diff --git a/examples/clue_solver.ds b/examples/clue_solver.ds
index 0f1e5c3..07b0a22 100644
--- a/examples/clue_solver.ds
+++ b/examples/clue_solver.ds
@@ -5,9 +5,9 @@ all_cards = {
 }
 
 is_ready_to_solve <fn map -> bool> |cards| {
-	(length cards:suspects) == 1
-	&& (length cards:rooms) == 1
-	&& (length cards:weapons) == 1
+	((length cards:suspects) == 1)
+	&& ((length cards:rooms) == 1)
+	&& ((length cards:weapons) == 1)
 }
 
 take_turn <fn str str map -> map> |opponent_card current_room cards| {
@@ -17,6 +17,7 @@ take_turn <fn str str map -> map> |opponent_card current_room cards| {
 }
 
 remove_card <fn str map> |opponent_card cards| {
+	(output opponent_card cards)
 	cards:rooms -= opponent_card
 	cards:suspects -= opponent_card
 	cards:weapons -= opponent_card
diff --git a/src/abstract_tree/function_call.rs b/src/abstract_tree/function_call.rs
index c5743e8..34bb1f3 100644
--- a/src/abstract_tree/function_call.rs
+++ b/src/abstract_tree/function_call.rs
@@ -41,10 +41,20 @@ impl AbstractTree for FunctionCall {
             }
         }
 
-        Ok(FunctionCall {
+        let function_type = function_name.expected_type(context)?;
+        let function_call = FunctionCall {
             function_name,
             arguments,
-        })
+        };
+
+        function_type.abstract_check(
+            &function_call.expected_type(context)?,
+            context,
+            node,
+            source,
+        )?;
+
+        Ok(function_call)
     }
 
     fn run(&self, source: &str, context: &Map) -> Result<Value> {
@@ -72,21 +82,20 @@ impl AbstractTree for FunctionCall {
                 self.function_name.clone(),
             ));
         };
-        let mut function_context = Map::clone_from(context)?;
-        let parameter_expression_pairs = function.parameters().iter().zip(self.arguments.iter());
 
-        for ((identifier, _type), expression) in parameter_expression_pairs {
-            let key = identifier.clone().take_inner();
-            let value = expression.run(source, context)?;
-
-            function_context.variables_mut()?.insert(key, value);
-        }
-
-        function.body().run(source, &mut function_context)
+        function.call(&self.arguments, source, context)
     }
 
     fn expected_type(&self, context: &Map) -> Result<TypeDefinition> {
-        self.function_name.expected_type(context)
+        let function_name = self.function_name.inner();
+
+        if let Some(value) = context.variables()?.get(function_name) {
+            let return_type = value.as_function()?.return_type();
+
+            Ok(return_type.clone())
+        } else {
+            self.function_name.expected_type(context)
+        }
     }
 }
 
diff --git a/src/abstract_tree/function_declaration.rs b/src/abstract_tree/function_declaration.rs
index 7f21e3c..58a706c 100644
--- a/src/abstract_tree/function_declaration.rs
+++ b/src/abstract_tree/function_declaration.rs
@@ -8,7 +8,7 @@ use crate::{
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
 pub struct FunctionDeclaration {
     name: Identifier,
-    _type_definition: TypeDefinition,
+    type_definition: TypeDefinition,
     function: Function,
 }
 
@@ -72,7 +72,7 @@ impl AbstractTree for FunctionDeclaration {
 
         Ok(FunctionDeclaration {
             name,
-            _type_definition: type_definition,
+            type_definition,
             function,
         })
     }
diff --git a/src/abstract_tree/identifier.rs b/src/abstract_tree/identifier.rs
index 7b23c77..e01f2a1 100644
--- a/src/abstract_tree/identifier.rs
+++ b/src/abstract_tree/identifier.rs
@@ -1,7 +1,7 @@
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
-use crate::{AbstractTree, Error, Map, Result, Type, TypeDefinition, Value};
+use crate::{AbstractTree, Error, Map, Result, Type, TypeDefinition, Value, BUILT_IN_FUNCTIONS};
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
 pub struct Identifier(String);
@@ -41,6 +41,12 @@ impl AbstractTree for Identifier {
         if let Some(value) = context.variables()?.get(&self.0) {
             value.r#type(context)
         } else {
+            for built_in_function in BUILT_IN_FUNCTIONS {
+                if self.0 == built_in_function.name() {
+                    return Ok(built_in_function.type_definition());
+                }
+            }
+
             Ok(TypeDefinition::new(Type::Empty))
         }
     }
diff --git a/src/abstract_tree/logic.rs b/src/abstract_tree/logic.rs
index ab97319..a26340c 100644
--- a/src/abstract_tree/logic.rs
+++ b/src/abstract_tree/logic.rs
@@ -12,10 +12,23 @@ pub struct Logic {
 
