#12224: Replace placeHolderPunRhs with a PlaceHolder value -------------------------------------+------------------------------------- Reporter: alanz | Owner: alanz Type: task | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 (Parser) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): alanz, presumably you're talking this datatype in `HsPat`? {{{#!hs -- For details on above see note [Api annotations] in ApiAnnotation data HsRecField' id arg = HsRecField { hsRecFieldLbl :: Located id, hsRecFieldArg :: arg, -- ^ Filled in by renamer when punning hsRecPun :: Bool -- ^ Note [Punning] } deriving (Data, Functor, Foldable, Traversable) }}} And you want to change `hsRecFieldArg` to be of type `PostRn id arg`? If so, I don't think you'll be able to derive `Functor`, `Foldable`, or `Traversable` anymore, since GHC won't know what code to fill in for the irreducible type family `PostRn id arg`. One possible workaround is to define your `Functor` instances manually: {{{#!hs instance Functor (HsRecField' Id) where fmap f (HsRecField a b c) = HsRecField a (f b) c instance Functor (HsRecField' Name) where fmap _ (HsRecField a b c) = HsRecField a b c instance Functor (HsRecField' RdrName) where fmap _ (HsRecField a b c) = HsRecField a b c }}} and similarly for `Foldable` and `Traversable`. It's not the prettiest solution, but I think it might work for GHC's purposes. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12224#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler