[Haskell-cafe] Why the reluctance to introduce the Functor requirement on Monad?

[Felipe Almeida Lessa]

On Thu, Jul 21, 2011 at 8:31 AM, Ivan Lazar Miljenovic
<ivan.miljenovic at gmail.com> wrote:
> Well, for fmap vs liftM, you have that liftM is automatically defined
> for you rather than needing to make the Functor instance, so if you're
> quickly defining a Monad for internal use then you can just use liftM,
> etc. without needing to also make Functor and Applicative instances
> (note that AFAIK, return  and pure are the same thing, in that return
> isn't automatically defined like liftM is).

Note that even if we had "class Applicative m => Monad m where ...",
we could say

  data X a = ...

  instance Functor X where
    fmap = liftM

  instance Applicative X where
    pure = return
    (<*>) = ap

  instance Monad X where
    return = ...
    x >>= f = ...

So you just need five more lines of boilerplate to define both Functor
and Applicative.

Cheers,

-- 
Felipe.