#11334: GHC panic when calling typeOf on a promoted data constructor -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: bgamari Type: bug | Status: new Priority: highest | Milestone: 8.0.1 Component: Compiler (Type | Version: 8.0.1-rc1 checker) | Resolution: | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D1757 Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): Yech. First off, let's rename the data constructor and type constructor differently. I got quite confused about that! {{{ data Ty (f :: k -> *) (a :: k) = Con (f a) }}} This produces {{{ Ty :: forall k. (k -> *) -> k -> * Con :: forall k (f :: k -> *) (a :: k). (f a) -> Ty k f a }}} The question is: how should we interpret `Proxy 'Con` with `-XNoPolyKinds`? The old rule for `-XNoPolyKinds` was "set all kind variables to `*`", which is still what `quantifyTyVars` is doing. This used to make sense, because all kind variables used to have sort `BOX`, never something like `BOX -> BOX`. But those halcyon days are now gone. I suppose it would be reasonable to set all kind variables of kind `*` to be `*`, and use `Any` for the others. We don't want to just use `Any` all the time, because that wouldn't be backwards compatible and defaulting to `*` is quite sensible. The other reasonable way forward is to issue an error at `Proxy 'Con` without having more direction. For example, the user could say `Proxy ('Con :: Maybe Bool -> Ty * Maybe Bool)` to get the right instantiation. I actually prefer the "issue an error" option. The user could use a kind signature, but more likely should just enable `-XPolyKinds`. Using a promoted data constructor taken from a polykinded datatype without `-XPolyKinds` is asking for trouble. I'm happy to put this change in when I get to rounding up these tickets. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11334#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler