Aha. See http://hackage.haskell.org/trac/ghc/ticket/7205. I don't think there's a workaround, I'm afraid Simon | -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe- | bounces@haskell.org] On Behalf Of Paul Liu | Sent: 30 August 2012 20:52 | To: Haskell Cafe | Subject: [Haskell-cafe] why do I need class context in declaring data | constructor? | | 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