data families and TH names do not mix well (e.g. cannot use TH deriving)
lots of Haskell libraries have TH functions like:
deriveJSON :: Name -> Q [Dec]
(e.g. aeson package for deriving JSON conversion)
They're used like
data Foo = Foo This That $(deriveJSON ''Foo)
deriveJSON reifies the Name and expects to find that the name refers to a data type, from which it finds the data constructors and generates the appropriate code.
Now, this doesn't work when the data type we want to derive an instance for is defined using a data family:
class FooClass a where data Foo a ... instance FooClass Bar where data Foo Bar = Bar This That $(deriveJSON ''Foo Bar) -- ???
We'd like to make the type Foo Bar an instance of the class (e.g. To/FromJSON) but
deriveJSON expects a Name, and there is no name for this data type, its "name" is
Foo Bar but obviously that's not a
Of course in general a name for a type application doesn't make sense, but for data family instances what other way do we have to refer to them?