|Version 37 (modified by dreixel, 5 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. The HaskellWiki page gives a more general overview.
- TcDeriv.tcDeriving generates an InstInfo for each data type that fulfills the isRep0 predicate. This InstInfo is the Generic 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
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.
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.
- Generate Generic1 instances
- Print also the Rep type instance when -ddump-deriving is on
- Give better error messages when we cannot derive Generic (currently we say only Cannot derive Generic)
- Register the DeriveGeneric and DefaultSignatures extensions with Cabal.
- Do we want Show, etc. instances for types like U1, :+:, ...?
- Tests are available under the generics directory of the testsuite.