On Tue, 2007-12-11 at 16:38 +0000, Ross Paterson wrote: > On Tue, Dec 11, 2007 at 04:26:52PM +0000, Simon Marlow wrote: > > Duncan Coutts wrote: > >> On Tue, 2007-12-11 at 07:07 -0800, Stefan O'Rear wrote: > >>> This is almost exactly the > >>> http://haskell.org/haskellwiki/Class_system_extension_proposal; that > >>> page has some discussion of implementation issues. > >> > >> Oh yes, so it is. Did this proposal get discussed on any mailing list? > >> I'd like to see what people thought. Was there any conclusion about > >> feasibility? > > > > Ross proposed this on the libraries list in 2005: > > > > http://www.haskell.org//pipermail/libraries/2005-March/003494.html > > and again in 2003: > > http://www.haskell.org/pipermail/haskell-cafe/2003-July/004654.html
Ross, you need to shout louder! :-) If it really would work ok we should get it fully specified and implemented so we can fix the most obvious class hierarchy problems in a nice backwards compatible way. Things are only supposed to be candidates for Haskell' if they're already implemented. So how about the objection that two sub classes could try and define conflicting defaults for a superclass method? David Menendez had the example of Monad and CoMonad defining Functor's fmap. Can that easily be rejected? I suppose it gives rise to duplicate instance declarations so it'd be an error in the same way that defining clashing instances in two different modules and importing both into a third module. Another error case would be: module A where data Foo module B where instance Functor Foo module C where instance Monad Foo module D import Bar import Baz Now we get slashing instances for Functor, since both Bar and Baz export Functor instances for Foo. Since the instance for Functor Foo was not visible in module C, so we get the default instance defined in C. So the one slightly surprising thing about this suggestion is that we get an instance defined or not depending on whether there is already an instance in scope. In the Functor, Applicative, Monad case I don't see that causing a problem in practise but is it worse more generally? Duncan _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
