I know it's just anecdata, but from my experience, Haskell forces me (due to strict control over side effects) to write really short functions. And while that can be harder to do when you're writing the code, it seems to really help readability, testability and reusability.

It's not like you cannot write short functions in other languages - you can - but it's often too easy not to do it.

It would be nice to have some empirical evidence for this, unfortunately I am not aware of any.