A generic complete dynamic logic for reasoning about purity and effects

  • Authors:
  • Till Mossakowski;Lutz Schröder;Sergey Goncharov

  • Affiliations:
  • DFKI Laboratory, Bremen and Department of Computer Science, University of Bremen;DFKI Laboratory, Bremen and Department of Computer Science, University of Bremen;Department of Computer Science, University of Bremen

  • Venue:
  • FASE'08/ETAPS'08 Proceedings of the Theory and practice of software, 11th international conference on Fundamental approaches to software engineering
  • Year:
  • 2008

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Abstract

For a number of programming languages, among them Eiffel, C, Java and Ruby, Hoare-style logics and dynamic logics have been developed. In these logics, pre- and postconditions are typically formulated using potentially effectful programs. In order to ensure that these pre- and postconditions behave like logical formulae (that is, enjoy some kind of referential transparency), a notion of purity is needed. Here, we introduce a generic framework for reasoning about purity and effects. Effects are modeled abstractly and axiomatically, using Moggi's idea of encapsulation of effects as monads. We introduce a dynamic logic (from which, as usual, a Hoare logic can be derived) whose logical formulae are pure programs in a strong sense. We formulate a set of proof rules for this logic, and prove it to be complete with respect to a categorical semantics. Using dynamic logic, we then develop a relaxed notion of purity which allows for observationally neutral effects such writing on newly allocated memory.