Doesn't this require TH? A primary constraint here is using generics only, since TH has many undesirable properties.
What syntax the metadata comes in is orthogonal to whether it's provided and processed using TH. That goes back to this previous question of yours:
Would it be technically feasible to add such a thing to GHC's Generics support?
Yes, it is technically feasible to extend GHC generics to reflect as much metadata as you want. It's a matter of deciding what syntax it's going to come in (whether in types, comments, or whatever else), how it's going to be encoded (via an extra `Meta` field, constructor, or whatever), and correspondingly adapting the code that generates Generic instances. However, changing GHC is something that takes a lot of effort. A short-term compromise is to reimplement the deriving of Generic instances in TH. TH would only be used for that, then the processing of that metadata by applications can be done purely in Haskell. Li-yao