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

[Niklas Broberg]

>  >  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...

/Niklas