|Version 43 (modified by dreixel, 2 years ago) (diff)|
Support for generic programming
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
The current implementation supports defining both functions over types of kind * (such as show) and functions over types of kind * -> * (such as fmap). Although care has been taken to reduce code duplication, we still need two generic classes, one for each kind (Generic and Generic1).
With the new -XPolyKinds functionality, we can make the support for generic programming better typed and more general. The basic idea is to define the universe codes (M1, :+:, etc.) as constructors of a datatype. Promotion then lifts these constructors to types, which we can use as before, only that now we have them all classified under a new kind:
data Universe x = U | K x | P Nat | Universe x :+: Universe x | Universe x :*: Universe x | M MetaData (Universe x) data MetaData = C | D | S