Side Effects in Steering Fragments

In this thesis I will give a formal definition of side effects. I will do so by modifying a system for modelling program instructions and program states, Quantified Dynamic Logic, to a system called DLAf (for Dynamic Logic with Assignments as Formulas), which in contrast to QDL allows assignments in formulas and makes use of short-circuit evaluation. I will show the underlying logic in those formulas to be a variant of short-circuit logic called repetitionproof short-circuit logic. Using DLAf I will define the actual and the expected evaluation of a single instruction. The side effects are then defined to be the difference between the two. I will give rules for composing those side effects in single instructions, thus scaling up our definition of side effects to a definition of side effects in deterministic DLAf-programs. Using this definition I will give a classification of side effects, introducing as most important class that of marginal side effects. Finally, I will show how to use our system for calculating the side effects in a real system such as Program Algebra (PGA).