On Fri, Apr 25, 2008 at 12:02 AM, Ben
Luke,
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) [1,2,3] *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 slowdown?
Tail recursion can make a huge difference in strict settings: the difference between a loop and recursion in C. Luke