uniplate-1.0.1: Uniform type generic traversals.

Data.Generics.PlateDirect

Contents

Description

This module supplies a method for writing `Biplate` instances more easily. This module requires fewest extensions, highest performance, and most instance definitions.

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 PlateOne Expr where
plateOne (Var x  ) = plate Var |- x
plateOne (Pos x y) = plate Pos |* x |- y
plateOne (Neg x  ) = plate Neg |* x
plateOne (Add x y) = plate Add |* x |* y

instance PlateAll Expr Expr where
plateAll = plateSelf

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

instance PlateAll Stmt Stmt where
plateAll = plateSelf

instance PlateAll Stmt Expr where
plateAll (Seq x  ) = plate Seq ||+ x
plateAll (Sel x  ) = plate Sel ||* x
plateAll (Let x y) = plate Let |-  x |* y
```

Synopsis

# The Classes

class PlateAll from to whereSource

This class represents going from the container type to the target.

If `from == to` then use `plateSelf`, otherwise use `plate` and the other combinators.

Methods

plateAll :: from -> Type from toSource

class PlateOne to whereSource

This class is for when the target and container are the same type.

Methods

plateOne :: to -> Type to toSource

# The Combinators

plate :: from -> Type from toSource

The main combinator used to start the chain.

The following rule can be used for optimisation:

``` plate Ctor |- x == plate (Ctor x)
```

plateSelf :: to -> Type to toSource

Used for `PlayAll` definitions where both types are the same.

(|+) :: PlateAll item to => Type (item -> from) to -> item -> Type from toSource

The field to the right may contain the target.

(|-) :: Type (item -> from) to -> item -> Type from toSource

The field to the right does not contain the target.

(|*) :: Type (to -> from) to -> to -> Type from toSource

The field to the right is the target.

(||+) :: PlateAll item to => Type ([item] -> from) to -> [item] -> Type from toSource

The field to the right is a list of types which may contain the target

(||*) :: Type ([to] -> from) to -> [to] -> Type from toSource

The field to the right is a list of the type of the target