This is exactly what GADTs are for.
-Edward
On Thu, Jul 18, 2013 at 6:54 AM, harry
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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