On Tue, Feb 15, 2011 at 12:48 AM, Max Bolingbroke <batterseapo...@hotmail.com> wrote: > On 14 February 2011 21:43, Patrick Bahr <pa...@arcor.de> wrote: >> Am I doing something wrong or is it not possible for GHC to dispatch a rule >> according to type class constraints? > > As you have discovered this is not possible. You can write the rule > for as many *particular* types as you like, but you can't write it in > a way that abstracts over the exact type class instance you mean. This > is a well known and somewhat tiresome issue. > > I think the reason that this is not implemented is because it would > require the rule matcher to call back into the type checking machinery > to do instance lookup.
This is a semi-related question I've been meaning to ask at some point: I suppose this also means it's not possible to write a class, write some rules for the class, and then have the rules be applied to every instance? (I.e. you'd have to write them separately for each?) > > Cheers, > Max > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Work is punishment for failing to procrastinate effectively. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
