Can you give a small example? Certainly any instance like instance Data x => Data (F x) is not allowed, of course, if F is a type function. It’s like not allowing f (g x) = x in the term language. Only constructors in patterns! S From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Alan & Kim Zimmerman Sent: 25 May 2016 18:39 To: ghc-devs@haskell.org Subject: instances for closed type families I am working on https://ghc.haskell.org/trac/ghc/ticket/12105, and have a type to ensure that the `HsMatchContext` has either a `RdrName` or a `Name`, not an `Id`. type family NameOrRdrName id where NameOrRdrName Id = Name NameOrRdrName Name = Name NameOrRdrName RdrName = RdrName Is there any way to declare `Data` and `OutputableBndr` instances for this? Without it I am having to do something like instance (OutputableBndr name, OutputableBndr (NameOrRdrName name)) => Outputable (HsDecl name) where which requires UndecidableInstances. I get compiler/hsSyn/PlaceHolder.hs:114:19: Illegal type synonym family application in instance: NameOrRdrName id In the stand-alone deriving instance for ‘Data (NameOrRdrName id)’ for the parameterised version and compiler/hsSyn/PlaceHolder.hs:115:19: Illegal type synonym family application in instance: NameOrRdrName RdrName In the stand-alone deriving instance for ‘Data (NameOrRdrName RdrName)’ for the one specific to `RdrName`. Regards Alan