Functional programming is a radical and elegant attack on the whole enterprise of writing programs. It’s very different from the “do this and then do that” programming mentality. You have to rewire your brain in quite a different way. For a long time it was well understood theoretically—there was lots of stuff about semantics and it had these very deep foundations in logic. But in terms of a practical programming medium it seemed like a completely virgin field. — Interview with Simon Peyton-Jones