A purely functional programming language based on lambda calculus and de Bruijn indices written in Haskell.

Pronunciation: /bɹaʊn/.

Wiki, docs, articles, examples and more: website. Also: Rosetta Code.

Features

Substantial standard library with 700+ useful functions (see std/)
No primitive functions - every function is implemented in Bruijn itself
1:1 correspondence to lambda calculus (e.g. space-efficient compilation to binary lambda calculus (BLC))
de Bruijn indices instead of named variables
Lazy evaluation by default (call-by-need reduction)
Syntactic sugar makes writing terms simpler (e.g. numbers, strings, chars, meta terms)
Mixfix and prefix operators
Recursion can be implemented using combinators such as Y, Z or ω

Why

By having a very small core (the reducer), bruijn is safe, consistent, and (potentially) proven to be correct!
Since it doesn’t have builtin functions, bruijn is independent of hardware internals and could easily be run on almost any architecture.
Compiled binary lambda calculus is incredibly expressive and tiny. Read the articles by Jart and Tromp.
Exploring different encodings of data as function abstractions is really fascinating.
Naming parameters of functions is annoying. De Bruijn indices are a universal reference independent of the function and can actually help readability!
Really, just for fun.

Learn anything about bruijn in the wiki (also found in docs/wiki_src/).

De Bruijn, Nicolaas Govert. “Lambda calculus notation with nameless dummies, a tool for automatic formula manipulation, with application to the Church-Rosser theorem.” Indagationes Mathematicae (Proceedings). Vol. 75. No. 5. North-Holland, 1972.
Mogensen, Torben. “An investigation of compact and efficient number representations in the pure lambda calculus.” International Andrei Ershov Memorial Conference on Perspectives of System Informatics. Springer, Berlin, Heidelberg, 2001.
Wadsworth, Christopher. “Some unusual λ-calculus numeral systems.” (1980): 215-230.
Tromp, John. “Binary lambda calculus and combinatory logic.” Randomness and Complexity, from Leibniz to Chaitin. 2007. 237-260.
Tromp, John. “Functional Bits: Lambda Calculus based Algorithmic Information Theory.” (2022).
Biernacka, M., Charatonik, W., & Drab, T. (2022). A simple and efficient implementation of strong call by need by an abstract machine. Proceedings of the ACM on Programming Languages, 6(ICFP), 109-136.