Several Traversable instances have an extra fmap

For example, we define

instance Traversable ZipList where
  traverse f (ZipList xs) = ZipList <$> traverse f xs

If the list is very short, the extra fmap could be bad. We can fix this by inlining the inner traverse. However, I suspect a better approach would be to add a method to Traversable:

mapTraverse :: Applicative f
            => (t b -> r) -> (a -> f b) -> t a -> f r
mapTraverse p f xs = p <$> traverse f xs

but I need to work through whether this is enough power to solve enough problems.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Core Libraries
Test case
Differential revisions
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Blocking
CC	ekmett
Operating system
Architecture