How much of this derivation machinery could NOT be implemented by means of some kind of a (hypothetical) type-backed metaprogramming facility?
I think this would be a wonderful thing to have. Matthew Pickering (cc'd) has expressed a desire to have all the logic for the `bespoke` deriving strategies compartmentalized into a library that could easily be expanded on in the future to support more typeclasses in base. (Bifunctor, anyone?) Unfortunately, each of the major players in today's Haskell metaprogramming scene that I'm aware of have some downfalls that make them unsuitable as `deriving` replacements: * Template Haskell: Not portable. Staging issues make it hard to use as a drop-in replacement for the `deriving` keyword * GHC generics: Can't express all the optimizations that the bespoke `deriving` algorithms perform. Unperformant. * Haskell preprocessors: Difficult to integrate in a typical GHC workflow. Probably wouldn't have all the metadata you'd need to be feature-complete with what `deriving` does today. The way I see it, the whole `deriving` business as it currently stands today is a somewhat-grotesque-but-darn-useful hack that gets around the lack of a truly nice metaprogramming facility in Haskell. I'm holding out hope that the work in https://github.com/shayan-najd/NativeMetaprogramming makes things nicer soon, and then we can revisit this idea. Until then, -XDerivingStrategies provides a way to contain some of the madness of `deriving` after having many features tacked onto it in recent GHC releases. Ryan S.