On Thu, Jul 21, 2011 at 8:31 AM, Ivan Lazar Miljenovic
<ivan.miljenovic@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.

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