I had a toy program that encodes simply typed lambda in types. It used to work fine with GHC prior to 7.2. But now it no longer compiles. Here is a minimal fragment that demonstrates this problem.
> {-# LANGUAGE GADTs, > MultiParamTypeClasses, > FlexibleInstances, > FlexibleContexts #-} > > data Abs env t v where > Abs :: g (a, env) h v -> Abs env (g (a, env) h v) (a -> v) > > class Eval g env t v where > eval :: env -> g env t v -> v > > instance Eval g (a, env) h v => > Eval Abs env (g (a, env) h v) (a -> v) where > eval env (Abs e) = \x -> eval (x, env) e The type Abs has 3 parameters: its environment, sub term (encoded in types), and type. The constructor Abs has 1 parameter: its sub term. The code loads fine in GHC 7.0.3. Here is the error reported by GHC 7.2.2 (and later): test.lhs:14:30: Could not deduce (Eval g1 (a1, env) h1 v1) arising from a use of `eval' from the context (Eval g (a, env) h v) bound by the instance declaration at test.lhs:(12,12)-(13,49) or from (g (a, env) h v ~ g1 (a1, env) h1 v1, (a -> v) ~ (a1 -> v1)) bound by a pattern with constructor Abs :: forall env (g :: * -> * -> * -> *) a h v. g (a, env) h v -> Abs env (g (a, env) h v) (a -> v), in an equation for `eval' at test.lhs:14:15-19 Possible fix: add (Eval g1 (a1, env) h1 v1) to the context of the data constructor `Abs' or the instance declaration or add an instance declaration for (Eval g1 (a1, env) h1 v1) In the expression: eval (x, env) e In the expression: \ x -> eval (x, env) e In an equation for `eval': eval env (Abs e) = \ x -> eval (x, env) e However, if I move the class context to the data constructor of definition, then it compiles fine in GHC 7.2.2 (and later): > data Abs env t v where > Abs :: Eval g (a, env) h v => g (a, env) h v -> Abs env (g (a, env) h v) (a > -> v) But this is very troublesome because for every new class instance I want to make Abs of, I have to make a new class context to the data constructor. It totally defeats the purpose of making class instances to extend usage of data types. Did I missed a language extension when moving code from GHC 7.0.3 to GHC 7.2.2? What can I do to fix it for newer GHCs? -- Regards, Paul Liu _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe