[Haskell-cafe] Re: Comparing GADTs for Eq and Ord
apfelmus
apfelmus at quantentunnel.de
Tue Sep 16 03:55:40 EDT 2008
Tom Hawkins wrote:
> apfelmus 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 = ...
>
Well,
Const :: a -> Term a
is too general anyway, you do need some information on a to be able to compare
different Const terms. An Eq constraint on a is the minimum:
Const :: Eq a => a -> Term a
But that's not enough for (===) of course, additional information as suggested
by others is needed.
Regards,
apfelmus
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