Running :kind! twice in a row does nothing, and your hunch was correct: it does appear to be a bug. Specifically, it's a bug that affects type families with polymorphic return kinds (like Extract, which has kind y). I've opened Trac #11275 [1] for this issue.
Ryan S. ----- [1] https://ghc.haskell.org/trac/ghc/ticket/11275 _______________________________________________ ghc-devs mailing list [email protected] http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
