I don't see why Typeable (Foo 'Just) requires that. I'd expect to get back a TrTyCon, not a TrApp. Some modifications to the structure of TrTyCon might be required.



David Feuer
Well-Typed, LLP

-------- Original message --------
From: Ryan Scott <ryan.gl.scott@gmail.com>
Date: 9/24/17 10:08 AM (GMT-05:00)
To: ghc-devs@haskell.org
Subject: Re: Why isn't this Typeable?

Trying to conclude Typeable Foo (or, if expanded with
-fprint-explicit-kinds, Typeable ((forall a. a -> Maybe a) -> Type)
Foo) is beyond GHC's capabilities at the moment, as that would require
impredicative polymorphism. This problem has arose in other contexts
too—see Trac #13895 [1] for one example.

I don't think you can conclude Typeable (Foo 'Just) either, since that
requires concluding both Typeable Foo and Typeable 'Just, so you
ultimately run into the same problem.

While there an in-the-works plan to allow a limited form of
impredicativity through explicit use of visible type application [2],
my fear is that that wouldn't be enough to address the problem you've
encountered, since there's no way to visibly apply @((forall a. a ->
Maybe a) -> Type) to Typeable at the moment. To accomplish this, you
would need visible kind application [3].

Ryan S.
-----
[1] https://ghc.haskell.org/trac/ghc/ticket/13895
[2] https://ghc.haskell.org/trac/ghc/ticket/11319#comment:11
[3] https://ghc.haskell.org/trac/ghc/ticket/12045
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