How to fix DatatypeContexts?

[Edward Kmett]

This is exactly what GADTs are for.

-Edward

On Thu, Jul 18, 2013 at 6:54 AM, harry <voldermort at hotmail.com> wrote:

> data Eq a => Pair a = Pair {x::a, y::a}
>
> equal :: Pair a -> Bool
> equal pair = (x pair) == (y pair)
>
> This code will fail to compile, even with the deprecated DatatypeContexts
> extension, because equal must be given the Eq a => constraint, even though
> this has already been declared on the Pair type.
>
> Some of my code is littered with such redundant type constraints (although
> some of them could be replaced with a redundant pattern match). A proposal
> to enhance DatatypeContexts (#8026) in this way was rejected as unsound, as
> some nefarious uses of undefined would break the type system.
>
> Is there any way that the type system could be enhanced to avoid this
> redundancy?
>
>
>
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