Hello Bulat, 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 <bulat.ziganshin@gmail.com> wrote:

Hello Christophe,

Wednesday, June 7, 2006, 12:27:22 PM, you wrote:

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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 :)

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-- 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'

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Christophe(vincenz)

-- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com