Package net.sourceforge.czt.base.visitor

Base visitors for the AST for Z.

See: Description

Interface Summary

Interface	Description
ListTermVisitor<R>	A visitor for visiting annotated terms.
TermVisitor<R>	A visitor for visiting terms.

Class Summary

Class	Description
VisitorUtils	Static methods for visiting Z terms.

Package net.sourceforge.czt.base.visitor Description

Base visitors for the AST for Z.

The interfaces in this package provide support for the visitor design pattern. The purpose of the visitor pattern is to encapsulate an operation that you want to perform on the elements of the AST for Z. In this way, you can provide operations on the AST without the need of changing the AST classes itself.

Each Z term accepts a visitor via the accept-method, which in turn calls the correct visitor method (double dispatch). The visitor will then execute its algorithm for that element.

The AST for Z supports a variant of the so-called default visitor introduced by Nordberg. The default visitor adds another level of inheritance to the visitor pattern. It provides the possibility to implement default behaviour by taking advantage of the inheritance relationship of the classes to be visited. This is achieved by adding visit methods for abstract AST classes as well.

While Nordberg suggests to derive the default visitor from the standard visitor and to implement all visit-methods in a way that its default behaviour is to call the visit method for the base class of the visited class, the AST classes for Z handle that automatically.

Each AST object, whether abstract or not, has a corresponding visitor interface that defines the visit method for that object. If a visitor implementation wants to visit that object, it has to implement this interface. When the accept-method of the AST object is called, the corresponding visit-method will be executed. If a visitor does not implement the visitor interface for an AST object, the visit-method of the base classes for that object are tried instead, starting with the base class, followed by the base class of the base class, etc., untill Term is reached.

The following code provides a visitor that traverses over a tree of Z terms:

public class DepthFirstVisitor implements TermVisitor
{
  public Object visitTerm(Term term)
  {
    Object[] children = term.getChildren();
    for (int i=0; i < children.length; i++) {
      if (children[i] instanceof Term) {
        ((Term) children[i]).accept(this);
      }
    }
  }
}

Note that the annotations of a term are not visited by this visitor since they are not returned by the getChildren-method.

References

[NorIII] Martin E. Nordberg III. Variations of the Visitor Pattern In: PLoP '96