Unlambda is a minimal, "nearly pure" 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 (<code>s</code> and <code>k</code>) and an apply operator (written <code>`</code>, 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.
Basic principles
As an esoteric programming language, Unlambda is meant as a demonstration of very pure functional programming rather than for practical use. Its main feature is the lack of conventional operators and data types—the only kind of data in the program are one-parameter functions. Data can nevertheless be simulated with appropriate functions as in the lambda calculus. Multi-parameter functions can be represented via the method of currying.
Unlambda is based on the principle of abstraction elimination, or the elimination of all saved variables, including functions. As a purely functional language, Unlambda's functions are first-class objects, and are the only such objects.
Here is an implementation of a hello world program in Unlambda: