#15515: Bogus "No instance" error when type families appear in kinds -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler (Type | Version: 8.4.3 checker) | Keywords: TypeInType, Resolution: | TypeFamilies Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: 12564 | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * blockedby: => 12564 Comment: So as far as why GHC doesn't simply error when you define `instance C (D :: F -> Type)`, I think it might be due to these lines in `check_valid_inst_head`: {{{#!hs | otherwise = mapM_ checkValidTypePat ty_args where ... ty_args = filterOutInvisibleTypes (classTyCon clas) cls_args }}} Where `checkValidTypePat` is what throws the `Illegal type synonym family application in instance` error seen in #12564. Because `ty_args` has filtered out kinds, it won't catch any type families in kinds, like in the original program. I think we could extend this error message to kinds by simply mapping `checkValidTypePat` over all `cls_args`, and not just `ty_args`. Do you agree? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15515#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler