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.
Greetings, Joachim -- Joachim "nomeata" Breitner mail: mail@joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nomeata@joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nomeata@debian.org