Possible bug with GADTs?

[Brandon S Allbery KF8NH]

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On 08/17/2010 03:22 PM, Ben Moseley wrote:
> {-# LANGUAGE GADTs #-}
> module Foo where
>
> data TemplateValue t where
>   TemplateList :: [x] -> TemplateValue [x]
>
> instance (Eq a) => Eq (TemplateValue a) where
>   (==) (TemplateList b) (TemplateList c) = (==) b c  -- here we have a
== [x]
>
>     Could not deduce (Eq x) from the context (a ~ [x1])
>
> It looks as though it has decided to use the (instance Eq x => Eq [x])
instance, and hence is searching for Eq x (which can't be deduced)
rather than using the (Eq a / Eq [x]) directly.  So, I guess that's an
interesting question why that happens...

Hm... isn't that a case of overlapping and/or incoherent instances?
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