Add mapMaybeM

[vrom911]

With the type

mapMaybeM :: (a -> m (Maybe b)) -> f a -> m (f b)

[josephcsible]

That looks exactly like wither. How do you plan to implement it without the Filterable or Witherable typeclasses?

[chshersh]

@josephcsible Specialization to [] should work for now 👍 And once something like Witherable gets into base (or at least boot libraries), we can easily generalise this function.

[josephcsible]

Can we change this to use traverse instead of mapM, so that it can support applicatives that aren't monads?

[chshersh]

@josephcsible Do you have a particular use case in mind?

[josephcsible]

Not offhand. I just ask because that's how wither is, and I can't think of a good reason to have a new function be less general than it could otherwise be.