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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