WIP: Add quantified constraint superclasses

[Ericson2314]

This will not work unless haskell/core-libraries-committee#10 is accepted.

[phadej]

i won't accept PRs making API conditional to GHC version.

I'm also don't feel bumping GHC support to GHC-8.6 just yet.

I also don't understand the motivation. I'd recommend to not use this class for anything else other than an auxiliary class for Show (Some f) instance.

[phadej]

i don't understand this. shouldn't GShow a imply forall x. Show (a x), because it's class forall x. Show (t x)) => GShow t. Why GHC doesn't accept this?

[Ericson2314]

So since this PR was written, I made haskell/core-libraries-committee#10. That has has the superclass for the same reason.

Show1 f is slightly stronger than forall a. Show (f a). GShow f however is identical ind expressive power to forall a. Show (f a) and thus I am not whether it ought to existing if 8.6 ever become a lower bound.

[phadej]

It's not identifical to Show1. GShow doesn't need Show a / a -> Int -> ShowS. GShow is unrestricted.

[phadej]

Yes. Let's reconsider this when i bump the lower bound to GHC-8.6. i don't see a need to do that.

(if that going to happen I'd probably just remove the class all-together).

[Ericson2314]

I'll just turn this to a 8.6+ PR that does quantified constraints for all the classes. And mark it draft.

[Ericson2314]

Really should be

Suggested change

	-- (forall a. Read (t a)) =>
	-- (exists a. Read (t a)) =>

GRead ((:~:) a) only reads a :~: a, we're not going to create non-trivial :~: out of nothing. Likewise, in #45 we have

instance forall k1 k2 (a :: k1). k1 ~ k2 => GRead ((:~~:) a :: k2 -> Type)

which is more clearly written (at the cost of less back compat) as

instance forall k1 k2 (a :: k1). k1 ~ k2 => GRead ((:~~:) @k1 @k2 a)

We can only attempt to read when the kinds are the same and there isn't, in fact, any heterogeneity.

[Ericson2314]

Once GHC 9.6 comes out we can give this another look.