accumulating mapM (was Re: [Haskell-cafe] mapM vs mapM_ performance)
midfield at gmail.com
Thu Apr 24 20:02:18 EDT 2008
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
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]
> *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
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