return package:constrained-categories

This is monomorphic in the category Hask, thus exactly the same as return from the standard prelude. This allows writing expressions like return $ case x of ..., which would always be ambiguous with the more general signature Monad m k => k a (m a). Use pure when you want to "return" in categories other than (->); this always works since Applicative is a superclass of Monad.
Inject a value into the monadic type.