There is an example at http://lpaste.net/164532 (attached as well) The derived Data instance fails with /home/alanz/Example.hs:19:19: Illegal type synonym family application in instance: NameOrRdrName id In the stand-alone deriving instance for ‘Data (NameOrRdrName id)’ And how would an instance for Outputable be defined in terms of the existing ones? Alan On Wed, May 25, 2016 at 8:03 PM, Simon Peyton Jones wrote:

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