Experiments with Generic Visitors

This paper gives an overview of our work on generic visitors [12], and it gives a comparison of two implementations, in Java and Smalltalk. For object-oriented programming, the Visitor pattern [3] enables the definition of a new operation on an object structure without changing the classes of the objects. The discovery of generic visitors is due to striking parallels with shape polymorphic [5], or polytypic [4, 10] programming. This illustrates the connections among the various programming styles. The standard way of creating a new operation on an object is to create a new method for it by editing and recompiling its class. It often happens that new methods in many classes must be created to support what is conceptually a single action, such as a search for a particular field in a particular class of objects. This is rather tedious. Analogous problems arise in functional programming where the standard pattern-matching approach means that an operation like mapping of a function must be redefined for each new data type. The Visitor pattern provides an alternative means of creating new operations without changing any classes. The operation is decomposed into a description of how to determine the class of an object, and the actions to be taken when the class is found. The former need only be specified by, once and for all, insertin