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

[Ben]

Luke,

Thanks for the nice answer.  So maybe I'll give mapM3 the name mapM'
and put it in my personal library.

But I'm still a bit curious about the performance profile of mapM.
The profiler is telling me they're allocating around the same amount
of memory, so I am not clear what is making it slow.  I am guessing it
has something to do with extra thunks due to laziness, but a 10x
slowdown?

Thanks again, B

On Thu, Apr 24, 2008 at 4:45 PM, Luke Palmer <lrpalmer at gmail.com> wrote:
> On Thu, Apr 24, 2008 at 11:28 PM, Ben <midfield at gmail.com> wrote:
>  >  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.
>
>  Luke
>