Re: [Haskell] Types of when and unless in Control.Monad

Btw. the mapM memory leak can be easily and accidentally resurrected by writing

 when_ b (mapM f xs)

with

 f :: a -> m ()  when_ :: Bool -> m a -> m ()

I don't understand the whole mapM analogy (yet). I don't need when_ to get into trouble with using mapM. Also, whether a space leak with mapM arises or not typically doesn't depend on whether you use its result or not. It's the other way round: if you run mapM on a huge list but aren't interested in the results, then you should use mapM_ instead. So the problem here is accidentally producing a (large) result rather than accidentally ignoring it. Cheers, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com