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 at haskell.org [mailto:haskell-cafe-
| bounces at 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
|
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