diff --git a/docs/ROADMAP.md b/docs/ROADMAP.md
index 3fac9ee..1af86f2 100644
--- a/docs/ROADMAP.md
+++ b/docs/ROADMAP.md
@@ -14,6 +14,9 @@
   - [x] integers
   - [ ] floating point numbers
   - [ ] text
+- [ ] items
+  - [x] function definitions
+  - [ ] type definitions
 - [ ] functions
   - [x] lambdas / closures
   - [ ] generic functions
diff --git a/src/builtins.rs b/src/builtins.rs
new file mode 100644
index 0000000..267349b
--- /dev/null
+++ b/src/builtins.rs
@@ -0,0 +1,132 @@
+use std::fmt::Write;
+
+use thiserror::Error;
+
+use crate::evaluate::{Dictionary, Value};
+
+#[derive(Debug, Copy, Clone)]
+pub enum Builtin {
+    Print,
+    Add,
+    Sub,
+    Mul,
+    Div,
+    Equals,
+    GreaterThan,
+    LessThan,
+}
+
+#[derive(Debug, Error)]
+pub enum Error {
+    #[error("Tried to {0} a non-integer: {1}")]
+    NonInteger(&'static str, Value),
+}
+
+type Result<T> = std::result::Result<T, Error>;
+
+fn unwrap_int_value(op_name: &'static str) -> impl Fn(Value) -> Result<i64> {
+    move |value| match value {
+        Value::Integer(i) => Ok(i),
+        _ => Err(Error::NonInteger(op_name, value)),
+    }
+}
+
+macro_rules! int_op {
+    ($arguments:expr, $op:tt, $op_name:expr) => {
+        Value::Integer(
+            $arguments
+                .into_iter()
+                .map(unwrap_int_value($op_name))
+                .reduce(|acc, v| Ok(acc? $op v?))
+                .unwrap()?,
+        )
+    };
+}
+
+macro_rules! int_cmp {
+    ($arguments:expr, $op:tt) => {{
+        let mut arguments = $arguments;
+        let unwrap = unwrap_int_value("compare");
+        let y = unwrap(arguments.pop().unwrap())?;
+        let x = unwrap(arguments.pop().unwrap())?;
+        Value::Boolean(x $op y)
+    }};
+}
+
+impl Builtin {
+    pub fn dictionary() -> Dictionary<Value> {
+        [
+            ("_print", Builtin::Print),
+            ("_add", Builtin::Add),
+            ("_sub", Builtin::Sub),
+            ("_mul", Builtin::Mul),
+            ("_div", Builtin::Div),
+            ("_equals", Builtin::Equals),
+            ("_greaterthan", Builtin::GreaterThan),
+            ("_lessthan", Builtin::LessThan),
+        ]
+        .map(|(name, builtin)| (name.to_string(), Value::Builtin(builtin)))
+        .into()
+    }
+
+    pub fn min_parameters(&self) -> usize {
+        match self {
+            Builtin::Print => 0,
+            Builtin::Add
+            | Builtin::Sub
+            | Builtin::Mul
+            | Builtin::Div
+            | Builtin::Equals
+            | Builtin::GreaterThan
+            | Builtin::LessThan => 2,
+        }
+    }
+    pub fn max_parameters(&self) -> Option<usize> {
+        match self {
+            Builtin::Print
+            | Builtin::Add
+            | Builtin::Sub
+            | Builtin::Mul
+            | Builtin::Div
+            | Builtin::Equals => None,
+            Builtin::GreaterThan | Builtin::LessThan => Some(2),
+        }
+    }
+
+    pub fn call(&self, arguments: Vec<Value>) -> Result<Value> {
+        let result = match self {
+            Builtin::Print => {
+                let mut output = String::new();
+                let mut arguments = arguments.into_iter().peekable();
+                while let Some(term) = arguments.next() {
+                    if arguments.peek().is_some() {
+                        write!(output, "{term} ").expect("Error during printing");
+                    } else {
+                        write!(output, "{term}").expect("Error during printing");
+                    }
+                }
+                println!("{output}");
+                Value::Unit
+            }
+            Builtin::Add => int_op!(arguments, +, "add"),
+            Builtin::Sub => int_op!(arguments, -, "subtract"),
+            Builtin::Mul => int_op!(arguments, *, "multiply"),
+            Builtin::Div => int_op!(arguments, /, "divide"),
+            Builtin::Equals => {
+                let mut arguments = arguments;
+                let y = arguments.pop().unwrap();
+                let x = arguments.pop().unwrap();
+                let b = match (x, y) {
+                    (Value::Integer(x), Value::Integer(y)) => x == y,
+                    (Value::Boolean(x), Value::Boolean(y)) => x == y,
+                    (Value::Unit, Value::Unit) => true,
+                    _ => false,
+                };
+                Value::Boolean(b)
+            }
+            Builtin::GreaterThan => int_cmp!(arguments, >),
+            Builtin::LessThan => int_cmp!(arguments, <),
+        };
+        Ok(result)
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index 5b628be..6be69f6 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -5,8 +5,8 @@ use std::path::Path;
 use anyhow::{anyhow, Context, Result};
 
 use mul::cli::{Command, USAGE};
-use mul::phase::evaluate::Interpreter;
-use mul::phase::parse::{parse_interactive_entry, parse_program};
+use mul::evaluate::Interpreter;
+use mul::parse::{parse_interactive_entry, parse_program};
 use mul::syntax::Program;
 
 fn main() -> Result<()> {
@@ -23,8 +23,7 @@ fn main() -> Result<()> {
         Command::Run { path } => {
             let program = parse(path.as_path())?;
             let mut interpreter = Interpreter::new();
-            let value = interpreter.run(&program)?;
-            println!("{value:#?}");
+            let _result = interpreter.run(&program)?;
             Ok(())
         }
         Command::Repl => repl(),
@@ -64,7 +63,7 @@ fn repl() -> Result<()> {
             }
         };
         let value = interpreter.evaluate_interactive_entry(&parsed)?;
-        println!("{value:?}");
+        println!("{value}");
     }
     Ok(())
 }
diff --git a/src/syntax.rs b/src/syntax.rs
index 20cd2cf..312aeeb 100644
--- a/src/syntax.rs
+++ b/src/syntax.rs
@@ -46,6 +46,7 @@ pub enum Expression {
     },
     Boolean(bool),
     Integer(i64),
+    Unit,
 }
 
 /// A sequence of bindings with a final result.