instances for closed type families

[Alan & Kim Zimmerman]

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 <simonpj at microsoft.com>
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 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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