Reification of Foreign Type Systems

Building systems from existing applications and data sources is common practice. Semi-structured data sources, such as XML, HTML, and databases, and programming languages, such as C# and Java, conform to welldefined, albeit different, type systems, each with their own unique underlying representations. As a consequence, writing programs that access and update data in foreign type systems (FTSs), i.e., type systems that are different from the host programming language, is a notoriously difficult task. In this paper, we present a simple, practical, and effective way to develop and maintain FTS-based systems. We accomplish this by abstracting foreign data as graphs and using path expressions for traversing and accessing data. Path expressions are implemented by type reification — turning foreign types into first-class objects and enabling access to and manipulation of their instances. Doing this results in multiple benefits, including coding simplicity and uniformity (neither of which was present before), that have been demonstrated in a complex commercial project. The contribution of this paper is an approach that allows programmers to operate on foreign types and their instances without writing or generating additional code. We know of no other approach with comparable benefits.