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. The  HaskellWiki page gives a more general overview.

This mechanism replaces the previous generic classes implementation. The code is in the ghc-generics branch of the  ghc,  base,  ghc-prim,  haddock2, and  testsuite repos.

Main components

• TcDeriv.tcDeriving now allows deriving Generic instances.

• 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

Things that have been removed

• All of the generic classes stuff. In particular, the following have been removed:
  • hasGenerics field from TyCon;
  • HsNumTy constructor from HsType;
  • TypePat constructor from Pat.

• The -XGenerics flag is now deprecated.

What already works

• Generic instances can be derived when -XDeriveGeneric is enabled.

• The default keyword can used for generic default method signatures when -XDefaultSignatures is enabled.

• Generic defaults are properly instantiated when giving an instance without defining the generic default method.

• Base types like [], Maybe, tuples, come with Generic instances.

To do

• Generate Generic1 instances

• Print also the Rep type instance when -ddump-deriving is on

• Register the DeriveGeneric and DefaultSignatures extensions with Cabal.

• Do we want Show, etc. instances for types like U1, :+:, ...?

Testing

• Tests are available under the generics directory of the testsuite.