At this moment there are no plans of making it part of GHC -- some details about the integration with other features need further thought. If you are really interested in trying, we built a prototype at https://git.science.uu.nl/f100183/ghc-invariant, which incorporates some advances in the "backward compatibility" section of the paper, but not guarded impredicativity per se.
2018-02-26 23:36 GMT+01:00 Simon Peyton Jones via ghc-devs <[hidden email]>:
Alejandro may want to comment…
From: ghc-devs [mailto:[hidden email]]
On Behalf Of Artem Pelenitsyn Sent: 26 February 2018 17:05 To:[hidden email] Subject: Guarded Impredicativity implementation
Is there any work has been done on the latest (ttbomk) proposal for impredicative types in Haskell: