Version 6 (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

To do

  • Generate meta-information empty datatypes and instances (Datatype, Constructor, and Selector instances)
  • Generate Representable1 instances
  • 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.


  • When representations are generated for more than one datatype, assembler errors appear: symbol `ghczmprim_GHCziGenerics_Representable0_closure' is already defined
  • For meta-information, we need to generate a bunch of empty datatypes per user datatype declaration. Where can/should we do this? TcDeriv seems to be "too late", since we can only add instances or top-level value bindings (as the old generic mechanism did)...
  • Currently, in TcDeriv.genGenericRepBind we generate instances using mkLocalInstance. Is this right, or should we use mkImportedInstance instead?