a faster, accumulating mapM (was Re: [Haskell-cafe] mapM vs mapM_ performance)

Niklas Broberg niklas.broberg at gmail.com
Thu Apr 24 20:25:13 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...


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