 impl AbstractTree for Logic {
     fn from_syntax_node(source: &str, node: Node, context: &Map) -> Result<Self> {
-        let left_node = node.child(0).unwrap();
+        let first_node = node.child(0).unwrap();
+        let (left_node, operator_node, right_node) = {
+            if first_node.is_named() {
+                (
+                    first_node,
+                    node.child(1).unwrap().child(0).unwrap(),
+                    node.child(2).unwrap(),
+                )
+            } else {
+                (
+                    node.child(1).unwrap(),
+                    node.child(2).unwrap().child(0).unwrap(),
+                    node.child(3).unwrap(),
+                )
+            }
+        };
         let left = Expression::from_syntax_node(source, left_node, context)?;
-
-        let operator_node = node.child(1).unwrap().child(0).unwrap();
         let operator = match operator_node.kind() {
             "==" => LogicOperator::Equal,
             "!=" => LogicOperator::NotEqual,
@@ -34,8 +47,6 @@ impl AbstractTree for Logic {
                 })
             }
         };
-
-        let right_node = node.child(2).unwrap();
         let right = Expression::from_syntax_node(source, right_node, context)?;
 
         Ok(Logic {
diff --git a/src/built_in_functions/assert.rs b/src/built_in_functions/assert.rs
index 0a52eda..1b9516d 100644
--- a/src/built_in_functions/assert.rs
+++ b/src/built_in_functions/assert.rs
@@ -1,4 +1,4 @@
-use crate::{BuiltInFunction, Error, Map, Result, Value};
+use crate::{BuiltInFunction, Error, Map, Result, Type, TypeDefinition, Value};
 
 pub struct Assert;
 
@@ -16,6 +16,13 @@ impl BuiltInFunction for Assert {
 
         Ok(Value::Empty)
     }
+
+    fn type_definition(&self) -> TypeDefinition {
+        TypeDefinition::new(Type::Function {
+            parameter_types: vec![Type::Any],
+            return_type: Box::new(Type::Empty),
+        })
+    }
 }
 
 pub struct AssertEqual;
@@ -40,4 +47,11 @@ impl BuiltInFunction for AssertEqual {
             })
         }
     }
+
+    fn type_definition(&self) -> TypeDefinition {
+        TypeDefinition::new(Type::Function {
+            parameter_types: vec![Type::Any, Type::Any],
+            return_type: Box::new(Type::Boolean),
+        })
+    }
 }
diff --git a/src/built_in_functions/data_formats.rs b/src/built_in_functions/data_formats.rs
index 02888bb..846b838 100644
--- a/src/built_in_functions/data_formats.rs
+++ b/src/built_in_functions/data_formats.rs
@@ -15,6 +15,10 @@ impl BuiltInFunction for FromJson {
 
         Ok(value)
     }
+
+    fn type_definition(&self) -> crate::TypeDefinition {
+        todo!()
+    }
 }
 
 pub struct ToJson;
@@ -32,4 +36,8 @@ impl BuiltInFunction for ToJson {
 
         Ok(Value::String(json_string))
     }
+
+    fn type_definition(&self) -> crate::TypeDefinition {
+        todo!()
+    }
 }
diff --git a/src/built_in_functions/fs.rs b/src/built_in_functions/fs.rs
index 34b2d9a..22b7e1b 100644
--- a/src/built_in_functions/fs.rs
+++ b/src/built_in_functions/fs.rs
@@ -46,6 +46,10 @@ impl BuiltInFunction for Read {
 
         Ok(Value::String(file_content))
     }
+
+    fn type_definition(&self) -> crate::TypeDefinition {
+        todo!()
+    }
 }
 
 pub struct Write;
@@ -63,6 +67,10 @@ impl BuiltInFunction for Write {
 
         Ok(Value::Empty)
     }
+
+    fn type_definition(&self) -> crate::TypeDefinition {
+        todo!()
+    }
 }
 
 pub struct Append;
@@ -84,4 +92,8 @@ impl BuiltInFunction for Append {
 
         Ok(Value::Empty)
     }
+
+    fn type_definition(&self) -> crate::TypeDefinition {
+        todo!()
+    }
 }
diff --git a/src/built_in_functions/mod.rs b/src/built_in_functions/mod.rs
index 8be8f65..3d8f8ff 100644
--- a/src/built_in_functions/mod.rs
+++ b/src/built_in_functions/mod.rs
@@ -1,4 +1,4 @@
-use crate::{Map, Result, Value};
+use crate::{Map, Result, TypeDefinition, Value};
 
 mod assert;
 mod data_formats;
@@ -26,4 +26,5 @@ pub const BUILT_IN_FUNCTIONS: [&dyn BuiltInFunction; 13] = [
 pub trait BuiltInFunction {
     fn name(&self) -> &'static str;
     fn run(&self, arguments: &[Value], context: &Map) -> Result<Value>;
+    fn type_definition(&self) -> TypeDefinition;
 }
diff --git a/src/built_in_functions/output.rs b/src/built_in_functions/output.rs
index 859edb2..44105cb 100644
--- a/src/built_in_functions/output.rs
+++ b/src/built_in_functions/output.rs
@@ -1,4 +1,4 @@
-use crate::{BuiltInFunction, Map, Result, Value};
+use crate::{BuiltInFunction, Map, Result, Type, TypeDefinition, Value};
 
 pub struct Output;
 
