#13192: Ambiguity Caused By PolyKind and Not Helpful Error Messages -------------------------------------+------------------------------------- Reporter: Shayan-Najd | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): To add more information: this code is accepted without `PolyKinds`, because `F (I a)` can immediately reduce to `a`. With `PolyKinds`, `F (I a)` is stuck, as we don't know `a`'s kind. (The equation for `F` works only when `(a :: Type)`.) Perhaps the best fix is to add a check in !TcErrors for type families that match on invisible parameters (i.e., kinds) and emit a {{{NB: Type family `F' has equations that depend on the kind of its argument(s)}}}. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13192#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler