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. The code is in a branch of GHC; you can get it with darcs get http://darcs.haskell.org/ghc-generic-11Oct10/ghc.

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.

Testing

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

Problems/questions

• 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?