@@ -14,4 +14,11 @@ impl BuiltInFunction for Output {
 
         Ok(Value::Empty)
     }
+
+    fn type_definition(&self) -> crate::TypeDefinition {
+        TypeDefinition::new(Type::Function {
+            parameter_types: vec![Type::Any],
+            return_type: Box::new(Type::Empty),
+        })
+    }
 }
diff --git a/src/built_in_functions/random.rs b/src/built_in_functions/random.rs
index 94bd097..eb3f432 100644
--- a/src/built_in_functions/random.rs
+++ b/src/built_in_functions/random.rs
@@ -1,6 +1,6 @@
 use rand::{random, thread_rng, Rng};
 
-use crate::{BuiltInFunction, Error, Map, Result, Value};
+use crate::{BuiltInFunction, Error, Map, Result, Type, TypeDefinition, Value};
 
 pub struct Random;
 
@@ -10,13 +10,22 @@ impl BuiltInFunction for Random {
     }
 
     fn run(&self, arguments: &[Value], _context: &Map) -> Result<Value> {
-        Error::expect_argument_minimum(self, 2, arguments.len())?;
+        Error::expect_argument_minimum(self, 1, arguments.len())?;
 
-        let random_index = thread_rng().gen_range(0..arguments.len());
-        let random_argument = arguments.get(random_index).unwrap();
+        let list = arguments.first().unwrap().as_list()?;
+        let items = list.items();
+        let random_index = thread_rng().gen_range(0..items.len());
+        let random_argument = items.get(random_index).unwrap();
 
         Ok(random_argument.clone())
     }
+
+    fn type_definition(&self) -> TypeDefinition {
+        TypeDefinition::new(Type::Function {
+            parameter_types: vec![Type::List(Box::new(Type::Any))],
+            return_type: Box::new(Type::Any),
+        })
+    }
 }
 
 pub struct RandomInteger;
@@ -31,6 +40,10 @@ impl BuiltInFunction for RandomInteger {
 
         Ok(Value::Integer(random()))
     }
+
+    fn type_definition(&self) -> crate::TypeDefinition {
+        todo!()
+    }
 }
 
 pub struct RandomFloat;
@@ -45,6 +58,10 @@ impl BuiltInFunction for RandomFloat {
 
         Ok(Value::Float(random()))
     }
+
+    fn type_definition(&self) -> crate::TypeDefinition {
+        todo!()
+    }
 }
 
 pub struct RandomBoolean;
@@ -59,4 +76,8 @@ impl BuiltInFunction for RandomBoolean {
 
         Ok(Value::Boolean(random()))
     }
+
+    fn type_definition(&self) -> crate::TypeDefinition {
+        todo!()
+    }
 }
diff --git a/src/built_in_functions/type.rs b/src/built_in_functions/type.rs
index ff8caee..8ebffb1 100644
--- a/src/built_in_functions/type.rs
+++ b/src/built_in_functions/type.rs
@@ -31,4 +31,8 @@ impl BuiltInFunction for Type {
             Ok(Value::List(List::with_items(answers)))
         }
     }
+
+    fn type_definition(&self) -> crate::TypeDefinition {
+        todo!()
+    }
 }
diff --git a/src/value/function.rs b/src/value/function.rs
index 3d7fdb0..04f30a8 100644
--- a/src/value/function.rs
+++ b/src/value/function.rs
@@ -2,7 +2,7 @@ use std::fmt::{self, Display, Formatter};
 
 use serde::{Deserialize, Serialize};
 
-use crate::{Block, Identifier, TypeDefinition};
+use crate::{AbstractTree, Block, Expression, Identifier, Map, Result, TypeDefinition, Value};
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
 pub struct Function {
@@ -35,6 +35,28 @@ impl Function {
     pub fn return_type(&self) -> &TypeDefinition {
         &self.return_type
     }
+
+    pub fn call(&self, arguments: &[Expression], source: &str, context: &Map) -> Result<Value> {
+        let function_context = Map::new();
+        let parameter_argument_pairs = self.parameters.iter().zip(arguments.iter());
+
+        for ((identifier, type_definition), expression) in parameter_argument_pairs {
+            let key = identifier.inner();
+            let value = expression.run(source, context)?;
+
+            println!("{key} {value}");
+
+            type_definition.runtime_check(&value.r#type(context)?, context)?;
+            function_context.variables_mut()?.insert(key.clone(), value);
+        }
+
+        let return_value = self.body.run(source, &function_context)?;
+
+        self.return_type
+            .runtime_check(&return_value.r#type(context)?, context)?;
+
+        Ok(return_value)
+    }
 }
 
 impl Display for Function {