On Wed, Oct 29, 2008 at 10:20 PM, C Rodrigues
I discovered that closed type classes can be implicitly defined using GADTs The GADT value itself acts like a class dictionary. However, GHC (6.83) doesn't know anything about these type classes, and it won't infer any class memberships. In the example below, an instance of Eq is not recognized.
So is this within the domain of GHC's type inference, not something it shoud infer, or a bug?
{-# OPTIONS_GHC -XTypeFamilies -XGADTs -XEmptyDataDecls #-} module CaseAnalysisOnGADTs where
-- Commutative and associative operators. data CAOp a where Sum :: CAOp Int Disj :: CAOp Bool Concat :: CAOp String
I guess when you write something like this the type checker doesn't look at the closed set of types that 'a' can take on. I don't know why that would be a problem in your example, but if we did this: data CAOp a where Sum :: CAOp Int Disj :: CAOP Bool Concat :: CAOp String Weird :: Show a => CAOp a Now we have a problem, because the set of types for is { Int, Bool, String, forall a. Show a => a}. It's not a closed set anymore. And I think, but I'm just making this up, that the above set of type must be hard to reason about. I think it would essentially be: data Show a => CAOp a where ... And GHC won't allow that, presumably for a good reason. Neat example, and I hope someone sheds more light on this. By the way, since you mention ghc6.8.3, does that mean some other version of GHC permits noEvidenceOfEq to type check? I tried 6.6.1 but that didn't accept it either. Jason