#16255: Visible kind application defeats type family with higher-rank result kind -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.7 checker) | Keywords: Resolution: | TypeApplications, TypeFamilies Operating System: Unknown/Multiple | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: #15740 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): I have no idea what that `F` would mean if it were allowed. Thankfully, it's not. I don't think it's as simple as that arity check, though. Witness {{{#!hs type family F (x :: j) :: forall k. Either j k where F 5 @Symbol = Left 4 }}} That looks well-kinded to me, it has the right visible arity, and it has the right specified arity, but it's still wrong. To get this right, I think we have to look not at the overall specified arity, but the specified arity ''after the last visible argument''. If we get that right, I think we've gotten this aspect right. (Note that a problem before the last visible argument must necessarily be a kind error, not an arity error.) That's a bit gross, but I think it will work. This should probably be done in the validity checker. The spot you've edited above is in the kind-checker. The only reason the visible arity check is done there is because it gives a nicer error message. These checks morally ought to be later. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16255#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler