#10760: Conflicting typefamily instances not reported with polykinds -------------------------------------+------------------------------------- Reporter: steely | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Resolution: invalid | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by steely): I understand. This is indeed very subtle and ought to be documented. I am not sure whether this explanation should go in the section about type families alone, since this issue is related to their interaction with PolyKinds, but anyway it would be more precise to include something like: In the paragraph about "Type Family Declarations". The result kind annotation is optional and, as usual, defaults to * if omitted. Polykinded type families can be declared using a parameter in the kind annotation: {{{ type family F a :: k }}} Note that in this case the kind parameter k is an actual implicit parameter of the type family and as such is relevant when checking the apartness of type family equations. In the paragraph about "Compatibility and apartness of type family equations'": 1. all corresponding types and '''implicit kinds''' in the patterns are apart. The example of this ticket could also be included to show this. I am not sure whether in GHC 7.10 kind variables have to be explicitly given in type families, making it more intuitive. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10760#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler