Data.Generics.Uniplate.Direct

This module supplies a method for writing Uniplate and Biplate instances. This moulde gives the highest performance, but requires many instance definitions. The instances can be generated using Derive: http://community.haskell.org/~ndm/derive/.

To take an example:

data Expr = Var Int | Pos Expr String | Neg Expr | Add Expr Expr
data Stmt = Seq [Stmt] | Sel [Expr] | Let String Expr

instance Uniplate Expr where
    uniplate (Var x  ) = plate Var |- x
    uniplate (Pos x y) = plate Pos |* x |- y
    uniplate (Neg x  ) = plate Neg |* x
    uniplate (Add x y) = plate Add |* x |* y

instance Biplate Expr Expr where
    biplate = plateSelf

instance Uniplate Stmt where
    uniplate (Seq x  ) = plate Seq ||* x
    uniplate (Sel x  ) = plate Sel ||+ x
    uniplate (Let x y) = plate Let |-  x |- y

instance Biplate Stmt Stmt where
    biplate = plateSelf

instance Biplate Stmt Expr where
    biplate (Seq x  ) = plate Seq ||+ x
    biplate (Sel x  ) = plate Sel ||* x
    biplate (Let x y) = plate Let |-  x |* y

To define instances for abstract data types, such as Map or Set from the containers package, use plateProject.

This module provides a few monomorphic instances of Uniplate / Biplate for common types available in the base library, but does not provide any polymorphic instances. Given only monomorphic instances it is trivial to ensure that all instances are disjoint, making it easier to add your own instances.

When defining polymorphic instances, be carefully to mention all potential children. Consider Biplate Int (Int, a) - this instance cannot be correct because it will fail to return both Int values on (Int,Int). There are some legitimate polymorphic instances, such as Biplate a [a] and Biplate a a, but take care to avoid overlapping instances.