Modular reasoning about invariants over shared state with interposed data members

Reasoning about object-oriented programs is difficult since such programs usually involve aliasing, and it is not easy to identify the ways objects can relate to each other and thus to confine a program's heap. In this paper, we address this problem in the context of a relationship-based programming language. In relationship-based programming languages, relationships are first-class citizens and allow a precise description of inter-object relationships. Relationships enforce a modularization discipline that is closer to the natural modularity inherent to many problem domains and that yields, as a result, program heaps that are DAGs. We further describe a mechanism, member interposition, that leverages the new modularization discipline and supports encapsulation of fields of shared objects. We have implemented the described modularization discipline and the mechanism of member interposition in the context of Rumer, a relationship-based programming language with support for contract specifications. We discuss the implications of member interposition for the modular verification of object invariants with an example. Relationships and interposed members provide an alternative to ownership type systems.