Henning Thielemann wrote:
On Mon, 10 Mar 2008, Neil Mitchell wrote:
I would like to know if in fact there's any difference in practice between (), [()], i.e. if in practice the difference matters.
Usually, not so much. A lot of Monad functions have _ variants, i.e. mapM and mapM_. If you don't need the result, use the mapM_ version, as it will run faster and not space/stack leak in some circumstances.
In my opinion, mapM_ and sequence_ are in the wrong class, because they do not need much of Monads, or even Functors. They could well live, say, in Data.Monoid class. However, it's hard to integrate that in a hierarchy of type classes.
instance Monoid a => Monoid (M a) where mempty = return mempty mappend = liftM2 mappend
where M is a monad type.
Surely you mean to say: instance Monad m => Monoid (m ()) where mempty = return () mappend = (>>) ? That is the instance which is consistent with your text "don't need much of monads". Then sequence_ becomes mconcat, and mapM_ becomes foldMap (from Data.Foldable), or more directly mconcat $ map ... See also Control.Applicative, for things which can be sequence_'ed or even sequence'd without being Monads. Jules