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

[Ryan Ingram]

My guess is that nobody has put forward a clear enough design that solves
all the problems.  In particular, orphan instances are tricky.

Here's an example:

module Prelude where

class (Functor m, Applicative m) => Monad m where
    return :: a -> m a
    (>>=) :: m a -> (a -> m b) -> m b
    (>>) :: m a -> m b -> m b
    a >> b = a >>= const b

    pure = return
    (<*>) = ap
    fmap = liftM

module X where
data X a = ...

module Y where
instance Functor X where fmap = ...

module Z where
instance Monad X where
    return = ...
    (>>=) = ...
    -- default implementation of fmap brought in from Monad definition

module Main where
import X
import Z

foo :: X Int
foo = ...

bar :: X Int
bar = fmap (+1) foo  -- which implementation of fmap is used?  The one from
Y?


  -- ryan


On Sun, Jul 24, 2011 at 8:55 PM, Ivan Lazar Miljenovic <
ivan.miljenovic at gmail.com> wrote:

> On 25 July 2011 13:50, Sebastien Zany <sebastien at chaoticresearch.com>
> wrote:
> > I was thinking the reverse. We can already give default implementations
> of class operations that can be overridden by giving them explicitly when we
> declare instances, so why shouldn't we be able to give default
> implementations of operations of more general classes, which could be
> overridden by a separate instance declaration for these?
> >
> > Then I could say something like "a monad is also automatically a functor
> with fmap by default given by..." and if I wanted to give a more efficient
> fmap for a particular monad I would just instantiate it as a functor
> explicitly.
>
> I believe this has been proposed before, but a major problem is that
> you cannot do such overriding.
>
> --
> Ivan Lazar Miljenovic
> Ivan.Miljenovic at gmail.com
> IvanMiljenovic.wordpress.com
>
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