Using mapM instead of mapM_ can lead to a memory leak and I think the confusion between mapM and mapM_ was the initial reason to add the unused-binding warning to GHC. And it actually helped me spotting a lot of calls to mapM that have to be calls to mapM_.
The different space behaviour is a very good reason to make the distinction for mapM and mapM_. However, it's not a good reason to choose arbitrarily restrictive types for perfectly generic functions in other cases IMHO. At least I don't see that there's a performance reason not to generalize when or unless, as their implementation would stay the same. I'd argue that making functions more restrictive than necessary isn't an indication of safety, it's just confusing :) Cheers, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com