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.

Problems/questions

• 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? SLPJ: mkLocalInstance: it's as if the instance declaration was in this module, right?