[Haskell] Type problem

[Emil Axelsson]

Hello all,

Could I please get some guidance with this?

I'm working on implementing a simple relational language in Haskell.
I'm trying to construct a data type that can represent values and patterns for a 
small set of supported types. See code below.

HasX is a class of types that have an unconstrained value (xVal).

Number is a typical member of that class.

Val is my value/pattern data type.
P represents a primitive value and T2 is used to make structure.
X represents the unconstrained value or a wildcard pattern. It can only be used 
for types in HasX.

The problem is the commented line in the value function. I want to use the xVal 
method to get the value for X. This is only allowed if I add the constraint 
(HasX a => ). But I don't want value to have that constraint, since then I 
cannot run it on pairs.
Furthermore, it should be safe without the constraint. ex2 shows that we cannot 
use X to construct values that are not in HasX.

Is this just a limitation of the current GATDs, or is it unreasonable of me to 
expect this to work?

Is there any workaround, such as coercing the type of the value function?

-- 
/ Emil


********************************
{-# OPTIONS -fglasgow-exts #-}

class HasX a where
   xVal :: a

data Number = XN     -- Unconstrained
             | N Int  -- Constrained

instance HasX Number where
   xVal = XN



data Val a where
   P :: a -> Val a       -- Primitive

   T2 :: (Val a1, Val a2) -> Val (a1,a2)

   X :: HasX a => Val a  -- Unconstrained



value :: Val a -> a
-- value X            = xVal
value (P a)        = a
value (T2 (a1,a2)) = (value a1, value a2)



ex1 :: Val (Number,(Number,Number))
ex1 = T2 (P (N 3), T2 (X, P (N 5)))

-- ex2 :: Val (Number,Number)
-- ex2 = X