[Haskell] Type problem

Tomasz Zielonka tomasz.zielonka at gmail.com
Tue Dec 13 04:42:15 EST 2005

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

> 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

I am searching for a programmer who is good at least in some of
[Haskell, ML, C++, Linux, FreeBSD, math] for work in Warsaw, Poland
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