#12432: TypeInType: open type family application as type family return kind fails to compile -------------------------------------+------------------------------------- Reporter: j6carey | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: duplicate | Keywords: Operating System: Linux | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): That last test case is an unrelated issue, not yet fixed. The problem is that `-XTypeInType` can't use a kind equality "right away". Equality constraints in Haskell are ''lifted'', meaning that we must make sure they are not bottom before using them to cast anything. Normally, this is all optimized away. But it does mean that we can't immediately use a kind equality in a type unless there is a place to put a check for bottom. Your code has no such place. This is all a bit of a dark corner, and I'm afraid you shouldn't hold your breath for a solution. On the flip side, it would be very enlightening if you have a realistic scenario where this limitation bites. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12432#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler