Observational purity and encapsulation

  • Authors:

  • David A. Naumann

  • Affiliations:

  • Department of Computer Science, Stevens Institute of Technology

  • Venue:
  • FASE'05 Proceedings of the 8th international conference, held as part of the joint European Conference on Theory and Practice of Software conference on Fundamental Approaches to Software Engineering
  • Year:
  • 2005

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Abstract

Practical specification languages for imperative and object-oriented programs, such as JML, Eiffel, and Spec#, allow the use of program expressions including method calls in specification formulas. For coherent semantics of specifications, and to avoid anomalies with runtime assertion checking, expressions in specifications and assertions are typically required to be strongly pure in the sense that their evaluation has no effect on the state of preexisting objects. For specification of large systems using standard libraries this restriction is impractical: it disallows many standard methods that mutate state for purposes such as caching or lazy initialization. Calls of such methods can sensibly be used for specifications and annotations in contexts where their effects cannot be observed. This paper formalizes and extends a recently proposed notion of observational purity, reducing the proof obligation to a familiar one for equivalence of two class implementations.