"Bagatelle in C arranged for VDM SoLo"

Abstract

This paper sketches a engineering discipline which combines formal and semi-formal methods. Central to the former is denotational semantics, expressed in the ISO/IEC 13817-1 standard specification language (VDM-SL). This is strengthened with algebra of pro- gramming, which is applied in reverse order so as to reconstruct formal specifications from legacy code. The latter include code slicing, a shortcut which trims down the complexity of handling the formal semantics of all program variables at the same time. A key point of the approach is its constructive style. Reverse calculations go as far as absorbing auxiliary variables, introducing mutual recursion (if applicable) and reversing semantic denota- tions into standard generic programming schemata such as cata/paramorphisms. The approach is illustrated for a small piece of code already studied in the code-slicing literature: Kernighan and Richtie's word countC programming bagatelle.