[Haskell-cafe] Re: Comparing GADTs for Eq and Ord

[Dan Weston]

Take a look at

http://www.haskell.org/haskellwiki/GHC/AdvancedOverlap

Tom Hawkins wrote:
> On Mon, Sep 15, 2008 at 3:11 PM, apfelmus <apfelmus at quantentunnel.de> wrote:
>> So, in other words, in order to test whether terms constructed with  Equal  are
>> equal, you have to compare two terms of different type for equality. Well,
>> nothing easier than that:
>>
>>    (===) :: Expr a -> Expr b -> Bool
>>    Const       === Const         = True
>>    (Equal a b) === (Equal a' b') = a === a' && b === b'
>>    _           === _             = False
>>
>>    instance Eq (Expr a) where
>>        (==) = (===)
> 
> OK.  But let's modify Expr so that Const carries values of different types:
> 
> data Expr :: * -> * where
>   Const :: a -> Term a
>   Equal :: Term a -> Term a -> Term Bool
> 
> How would you then define:
> 
> Const a === Const b  = ...
> 
> 
> -Tom
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