Support for `exists a b. Either a b` and `exists a. Either a Int`

[gelisam]

Some can only express exists a. Either Int a, but not the other two (at least not very easily).

I stumbled upon some old code of mine which supports all three and more, using two transformers I call SomeT and ThisT:

• exists a. Either Int a ~ Some Either
• exists a b. Either a b ~ SomeT Some Either
• exists a. Either a Int ~ SomeT (This Int) Either

I thought you might want to steal those ideas to make this package useful in more cases. I bet it would be easy to implement a third transformer which would support constraints too:

• exists a. Show a => Either a Int ~ ConstrainedT Show (This Int) Either

[masaeedu]

The latter is supported by Some (Flip Either Int) I believe. The former is "supported" to the extent that we can uncurry data types:

type Uncurry :: (x -> y -> *) -> (x, y) -> *
data Uncurry f
  where
  Uncurry :: f a b -> Uncurry f '(a, b)

type TheFirstThing = Some (Uncurry Either)

Although this sometimes tends to run into issues.

[Ericson2314]

Yeah, I think such Flip and Uncurry tricks are good enough until https://github.com/ghc-proposals/ghc-proposals/pull/473 exists.