Hi all, I've been playing with multiparameter typeclasses recently and have written a few "uncallable methods" in the process. For example, in
class Moo a b where moo :: a -> a the moo function is effectively impossible to call (no amount of type annotations can tell the compiler what you intended b to be there). Some might suggest adding an a -> b functional dependency, but in some cases that is not appropriate, as there are multiple possible instances. Another solution would be to artificially force moo to take a "dummy" b so that the compiler can figure out which instance you meant. That's what I've been doing in the mean time, but wouldn't it be simpler and less hackish to add a some form of "instance annotation", like a type annotation, that would make it possible to specify what instance you wanted when it's ambiguous? I'm not sure what syntax might be appropriate here, but it could also be seen as "opening" a particular instance, so something "open"-like might be good. I don't know whether this has already been discussed much, but I was unable to find anything that seemed relevant (beyond discussing relationships between parametrized modules and typeclasses) but I wanted to know if anyone had any opinion on adding such a feature to (a future revision of) Haskell? I know people are generally reluctant to add new syntax elements to a language, but considering the lack of such a feature and the impossibility of writing such a thing in the language itself, it seems like it'd be useful to add to the language itself. Thanks, Dan _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
