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 deeni-tion 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 eld 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 redeened 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 speciied by, once and for all, inserting accept methods into the class deenitions, while the latter vary from visitor to visitor 13]. The analogy with mapping becomes clearer, as the visitor corresponds to the function being mapped, whose application is controlled by the class. The analogous functional approach is captured in PolyP 4] which improves on the earlier work in Charity 2]. The compiler determines the type, and uses this information to specialise the map algorithm automatically. The functional approach is a little smoother here, because the limited set of combinators used all have standard deenitions, so that there is no need to supply explicit accept methods. A generic visitor, or walkabout, does away with the need for accept methods, so that there is no need to change the classes at all. This is achieved by using reeection to determine the elds of an object at run-time, and then visiting them all unless explicitly diverted by the visitor. Here the analogy was put to work, explicitly transferring ideas from the functional setting. A shape polymorphic map function was proposed in 7] and implemented in Functorial …