#11152: GND accepts ill-roled coercion when manually defining it won't typecheck -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.10.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by rwbarton): Don't think there is a bug here. The GND is okay because `MyMaybe` is known to have a type parameter of representational role. The problem arises with types involving a variable monad, since we cannot or do not want to require all monads be representational. Your manual implementation is wrong because `a` does not refer to the type you want it to (compare `myid :: a -> a; myid x = x :: a`). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11152#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler