Building program understanding tools using visitor combinators

Program understanding tools manipulate program representations, such as abstract syntax trees, control-flow graphs, or data-flow graphs. This paper deals with the use of <em>visitor combinators</em> to conduct such manipulations. Visitor combinators are an extension of the well-known visitor design pattern. They are small, reusable classes that carry out specific visiting steps. They can be composed in different constellations to build more complex visitors. We evaluate the expressiveness, reusability, ease of development, and applicability of visitor combinators to the construction of program understanding tools. To that end, we conduct a case study in the use of visitor combinators for control-flow analysis and visualization as used in a commercial Cobol program understanding tool.