Regular Object Types

Regular expression types have been proposed as a foundation for statically typed processing of XML and similar forms of tree-structured data. To date, however, regular expression types have been explored mainly in the setting of special-purpose languages (e.g., XDuce, CDuce, and XQuery) whose type systems were designed around regular expression types \from the ground up." The goal of the Xtatic language is to bring regular expression types to a broader audience by presenting them as a lightweight extension of a popular object-oriented language, C. We develop here the formal core of the Xtatic design|a combination of the tree-structured data model of XDuce with the classes-and-objects data model of a conventional object-oriented language. Our tool for this investigation is a tiny language called FX, whose features are drawn from Featherweight Java (FJ) and from the core of XDuce. Points of interest include a smooth interleaving of the two value spaces, in which XDuce's tree structures are grafted into of FJ's class hierarchy while objects and object types play the role of XDuce's label values and label types; a \semantic" de nition of the subtype relation, inherited from XDuce and extended to objects; and a natural encoding of XML documents and their schemas using a simple form of singleton classes.