Proposal: Add concatMapM function (#2042)

[Yitzchak Gale]

> the one thing that you miss - each time you add popular function to
> the base library, all programs that include this function
> becomes incompatible with next GHC version. i wonder whether anyone
> here has the experience of writing large programs and maintaining them
> through the years?

I import only the functions I need from libraries that I don't
own myself to minimize that effect.

That said, feature creep is definitely a problem. I think
the GHC team will be the first to agree. But this particular
function really is a natural.

One point to consider - perhaps nowadays the type ought to
be:

concatMapM :: (Monad m, Traversable t) => (a -> m (t b)) -> t a -> m (t b)

Regards,
Yitz