Version 13 (modified by dreixel, 6 years ago) (diff)


The new Generic Deriving mechanism (ongoing work)

GHC includes a new (in 2010) mechanism to let you write generic functions. It is described in A generic deriving mechanism for Haskell, by Magalhães, Dijkstra, Jeuring and Löh. This page sketches the specifics of the implementation; we assume you have read the paper.

This mechanism replaces the previous generic classes implementation.

Main components

  • TcDeriv.tcDeriving generates an InstInfo for each data type that fulfills the isRep0 predicate. This InstInfo is the Representable0 instance for that type, allowing it to be handled generically (by kind-* generic functions).
  • The representation types and core functionality of the library live on GHC.Generics (on the ghc-prim package).
  • Many names have been added as known in prelude/PrelNames
  • Most of the code generation is handled by types/Generics

What already works

  • Representable0 instances are automatically generated when -XGenerics is enabled.

To do

  • Remove all of the old deriving mechanism stuff
  • Properly deal with fixity and isTuple information for constructors
  • Generate Representable1 instances
  • What about base types like [], Maybe, etc.?
  • Generic instances
    • Add deriving as a keyword. This replaces the DERIVABLE pragma from the UHC implementation, and is attached to a default method on a class declaration.
    • Change the Class definition to allow for generic defaults (in addition to standard defaults).
    • Generate default instances for representable types which derive generic classes.


  • For temporary testing, a file test/Main.hs is available with sample datatypes.


  • Currently, in TcDeriv.genGenericRepBind we generate instances using mkLocalInstance. Is this right, or should we use mkImportedInstance instead? SLPJ: mkLocalInstance: it's as if the instance declaration was in this module, right?