Possible bug with GADTs?

[Ben Moseley]

Thinking about this a bit more, and renaming some of the variables:

{-# 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...

--Ben

On 17 Aug 2010, at 20:04, Ben Moseley wrote:

> 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)
>> _______________________________________________
>> Glasgow-haskell-users mailing list
>> Glasgow-haskell-users at haskell.org
>> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
> 
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