On 11/05/2015 22:41, Dan Doel wrote:

The reason I know of why mapM wasn't just made to be an alias for
traverse (assuming that's what you mean) was that it was thought that
particular definitions of mapM could be more efficient than traverse.
For instance:

     mapM :: Monad m => (a -> m b) -> [a] -> m [b]
     mapM f = go []
       where
       go ys [] = return (reverse ys)
       go ys (x:xs) = f x >>= \y -> go (y:ys) xs

This doesn't use stack for m = IO, for instance.

However, it has since been pointed out (to me and Ed, at least), that
this matters much less now. Stack overflows are now off by default, and
if you measure the overall time and memory usage, traverse compares
favorably to this custom mapM. So, as long as stack isn't an
artificially scarce resource, there's no reason to keep them distinct.
We didn't know this until after 7.10, though.

If you're just asking why the definition of 'mapM' for lists isn't
'traverse' with a more specific type, I don't know the answer to that.

Yes, I'm not really concerned that mapM is a method of Traversable rather than just being an alias for traverse, but I'm wondering why we define it in the list instance rather than using the default.

Cheers,
Simon

-- Dan


On Mon, May 11, 2015 at 3:15 PM, Simon Marlow <marlowsd@gmail.com
<mailto:marlowsd@gmail.com>> wrote:

    I was hoping that in GHC 7.10 we would make mapM = traverse for
    lists, but it appears this isn't the case: the Traversable instance
    for lists overrides mapM to be the manually-defined version in terms
    of foldr.

    Why is this?  Fusion?

    Unfortunately since I want mapM = traverse (for Haxl) I'll need to
    continue to redefine it in our custom Prelude.

    Cheers,
    Simon
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