Re: Superclasses of type families returning constraints?

On Jan 6, 2020, at 2:52 PM, Simon Peyton Jones via ghc-devs wrote:

| type family F a b :: Constraint where | F a a = () | | eq :: F a b => a :~: b | eq = Refl

This is rejected, and it's not in the least bit puzzling! You have evidence for (F a b) and you need to prove (a~b) -- for any a and b. Obviously you can't.

But how could you possibly have evidence for (F a b) without (a ~ b)? You can't.

And in Core you could write (eq @Int @Bool (error "urk")) and you jolly well don’t want it to return Refl.

Why not?

eq = /\ a b. \ (d :: F a b). case d of Something (co :: a ~# b) -> Refl @a @b co

This would obviously require extensions to Core and Haskell, but it's not a priori wrong to do so. Richard