Jón Fairbairn <[EMAIL PROTECTED]> writes: > John Meacham <[EMAIL PROTECTED]> writes: > > 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.
Or, alternatively, make my previous suggestion less automatic. I was suggesting that we could allow default instances of superclasses in class declarations; we'd say > class Functor m => > Monad m where > (>>=):: ... > instance Functor m where > fmap = ...<a> > ... what I'm proposing here is that we do that, but the "default" instances are never supplied automatically. Instead we allow a "deriving" clause on instance declarations: > instance Monad [] deriving Functor where > ... Now this would be exactly equivalent to > instance Monad [] where > ... > instance Functor [] where > fmap = ...<a> (and just as much an error if there's another instance declaration for Functor somwhere), while an instance declaration without the deriving clause would not say anything about a Functor instance at all. So, not fully automatic -- so avoiding the above problem, but more concise than a whole instance declaration. -- Jón Fairbairn [EMAIL PROTECTED] _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
