That's good enough until GHC 6.6.
Thank you very much!
/ Emil
Tomasz Zielonka skrev:
On Tue, Dec 13, 2005 at 09:46:31AM +0100, Emil Axelsson wrote:
Is this just a limitation of the current GATDs, or is it unreasonable of me
to expect this to work?
AFAIK it is a current limitation of GADTs, which will be removed in GHC
6.6.
Is there any workaround, such as coercing the type of the value function?
I've had the same problem myself. The workaround is to replace some of
type-class constraints with "witness" GADTs. The code I attached
shows how you can do it. I chose to make HasX a GADT, and introduce
the HasX' type-class, but the latter is only for convenience. Note
the subtle change in Val's definition:
data Val a where
...
X :: HasX a -> Val a -- Unconstrained
^^
Best regards
Tomasz
------------------------------------------------------------------------
{-# OPTIONS -fglasgow-exts #-}
data Number = XN -- Unconstrained
| N Int -- Constrained
data HasX a where
HasX_Number :: HasX Number
xVal :: HasX a -> a
xVal HasX_Number = XN
class HasX' a where
hasX :: HasX a
instance HasX' Number where
hasX = HasX_Number
x :: HasX' a => Val a
x = X hasX
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 hx) = xVal hx
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
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