accumulating mapM (was Re: [Haskell-cafe] mapM vs mapM_ performance)
lrpalmer at gmail.com
Thu Apr 24 20:19:03 EDT 2008
On Fri, Apr 25, 2008 at 12:02 AM, Ben <midfield at gmail.com> wrote:
> Thanks for the nice answer. So maybe I'll give mapM3 the name mapM'
> and put it in my personal library.
Except the answer was wrong. I forgot the reverse in my
implementation, so that undefined we were seeing was just the last
element of the list. But the conclusion is still true :-)
*Main> take 3 $ runIdentity $ mapM return (1:2:3:4:undefined)
*Main> take 3 $ runIdentity $ mapM3 return (1:2:3:4:undefined)
*** Exception: Prelude.undefined
> 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
Tail recursion can make a huge difference in strict settings: the
difference between a loop and recursion in C.
More information about the Haskell-Cafe