Unlambda

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Unlambda is a minimal, "nearly pure"^[1] functional programming language invented by David Madore. It is based on combinatory logic, an expression system without the lambda operator or free variables. It relies mainly on two built-in functions (s and k) and an apply operator (written `, the backquote character). These alone make it Turing-complete, but there are also some input/output (I/O) functions to enable interacting with the user, some shortcut functions, and a lazy evaluation function. Variables are unsupported.

Unlambda is free and open-source software distributed under a GNU General Public License (GPL) 2.0 or later.^{[clarification needed]}