diff --git a/ablescript/src/interpret.rs b/ablescript/src/interpret.rs
index bb79d33..af679d4 100644
--- a/ablescript/src/interpret.rs
+++ b/ablescript/src/interpret.rs
@@ -407,6 +407,7 @@ impl ExecEnv {
                 self.stack.pop();
                 res?;
             }
+            Functio::Builtin(b) => b.call(args).map_err(|e| Error::new(e, span.clone()))?,
             Functio::Chain { functios, kind } => {
                 let (left_functio, right_functio) = *functios;
                 match kind {
diff --git a/ablescript/src/variables.rs b/ablescript/src/variables.rs
index 545f2d4..05157d4 100644
--- a/ablescript/src/variables.rs
+++ b/ablescript/src/variables.rs
@@ -44,6 +44,7 @@ pub enum Functio {
         params: Vec<String>,
         body: Vec<Stmt>,
     },
+    Builtin(BuiltinFunctio),
     Chain {
         functios: Box<(Functio, Functio)>,
         kind: FunctioChainKind,
@@ -59,12 +60,57 @@ impl Functio {
                 tape_len: _,
             } => 0,
             Functio::Able { params, body: _ } => params.len(),
+            Functio::Builtin(b) => b.arity,
             Functio::Chain { functios, kind: _ } => functios.0.arity() + functios.1.arity(),
             Functio::Eval(_) => 0,
         }
     }
 }
 
+#[derive(Clone)]
+pub struct BuiltinFunctio {
+    function: Rc<dyn Fn(&[Rc<RefCell<Value>>]) -> Result<(), crate::error::ErrorKind>>,
+    arity: usize,
+}
+
+impl BuiltinFunctio {
+    pub fn new<F>(f: F, arity: usize) -> Self
+    where
+        F: Fn(&[Rc<RefCell<Value>>]) -> Result<(), crate::error::ErrorKind> + 'static,
+    {
+        Self {
+            function: Rc::new(f),
+            arity,
+        }
+    }
+
+    pub fn call(&self, args: &[Rc<RefCell<Value>>]) -> Result<(), crate::error::ErrorKind> {
+        (self.function)(args)
+    }
+}
+
+impl std::fmt::Debug for BuiltinFunctio {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("BuiltinFunctio")
+            .field("function", &"built-in")
+            .field("arity", &self.arity)
+            .finish()
+    }
+}
+
+impl PartialEq for BuiltinFunctio {
+    fn eq(&self, other: &Self) -> bool {
+        Rc::ptr_eq(&self.function, &other.function) && self.arity == other.arity
+    }
+}
+
+impl Hash for BuiltinFunctio {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        (Rc::as_ptr(&self.function) as *const () as usize).hash(state);
+        self.arity.hash(state);
+    }
+}
+
 #[derive(Debug, PartialEq, Copy, Clone, Hash)]
 pub enum FunctioChainKind {
     Equal,
@@ -134,6 +180,7 @@ impl Value {
                         .sum::<i32>()
                         + body.len() as i32
                 }
+                Functio::Builtin(_) => todo!(),
                 Functio::Chain { functios, kind } => {
                     let (lf, rf) = *functios;
                     Value::Functio(lf).into_i32()
@@ -222,6 +269,7 @@ impl Value {
 
                     Value::Int((params + body) % 3 - 1).into_abool()
                 }
+                Functio::Builtin(_) => todo!(),
                 Functio::Chain { functios, kind } => {
                     let (lhs, rhs) = *functios;
                     match kind {
@@ -391,6 +439,7 @@ impl Value {
                     );
                     cart
                 }
+                Functio::Builtin(_) => todo!(),
                 Functio::Chain { functios, kind } => {
                     let (lhs, rhs) = *functios;
                     match kind {
@@ -440,6 +489,7 @@ impl Value {
                     tape_len,
                 } => (instructions.len() + tape_len) as _,
                 Functio::Able { params, body } => (params.len() + format!("{:?}", body).len()) as _,
+                Functio::Builtin(_) => todo!(),
                 Functio::Chain { functios, kind } => {
                     let (lhs, rhs) = *functios.clone();
                     match kind {
@@ -564,6 +614,7 @@ impl ops::Sub for Value {
                     )
                     .into_functio(),
                 },
+                Functio::Builtin(_) => todo!(),
                 Functio::Chain { functios, .. } => {
                     let rhs = rhs.into_functio();
                     let (a, b) = *functios;
@@ -692,6 +743,7 @@ impl ops::Div for Value {
                             .collect(),
                     }
                 }
+                Functio::Builtin(_) => todo!(),
                 Functio::Chain { functios, kind } => {
                     let functios = *functios;
                     Functio::Chain {
@@ -765,6 +817,7 @@ impl ops::Not for Value {
 
                     Functio::Able { params, body }
                 }
+                Functio::Builtin(_) => todo!(),
                 Functio::Chain { functios, kind } => {
                     let (a, b) = *functios;
                     Functio::Chain {
@@ -858,6 +911,7 @@ impl Display for Value {
                         body,
                     )
                 }
+                Functio::Builtin(_) => todo!(),
                 Functio::Chain { functios, kind } => {
                     let (a, b) = *functios.clone();
                     write!(