#8566: Given kind equalities are discarded -------------------------------------+------------------------------------ Reporter: dreixel | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: polykinds/T8566 | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by goldfire): I can't find a case where I want an existentially quantified kind variable in an equality constraint, so I think that proposal is viable. ''But'', I think your comment that "'mentioned in an equality constraint' is not obvious" is an understatement. What if a context uses an open type family? Then, we can't know whether or not the type family will expand out to an equality constraint. We could forbid those, too, I suppose, but I don't like where this is going. I might prefer emitting a (suppressable) warning in this case, saying that the user is pushing the type system beyond its limits, really, and to expect the unexpected. Or, we could go the other way and allow such declarations only when the user specifies `-XAllowWonkyExistentialKinds` or similar. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8566#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler