[clean-list] Repeated type variable in multi-parameter type classes

[Arjen van Weelden]

Hi,

This is a restriction of the current implementation of multiple 
parameter classes in Clean. Could you use the following instead?

instance SubType a (Either b c) | SubType a b
where
	inj x        = Left (inj x)
	prj (Left x) = prj x
	prj _        = Nothing

The next restriction you will run into is that both instances
Subtype a (Either b c) overlap. In appears that in Haskell you can use 
type class instances to guide/steer the type inference. You cannot do 
such a thing in Clean, as far as I know.

kind regards,
	Arjen

romildo at iceb.ufop.br wrote:
> Hello.
> 
> I am porting a Haskell program to Clean, where type classes are used to
> introduce some kind of subtyping:
> 
> //-------------------------------------------------------------
> :: Either a b = Left a | Right b
> 
> class SubType sub sup
> where
> 	inj :: sub -> sup
> 	prj :: sup -> Maybe sub
> 
> instance SubType a (Either a b)
> where
> 	inj x        = Left x
> 	prj (Left x) = Just x
> 	prj _        = Nothing
> 
> instance SubType a (Either c b) | SubType a b
> where
> 	inj x         = Right (inj x)
> 	prj (Right x) = prj x
> 	prj _         = Nothing
> //-------------------------------------------------------------
> 
> But Clean rejects the first instance declaration because the type
> variable a occours more than once.
> 
> How can I define that instance in Clean? It is not a problem for
> Haskell.
> 
> Regards.
> 
> Romildo
> _______________________________________________
> clean-list mailing list
> clean-list at science.ru.nl
> http://mailman.science.ru.nl/mailman/listinfo/clean-list