I don't think this is correct. My understanding is that it's not a library issue, and in terms of Haskell types (<=<) is not a generalisation of (.) (although

I don't think this is correct. My understanding is that it's not a library issue, and in terms of Haskell types (<=<) is not a generalisation of (.) (although it is mathematically speaking). If you want a -> m b to be a Category instance so you can use (.) you need the Kleisli newtype wrapper. If you want a -> b to work in any monad so you can use (<=<) you need to compose with return to get Monad m => a -> m b. Either way you have to explicitly specify the isomorphism explicitly and I don't think there's any getting around that.