFailure("output does not exist"),
             ));
         };
-        let input = if let ValueInner::String(string) = value.inner().as_ref() {
+        let output = if let ValueInner::String(string) = value.inner().as_ref() {
             string
         } else {
             return Err(RuntimeError::ValidationFailure(
@@ -170,7 +207,7 @@ impl FunctionLogic for WriteLine {
             ));
         };
 
-        println!("{input}");
+        println!("{output}");
 
         Ok(None)
     }
@@ -181,13 +218,15 @@ struct JsonParse;
 
 impl FunctionLogic for JsonParse {
     fn r#type() -> Type {
+        let type_t = Type::Generic {
+            identifier: Identifier::new("T"),
+            concrete_type: None,
+        };
+
         Type::Function {
             type_parameters: None,
-            value_parameters: None,
-            return_type: Some(Box::new(Type::Generic {
-                identifier: Identifier::new("T"),
-                concrete_type: None,
-            })),
+            value_parameters: Some(vec![(Identifier::new("input"), type_t.clone())]),
+            return_type: Some(Box::new(type_t)),
         }
     }
 
diff --git a/dust-lang/src/abstract_tree/function_call.rs b/dust-lang/src/abstract_tree/function_call.rs
index d8a7e02..5de24a0 100644
--- a/dust-lang/src/abstract_tree/function_call.rs
+++ b/dust-lang/src/abstract_tree/function_call.rs
@@ -47,14 +47,6 @@ impl AbstractNode for FunctionCall {
     }
 
     fn validate(&self, context: &Context, manage_memory: bool) -> Result<(), ValidationError> {
-        if let Expression::Value(WithPosition {
-            node: ValueNode::BuiltInFunction(_),
-            ..
-        }) = self.function_expression.as_ref()
-        {
-            return Ok(());
-        }
-
         self.function_expression.validate(context, manage_memory)?;
 
         if let Some(value_arguments) = &self.value_arguments {
@@ -63,18 +55,23 @@ impl AbstractNode for FunctionCall {
             }
         }
 
-        let function_node_type =
-            if let Some(r#type) = self.function_expression.expected_type(context)? {
-                r#type
-            } else {
-                return Err(ValidationError::ExpectedExpression(
-                    self.function_expression.position(),
-                ));
-            };
+        let function_node_type = if let Expression::Value(WithPosition {
+            node: ValueNode::BuiltInFunction(function),
+            ..
+        }) = self.function_expression.as_ref()
+        {
+            function.r#type()
+        } else if let Some(r#type) = self.function_expression.expected_type(context)? {
+            r#type
+        } else {
+            return Err(ValidationError::ExpectedExpression(
+                self.function_expression.position(),
+            ));
+        };
 
         if let Type::Function {
             type_parameters,
-            value_parameters: _,
+            value_parameters,
             return_type: _,
         } = function_node_type
         {
@@ -90,6 +87,30 @@ impl AbstractNode for FunctionCall {
                 _ => {}
             }
 
+            match (value_parameters, &self.value_arguments) {
+                (Some(parameters), Some(arguments)) => {
+                    if parameters.len() != arguments.len() {
+                        return Err(ValidationError::WrongTypeArgumentCount {
+                            actual: parameters.len(),
+                            expected: arguments.len(),
+                        });
+                    }
+                }
+                (Some(parameters), None) => {
+                    return Err(ValidationError::WrongTypeArgumentCount {
+                        expected: parameters.len(),
+                        actual: 0,
+                    })
+                }
+                (None, Some(arguments)) => {
+                    return Err(ValidationError::WrongTypeArgumentCount {
+                        expected: 0,
+                        actual: arguments.len(),
+                    })
+                }
+                (None, None) => {}
+            }
+
             Ok(())
         } else {
             Err(ValidationError::ExpectedFunction {
diff --git a/dust-lang/src/lib.rs b/dust-lang/src/lib.rs
index a1089da..9d25111 100644
--- a/dust-lang/src/lib.rs
+++ b/dust-lang/src/lib.rs
@@ -452,7 +452,11 @@ impl InterpreterError {
                         ),
                     ),
                     ValidationError::WrongArguments { .. } => todo!(),
-                    ValidationError::WrongTypeArgumentCount { .. } => todo!(),
+                    ValidationError::WrongTypeArgumentCount { expected, actual } => {
+                        builder = builder.with_message(format!(
+                            "Expected {expected} arguments but got {actual}."
+                        ));
+                    }
                     ValidationError::ExpectedIntegerFloatOrString { actual, position } => {
                         builder = builder.with_message(format!(
                             "Expected an {}, {} or {}.",