Haskell - A purely functional programming language that is a general purpose language. It's often cited to be best for making language parsers using Parsec or Attoparsec, but I've found with Snap/Yesod/Scotty it is great for web development.

Purescript - A pure functional alternative to JavaScript that compiles down to very concise readable JavaScript code in the rare case you need to debug it or track down performance issues.