Unlambda - Your Functional Programming Nightmares Come True

The Lambda calculus is a formal system for expressing computation based on function abstraction. Functional programming languages are counterpart to lambda calculus in computer science. But also other languages have been extended with lambda abstraction.

Unlambda is an (obfuscated) functional programming language designed to make programming painful and difficult. Unlambda likes unfeatures (no variables, data structures, code constructs, ..). Everything is a unary function. As algebraist I cherry-picked the concept of Idempotence.

Example from the Unlambda home page: the following Unlambda program calculates and prints the Fibonacci numbers (as lines of asterisks)
Writing Unlambda programs may not be too hard, but reading them is practically impossible.