[Haskell-cafe] IO () and IO [()]
lemming at henning-thielemann.de
Tue Mar 11 07:43:00 EDT 2008
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
instance Monoid a => Monoid (M a) where
mempty = return mempty
mappend = liftM2 mappend
where M is a monad type.
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