accumulating mapM (was Re: [Haskell-cafe] mapM vs mapM_ performance)
dons at galois.com
Thu Apr 24 20:44:43 EDT 2008
> > > 2) Is there a reason to not use mapM3 above?
> > Yes, there certainly is. mapM3 is not equivalent to mapM; it is too strict:
> > *Main> take 3 $ head $ mapM return [1,2,3,4,undefined]
> > [1,2,3]
> > *Main> take 3 $ head $ mapM3 return [1,2,3,4,undefined]
> > [*** Exception: Prelude.undefined
> > So, like foldl', mapM3 seems a viable alternative for mapM, but not a
> > replacement.
> Wow. A 10x slowdown for a very commonly used function that in 99.8% of
> all use cases has no need for the extra laziness at all. No wonder
> some people say Haskell is a toy language...
mapM_ is far more common, and optimised specially.
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