|Version 41 (modified by dreixel, 3 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 paper A generic deriving mechanism for Haskell. 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. What we describe until the "Kind polymorphic overhaul" section is implemented and released in GHC 7.0.1.
- 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.
- Tests are available under the generics directory of the testsuite.
Kind polymorphic overhaul
Work in progress.