[Haskell-cafe] why do I need class context in declaring data constructor?

[Paul Liu]

Thanks, Simon! I'll be looking forward to its resolution.

On Fri, Aug 31, 2012 at 4:37 AM, Simon Peyton-Jones
<simonpj at microsoft.com> wrote:
> 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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> | http://www.haskell.org/mailman/listinfo/haskell-cafe

-- 
Regards,
Paul Liu