I've been very excited by many approaches before, then Haskell, and now I'm past it. While pure functional language will force you to cleanly separate computation and IO, with an app big enough you'll find yourself one day in a situation where independent data from 3 different places comes together in a way that the only reasonable way to proceed is to log a warning, tweak the value, and continue the computation. And that's what you'll do in any unpure language. In a pure one, you'll start to think how to redesign the computation to allow this... It's not wrong, but in many cases it's pointless.