[Haskell-cafe] mapM_ -> Monoid.Monad.map

[George Pollard]

On Fri, 2009-01-23 at 21:30 +0000, Joachim Breitner wrote:
> Hi,
> 
> Am Freitag, den 23.01.2009, 21:50 +0100 schrieb Henning Thielemann:
> >   However our recent Monoid discussion made me think about mapM_, 
> > sequence_, and friends. I think they could be useful for many monads if

> > they would have the type:
> >   mapM_ :: (Monoid b) => (a -> m b) -> [a] -> m b
> >    I expect that the Monoid instance of () would yield the same
efficiency 
> > as todays mapM_
> 
> will it? This is based on a naive, not well-founded understanding of
> haskell evaluation, but looking at
> > instance Monoid () where
> >	mempty        = ()
> >	_ `mappend` _ = ()
> >	mconcat _     = ()
> I’d assume that evaluating
> > mapM_ (putStrLn) lotsOfLargeStrings
> with your proposed mapM_ will leave a thunk equivalent to
> > () `mappend` () `mappend` () `mappend`...
> in memory until the mapM_ has completely finished, where each () is
> actually an unevalutated thunk that still has a reference to one of the
> elements in the lotsOfLargeStrings list.

Perhaps this is why the Monoid instance for () in GHC's source has the
comment "should this be strict?" :)
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