#10607: Auto derive from top to bottom -------------------------------------+------------------------------------- Reporter: songzh | Owner: Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.11 Component: Compiler | Operating System: Unknown/Multiple Keywords: deriving, | Type of failure: None/Unknown typeclass, auto | Blocked By: Architecture: | Related Tickets: Unknown/Multiple | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- It is very good to see that Template Haskell types are now `Generic` and `DeriveAnyClass` have been implemented. I suppose that this auto-deriving thing can go even further. The problem is like the following: When we manipulate a complex composite JSON object, we need to define a dozen of data types for each of the dozen we may need to derive `Eq`, `Show`, `Generic` etc. Another situaition can be manipulating the AST of a language, there may be dozens of data type definitions, also, for each of them, we need to derive `Show`, `Eq`, `Generic`. Take the old Template Haskell types without `Generic` as an example, I want to make them instances of `Generic` type class, I wrote: {{{#!hs deriving instance Generic FixityDirection deriving instance Generic Inline deriving instance Generic RuleBndr deriving instance Generic Match deriving instance Generic Name deriving instance Generic RuleMatch deriving instance Generic Pred deriving instance Generic Phases deriving instance Generic Con deriving instance Generic Module deriving instance Generic AnnTarget deriving instance Generic Type deriving instance Generic TyVarBndr deriving instance Generic TyLit deriving instance Generic Exp deriving instance Generic Lit deriving instance Generic Pat deriving instance Generic Dec deriving instance Generic Clause deriving instance Generic FunDep deriving instance Generic Foreign deriving instance Generic Fixity deriving instance Generic Pragma deriving instance Generic FamFlavour ... }}} How can we use `Generic`? Say we want to serialize Template Haskell data type with `Binary` type class, Of course, by writing `Binary` empty instances or just derive it with `DeriveAnyClass` extension. However, the problem is that we still need to write deriving Binary dozens of times, as many as the trivial `Generic` deriving declaration above. What if GHC can attemp to derive the instance from the top of the tree down to the leaf in the data type declarations? For example: {{{#!hs data Person = Person Names Address | Student Names Address deriving Generic data Names = Names String deriving Generic data Address = Address Gate deriving Generic type Gate = (String,Int) deriving topdown Eq Person }}} It will generate `Eq` instances of `Person`, `Names` and `Address`, since it is in the Person data declaration tree. I tried to implement a prototype see https://github.com/HaskellZhangSong /derive-topdown It is rather an experiment tryout than a formal implementation. If you think this feature is attractive, I would like to discuss it. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10607 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler