All Monads are Functors

[Jon Fairbairn]

On 2006-08-15 at 16:25CDT Taral wrote:
> On 8/15/06, Bulat Ziganshin <bulat.ziganshin at gmail.com> wrote:
> > in this case we lose "class Functor a => Monad a" base class
> > declaration. so what will be the meaning of this:
> 
> I don't see why that is the case.
> 
> class Functor m => Monad m where
>     return :: a -> m a
>     (>>=) :: m a -> (a -> m b) -> m b
>     instance Functor m where
>         fmap f = (>>= return . f)
> 
> What's wrong with this? All Monads are Functors. If you don't provide
> a Functor, it gets defined for you. The problem is working out whether
> to use the default Functor or an external Functor.

It seems obvious to me that we always use an external
definition if one exists, so I suppose the problem is
knowing whether an external instance exists -- so this
proposal would rely on doing something about scoping for
instances, I suppose.

-- 
Jón Fairbairn                              Jon.Fairbairn at cl.cam.ac.uk