Addendum to my previous reply from the end of EWD288 - "Concern for Correctness as a Guiding Principle for Program Composition";

My thesis is, that a helpful programming methodology should be closely tied to correctness concerns. I am perfectly willing to admit that I myself may be the most complete victim of my own propaganda, but that will not prevent me from preaching my gospel, which is as follows. When correctness concerns come as an afterthought and correctness proofs have to be given once the program is already completed, the programmer can indeed expect severe troubles. If, however, he adheres to the discipline to produce the correctness proofs as he programs along, he will produce program and proof with less effort than programming alone would have taken.

The framework of this contribution does not allow the inclusion of a worked-out example. Instead I shall try to sketch how the hand-in-hand construction of a program and its correctness proof guides the programming process.

I found what I regard as the quintessence of this methodology in the summer of 1968, when my attitude towards flowcharts changed radically. Up till that moment, I had regarded a flowchart as something incomplete, as a plan, as a (half-pictorial, but that is not essential) representation of my intentions, something that as a description would only make sense if all further details, down to the bottom, had indeed been supplied. But it was then that I saw that such a sketch of a program still to be made, could be regarded as an abstract version of the final program, or even, that it could be regarded as "the program", be it for a —in all probability hypothetical— machine with the proper repertoire of primitive actions operating on variables of the proper types. If such a machine were available, the "rough sketch" would solve the problem; usually it is not available, and actions and data types assumed have to be further detailed in the next levels of refinement. It is the function of these next levels to build the machine that has been assumed to be available at the top level, i.e. at the highest level of abstraction. This most abstract version is now no longer a draft of the final program, it is an essential part of the total program; its correctness is independent of the lower level refinements and can be established beforehand. To give this correctness proof before proceding with the lower level refinements serves many purposes. It establishes the correctness of the top level. Fine, but what is more important is that we do this by applying well-established theorems applicable to well-known sequencing clauses, and as a result it becomes most natural to avoid clever constructions like the plague. But the most important consequence is that the proof for one level ensures that the interface between itself and the lower levels has been given completely, as far as is relevant for that level; and also, by fixing in the interface only what the correctness proof really needs, the interface can be kept free from overspecification.

Finally, a word or two about a wide-spread superstition, viz. that correctness proofs can only be given if you know exactly what your program has to do, that in real life it is often not completely known what the program has to do and that, therefore, in real life correctness proofs are impractical. The fallacy in this argument is to be found in the confusion between "exact" and "complete": although the program requirements may still be "incomplete", a certain number of broad characteristics will be "exactly" known. The abstract program can see to it that these broad specifications are exactly met, while more detailed aspects of the problem specification are catered for in the lower levels. In the step-wise approach it is suggested that even in the case of a well-defined task, certain aspects of the given problem statement are ignored at the beginning. This means that the programmer does not regard the given task as an isolated thing to be done, but is invited to view the task as a member of a whole family; he is invited to make the suitable generalizations of the given problem statement. By successively adding more detail in the lower levels he eventually pins his program down to a solution for the given problem.