[Haskell-cafe] Monadic vs "pure" style (was: pros and cons of
sta tic typing and side effects)
Ben.Lippmeier at anu.edu.au
Tue Aug 30 20:55:29 EDT 2005
Perhaps you could write _everything_ in monadic style, and then derive
the non-monadic version by running it on an "empty" state monad. But
then if everything was already monadic you wouldn't need the non-monadic
> Perhaps the "inherent disadvantage" is that functions written in the
> monadic style must have different types compared with their conceptually
> similar non-monadic functions..
> mapM :: Monad m => (a -> m b) -> [a] -> m [b]
> map :: (a -> b) -> [a] -> [b]
> filter :: (a -> Bool) -> [a] -> [a]
> filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a]
> foldl :: (a -> b -> a) -> a -> [b] -> a
> foldM :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m a
> Some would say "but they're different functions!", others would say
> "close enough".
> I imagine this would be an absolute pain for library writers. Notice
> that we get Data.Map.map but no Data.Map.mapM - or perhaps there's some
> magical lifting combinator that I am not aware of?
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