instances for closed type families

[Ryan Scott]

Simon is right, you cannot use a type family as an instance head. But why
do you need to? Typically, if you're deriving a Data instance that involves
type families, the type families would be inside another data type. A
real-world example is HsBindLR [1]:

    data HsBindLR idL idR
      = FunBind {
          ...
          bind_fvs :: PostRn idL NameSet,
          ...
        } | ...

where PostRn is a type family [2]. Now, you can't simply derive Data for
HsBindLR, because GHC has no way of knowing what PostRn will evaluate to!
But you can use standalone deriving to get what you want:

    deriving instance (Data (PostRn idL NameSet), ...) => Data (HsBindLR
idL idR)

And in fact, this is what GHC does [3], using a convenient type synonyms
for the long, sprawling context you need [4].

So in your example, while you can't directly create a Data instance for
NameOrRdrName itself, you can quite easily create Data instances for
anything that might use NameOrRdrName. Does that work for your use cases?

Ryan S.
-----
[1]
http://git.haskell.org/ghc.git/blob/bdc555885b8898684549eca70053c9ce0ec7fa39:/compiler/hsSyn/HsBinds.hs#l111
[2]
http://git.haskell.org/ghc.git/blob/bdc555885b8898684549eca70053c9ce0ec7fa39:/compiler/hsSyn/PlaceHolder.hs#l47
[3]
http://git.haskell.org/ghc.git/blob/bdc555885b8898684549eca70053c9ce0ec7fa39:/compiler/hsSyn/HsBinds.hs#l264
[4]
http://git.haskell.org/ghc.git/blob/bdc555885b8898684549eca70053c9ce0ec7fa39:/compiler/hsSyn/PlaceHolder.hs#l102
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