instances for closed type families

[Simon Peyton Jones]

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 at haskell.org] On Behalf Of Alan & Kim Zimmerman
Sent: 25 May 2016 18:39
To: ghc-devs at 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
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