RECOMMENDATION: Use Data.Generics.Uniplate.Data instead - it usually performs faster (sometimes significantly so) and requires no special instance declarations.
This module supplies a method for writing
Biplate instances. One
instance declaration is required for each data type you wish to work with. The
instances can be generated using Derive: http://community.haskell.org/~ndm/derive/.
To take an example:
data Expr = Var Int | Neg Expr | Add Expr Expr deriving Typeable instance (Typeable a, Uniplate a) => PlateAll Expr a where plateAll (Var x ) = plate Var |+ x plateAll (Neg x ) = plate Neg |+ x plateAll (Add x y) = plate Add |+ x |+ y
- module Data.Generics.Uniplate.Operations
- module Data.Typeable
- class PlateAll from to where
- plateAll :: from -> Type from to
- plate :: from -> Type from to
- (|+) :: (Typeable item, Typeable to, PlateAll item to) => Type (item -> from) to -> item -> Type from to
- (|-) :: Type (item -> from) to -> item -> Type from to
- plateProject :: (Typeable item, Typeable to, PlateAll item to) => (from -> item) -> (item -> from) -> from -> Type from to
This class should be defined for each data type of interest.
The field to the right may contain the target.
The field to the right does not contain the target. This can be used as either an optimisation, or more commonly for excluding primitives such as Int.
Write an instance in terms of a projection/injection pair. Usually used to define instances for abstract containers such as Map:
instance (Ord a, Typeable a, PlateAll a c, Typeable b, PlateAll b c, Typeable c, PlateAll c c) => PlateAll (Map.Map a b) c where plateAll = plateProject Map.toList Map.fromList