Possible bug with GADTs?

[Ben Moseley]

It looks to me as though that wouldn't be expected to work because 'a' and 't' are different type variables... which seems to be essentially what the error msg is saying...

...am I missing something?

--Ben

On 17 Aug 2010, at 19:54, Dan Knapp wrote:

> Below, please find a snippet from a program I'm working on, and the
> error it produces.  I was told in #haskell that this was "pretty
> suspect" and could conceivably be a ghc bug.  So I'm reporting it
> here.  I'd also be grateful for workarounds.  This is on ghc
> 6.12.1.20100203, but if people can't reproduce it I'll install a newer
> one; I'm just not eager to do that because of course it means
> rebuilding quite a lot of things.
> 
> 
> {-# LANGUAGE GADTs #-}
> module Foo where
> 
> data TemplateValue t where
>  TemplateList :: [a] -> TemplateValue [a]
> instance (Eq a) => Eq (TemplateValue a) where
>    (==) (TemplateList a) (TemplateList b) = (==) a b
> 
> 
> Foo.hs:7:45:
>    Could not deduce (Eq a1) from the context (a ~ [a2])
>      arising from a use of `==' at Foo.hs:7:45-52
>    Possible fix:
>      add (Eq a1) to the context of the constructor `TemplateList'
>    In the expression: (==) a b
>    In the definition of `==':
>        == (TemplateList a) (TemplateList b) = (==) a b
>    In the instance declaration for `Eq (TemplateValue a)'
> 
> 
> 
> -- 
> Dan Knapp
> "An infallible method of conciliating a tiger is to allow oneself to
> be devoured." (Konrad Adenauer)
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