Why Monad instance of pair on Hackage has no implementation for return?

Question

While writing down this answer, mostly to get a better understanding of pairs as monads, I stumbed into this source code on Hackage where I read, in reference to the Monad instance of (,) a only this

instance Monoid a => Monad ((,) a) where
    (u, a) >>= k = case k a of (v, b) -> (u <> v, b)

where's return??? I expected to find something like this

    return a = (mempty, a)

in addition to the two lines above. Is this definition of return somehow implied by something else? Or maybe it's defined somewhere else?

Answer 1

In modern versions of Haskell (specifically, base version 4.8.0.0 and newer, corresponding to GHC version 7.10.1 and newer), the Monad class has the default implementation return = pure, so instances of it only need to define >>=. This was a result of the Functor-Applicative-Monad Proposal.