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

[Ben]

On the test case i'm running the performance impacts of reversing the
return list are negligible:

mapM3 :: Monad m => (a -> m b) -> [a] -> m [b]
{-# INLINE mapM3 #-}
mapM3 fn lst = mapM3accum fn lst []
    where mapM3accum _ [] accum = return $ reverse accum
          mapM3accum fn (x:xs) accum = do
            r <- fn x
            mapM3accum fn xs (r:accum)

main5 = do
  print $ length $ mapM_ (flip replicate ()) [1..11]

time ~ 18 seconds (about the same, faster on my machine probably due
to timing artifacts) and the memory was about the same (strangely less
than the non-reversing one though again that's probably an artifact.)

In any case, I have some questions:

1) Why is the Prelude mapM so slow?  It seems like running 10x slower
than mapM_ when generating only 50,000 return values is a problem.

2) Is there a reason to not use mapM3 above?

Thanks and take care, Ben

On Thu, Apr 24, 2008 at 2:33 PM, Bulat Ziganshin
<bulat.ziganshin at gmail.com> wrote:
> Hello Niklas,
>
>
>  Friday, April 25, 2008, 1:25:39 AM, you wrote:
>
>  > Not that it should matter for performance any, but you really ought to
>  > reverse the result list too, or compute the accumulator in the right
>  > order. :-)
>
>  unfortunately, this affects performance too. reverse costs one more scan
>  through the list and building lot of thunks has its own space and time
>  cost
>
>
>
>
>  --
>  Best regards,
>   Bulat                            mailto:Bulat.Ziganshin at gmail.com
>
>