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It is unfortunate that if we want to (efficiently) derive a new type class then we have to bake the derivation logic into GHC itself. It seems desirable to be able to implement new deriving methods in libraries in some manner.

Ryan Scott says:

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.

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Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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