Formal perspectives on software evolution: from refinement to retrenchment

The discipline of formal methods is concerned with the use of mathematical techniques to capture precise system specifications, and to transform these into verifiably correct programs. As these techniques mature, formal methods researchers are now taking a broader view of software development, and considering how evolution of such specifications and programs may be formally supported. This chapter discusses various ways in which formal methods can contribute to the evolutionary development of verifiably correct software. This discussion centres around model-based specification notations and associated development techniques, as exemplified by VDM, Z, B and the refinement calculus. The chapter begins by introducing the refinement calculus and outlining two ways in which program refinement can support evolutionary development. The chapter then introduces a recent generalisation of refinement, called retrenchment. Some of the factors motivating this generalisation are shown to be relevant to the evolution problem, and a sketch of a potential retrenchment-based method for modelling evolution is drawn. Discussion is supported by considering elements of realistic examples