instances for closed type families

Ryan Scott at
Wed May 25 18:42:42 UTC 2016

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.
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