[Haskell] Type problem
Emil Axelsson
emax at cs.chalmers.se
Tue Dec 13 04:58:12 EST 2005
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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