[ufo]

If you dont know anything about haskell (orat leastanother similar language). Then do yorself a favour and go learn more about it :)

If you already know what haskell is about, id say its killer feature is the rich type system. Not only is it very complete (including support for generics, overloading and much more) but the ecosystem as a whole is oriented towards static safety guarantees and having the compiler help you avoi errors. For example, option types make so you never ever get a nullpointerexception and after you get the hang of it you can also start encoding other proprties of your own. As a bonus, the language itself is quite clean and pretty, something I find surprising for a language designed by comitee.