John Meacham
On Tue, Aug 15, 2006 at 10:34:39PM +0100, Jon Fairbairn wrote:
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.
The problem is you can't have working code change its behavior because of a module import (other than failing), say, by bringing an instance into scope that wasn't before. There is no way to have a monad instance 'automatically' declare a functor instance without changing what the mo instance looks like somehow which would be a backwards incompatable change.
In that case we really ought just to accept that we have to declare the instances and swallow class Functor m => Monad m where ... without any cleverness. * * * another, more adventurous path would be to find a set of restrictions that allow one to define instance Monad m => Functor m where fmap f = (>>= f . return) without it being undecidable. (I don't really like this, because it seems to be backwards). -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2006-07-14)