Functional Visitors Revisited

In object-oriented programming the visitor design pattern allows for the addition of new operations on a data hierarchy, but lends itself to scattered traversal code and makes visitors difficult to combine. Previous attempts to solve these issues have separated traversal code from the data structure but still face a lack of modularity in visitor computation making it difficult– if not impossible– to duplicate the flexibility of hand written, monolithic methods. In this paper we present a reformulation of the visitor pattern that introduces functional style traversals and visitor methods. Our modifications increase the context information available to visitor methods by way of the implicit stack of the recursive traversal, removing the need for side effects in visitor definitions. This gives visitors more flexibility during traversal, while their functional nature allows for modular, compositional solutions, leading to more reusable designs. To utilize these ideas we introduce a simple new functional visitor-oriented language, which allows us to build functional visitors without the syntax burdens of most object-oriented languages. To support visitor composition in our language we introduce five useful visitor combinators that can be used to develop self contained visitors and discuss motivating examples in our new language.