[Haskell-cafe] Generic "annotations" on type definitions

[Li-yao Xia]

 > 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