OCaml

A programming language of the ML family of languages. Functional in spirit, flexible in practice: you can write straight-line mutable-state imperative subroutines just as easily as Haskell-esque pure functions. Just don't try to write a for loop.

Heavily used by a certain quantitative trading firm – and surprisingly not the only intersection of finance and esoteric programming languages.

Links

• Real World OCaml – a free online textbook