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

5 Jun 2012

      On Tue, 5 Jun 2012, Andres Löh wrote:
...
...
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.
If I write

   when b (mapM f xs)

with

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

then GHC will say something like: Cannot match [()] (result of 'mapM') 
with () (argument of 'when').

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

Henning Thielemann