Try the HEAD. GADTs + type classes are broken in 6.6., and will stay that way I'm afraid. Simon | -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Daniil | Elovkov | Sent: 22 March 2007 17:12 | To: haskell-cafe@haskell.org | Subject: [Haskell-cafe] type refinement to a typeclass constrained type | | Hello folks | | A section on GADTs in the GHC user's guide gives a simple example: | | data Term a where | Lit :: Int -> Term Int | Succ :: Term Int -> Term Int | IsZero :: Term Int -> Term Bool | | eval :: Term a -> a | eval (Lit i) = i | eval (Succ t) = 1 + eval t | eval (IsZero t) = eval t == 0 | | let's add | Sum :: Term Int -> Term Int -> Term Int | eval (Sum l r) = eval l + eval r | | it's ok | | But when I do this | Sum :: (Num a) => Term a -> Term a -> Term a | eval (Sum l r) = eval l + eval r --- same | | it doesn't typecheck with the error "no isntance Num a" | | writing | eval (Sum (l::Num b => Term b) (r::Num b => Term b)) | doesn't help. | | I wonder, if there's a way to do what I'm trying to do. After all, I'm | allowed to declare Term that way. Or is this a restriction of type | refinement? | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe