I have had my eye on functional programming languages for a while, but am hesitating to actually get into them. But I think it's about time I at least starting glancing that direction to make sure I'm ready for anything.

I've seen talk of Haskell, F#, Scala, and so on. But I have no clue the differences between the languages and their communities, nor do I particularly care; except in the context of game development.

So, from a game development standpoint, which functional programming language has the most features suited for game programming? For example, are there any functional game development libraries/engines/frameworks or graphics engines for functional languages? Is there a language that handles certain data structures which are commonly used in game development better?

Bottom line: what functional programming language is best for functional game programming, and why?

I believe/hope this question will declare a clear best language therefore I haven't marked it CW despite its subjective tendency.