Unlambda

Description #

Unlambda is a minimalist, functional esoteric programming language based entirely on combinatory logic. It eliminates variables altogether and uses only function application. It’s a parody of purely functional programming and is inspired by the SKI combinator calculus.

History #

Unlambda was created by David Madore in 1999. It was designed to challenge assumptions about programming by reducing it to the bare minimum: just functions and their applications. The language has since gained a cult following among functional programming fans and esolang explorers.

Hello World Code #

`r```````````.H.e.l.l.o. .w.o.r.l.d.!

This line uses the backtick (`) for function application and the dot (.) to output characters.

How to Run #

Option 1: Online

• Try It Online – Unlambda

Option 2: Local

Install a local interpreter:

• GitHub – Unlambda Interpreter

Save the file as hello.unl

Run:

./unlambda hello.unl

Key Concepts #

• No variables
• Uses combinators like s, k, and i
• Function application is done with backticks
• Output via .x prints character x
• Lazy evaluation
• Turing complete via combinator logic
• Unary functions only
• Can express recursion using the Y combinator
• Abstract and symbolic by design
• Excellent for demonstrating the fundamentals of computation

Try It Online #

• TIO – Unlambda

Fun Facts #

• Unlambda has no loops, if-statements, or variables—yet it’s Turing complete.
• Programs are often unreadable but mathematically elegant.
• It can be used to teach the basis of all functional languages (via SKI combinators).

Resources #

• Unlambda on Esolangs
• Unlambda GitHub Repository
• TIO Runner – Unlambda
• SKI Combinator Calculus