#10141: Kind inference regression in closed type families -------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire Type: bug | Status: closed Priority: highest | Milestone: 7.10.1 Component: Compiler | Version: 7.10.1-rc2 Resolution: invalid | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by rwbarton): In this particular case, given the third equation `G a = a`, I'm pretty sure the only possible return kind is `k`, no? But there was that whole long discussion about CUSKs that I didn't really follow, so I take it that in general a closed type family may have no principal kind. I would venture to say though that the error message GHC produces is IMHO not that helpful. In this case the fix is clearly to add a CUSK `type family G (a :: k) :: k where`; could GHC work out when it is appropriate to suggest adding a CUSK? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10141#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler