instances for closed type families

25 May 2016

      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

instances for closed type families

Alan & Kim Zimmerman