#14042: Datatypes cannot use a type family in their return kind -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Keywords: TypeInType, Resolution: | TypeFamilies Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I'm not sure either. But we do have worker/wrapper for ordinary data cons, so there is room to inject some coercions. Eg {{{ data T = MkT {-# UNPACK #-} !Int }}} We get a worker and a wrapper. {{{ MkT :: Int# -> T -- Worker -- No code; this is the real data constructor in Core $WMkT :: Int -> T -- Wrapper $WMkT x = case x of I# y -> MkT y }}} Occurrences of `MkT` in terms are replaced by `$WMkT`. In patterns we need some impedence matching. Can someone produce an example with some actual data constructors? I'm a bit lost as to the actual goal. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14042#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler