What Functional Programmers can Learn from the Visitor Pattern

This paper explores the practical potential for enhancing code reuse in functional languages by leveraging techniques and experiences from object-oriented programming. Since data types in typed functional languages do not support adding new variants, programming techniques derived from the object-oriented Visitor pattern can be readily applied to recursive functions on trees in functional languages. Specifically, we demonstrate how to define recursive functions on trees in such a way that they can be extended without modification through an inheritance-like mechanism. Furthermore, we demonstrate how the extensions themselves can be made highly reusable through a mechanism akin to mixin-based inheritance, which allows an extension to be applied to any suitable recursive function. These mechanisms combine well with the usual higher-order approach to reuse in functional languages. Although general encodings of inheritance in functional languages are not novel, such encodings lead to a particularly simple and elegant presentation when limited to recursive functions on trees. Because of their simplicity, we believe that these techniques have the potential to enhance code reuse in the practice of functional programming without requiring additional tool support.