I think you're on to something with your theory about wanting guarantees over expressivity. I'm no Haskell expert by any stretch, but it really pains me when one of my advanced students has to dive fully into monad land in order to do something that's otherwise painfully basic in most other languages. (Like, roll a die.) Certainly he's getting a lot out of the type system, in that he'll be able to structure his thoughts more clearly, but lately I'm wondering if it's worth it.