Why is GHCi typing this statement oddly?

Question

In answering a question on stackoverflow, I noticed that GHCi (interactive) is assigning a too-restrictive type in a let statement. Namely, given the code,<

Answer 1

See the extended defaulting rules used in GHCi for an explanation of where the () is coming from.

As for why the defaulting occurs in this case, compare the following:

> let f x = maximum &&& id >>> fst &&& (\(m,l) -> length $ filter (==m) l) $ x
> :t f
f :: (Ord a) => [a] -> (a, Int)

I assume this has something to do with bindings being monomorphic, but I'm not certain of the details.