You are indeed correct. However I fail to see how there is any undecideability. If instead one specified it as follows, it would be fine:
class Locatable a b | a -> b, b -> a where
value :: a -> b
wrap :: b -> a
instance Locatatable (Located a) a where
value (L a) = a
wrap a = L a
It compiles fine, yet this is semantically the same.
On 6/7/06, Bulat Ziganshin <[EMAIL PROTECTED]> wrote:
Hello Christophe,
Wednesday, June 7, 2006, 12:27:22 PM, you wrote:
> global instance of a typeclass (so there is no overlapping), GHC will
> still complain about overlapping. Is there any reason this is so or is
GHC compains here about undecidability, not overlapping :)
> -- Illegal instance declaration for `Locatable a'
> -- (There must be at least one non-type-variable in the instance head
> -- Use -fallow-undecidable-instances to permit this)
> -- In the instance declaration for `Locatable a'
> Christophe(vincenz)
--
Best regards,
Bulat mailto: [EMAIL PROTECTED]
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