Since GHC-7.10, the minimal complete definition for a Monad instance has been (>>=) (bind) only. So you can simply move the old definition for "return" into the

Since GHC-7.10, the minimal complete definition for a Monad instance has been (>>=) (bind) only. So you can simply move the old definition for "return" into the Applicative instance as "pure" and you're good.