I tried to post this to gmane.comp.lang.haskell.general, but it never got there - it may belong here anyway. Abraham Egnor wrote:
Is there any closer approximation [of GADTs] possible?
{-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} data Term a = forall b . (EqType a b, EqType b Int) => Lit Int | forall b . (EqType a b, EqType b Int) => Inc (Term Int) | forall b . (EqType a b, EqType b Bool) => IsZ (Term Int) | If (Term Bool) (Term a) (Term a) | forall b . Fst (Term (a,b)) | forall b . Snd (Term (b,a)) | forall b c . (EqType a (b,c))=> Pair (Term b) (Term c) class EqType a b | a->b, b->a instance EqType a a Unfortunately, I can't seem to write an eval function: eval (Lit a) = a gives Inferred type is less polymorphic than expected Quantified type variable `b' is unified with `Int' When checking an existential match that binds The pattern(s) have type(s): Term Int The body has type: Int In the definition of `eval': eval (Lit x) = x Any ideas? Jim