Dijkstra didn’t claim to consider the go to statement harmful, not in those words. The title of his letter to CACM was provided by the editor, Niklaus Wirth, who did such a great job that the entire industry knows that go to is “Considered Harmful”, and that you can quickly rack up the clicks by considering other things harmful.
A deeper reading of his short (~1400 words) article raises some interesting points, that did not as yet receive as much airing. Here, in the interests of writing an even shorter letter, is just one.
My first remark is that, although the programmer’s activity ends when he has constructed a correct program, the process taking place under control of his program is the true subject matter of his activity, for it is this process that has to accomplish the desired effect; it is this process that in its dynamic behavior has to satisfy the desired specifications. Yet, once the program has been made, the “making’ of the corresponding process is delegated to the machine.
There are many difficulties with this statement, including the presumed gender of the programmer. Let us also consider the idea of a “correct” program, which does not exist for the majority of programmers. Eight years after Dijkstra’s letter was published, Belady and Lehman published the first law of program evolution dynamics:
_ Law of continuing change_. A system that is used undergoes continuing change until it is judged more cost effective to freeze and recreate it. Software does not face the physical decay problems that hardware faces. But the power and logical flexibility of computing systems, the extending technology of computer applications, the ever-evolving hardware, and the pressures for the exploitation of new business opportunities all make demands. Manufacturers, therefore, encourage the continuous adaptation of programs to keep in step with increasing skill, insight, ambition, and opportunity. In addition to such external pressures for change, there is the constant need to repair system faults, whether they are errors that stem from faulty implementation or defects that relate to weaknesses in design or behavior. Thus a programming system undergoes continuous maintenance and development, driven by mutually stimulating changes in system capability and environmental usage. In fact, the evolution pattern of a large program is similar to that of any other complex system in that it stems from the closed-loop cyclic adaptation of environment to system changes and vice versa.
This model of programming looks much more familiar to me when I reflect on my experience than the Dijkstra model. If Dijkstra’s programmer stopped programming when they have “constructed a correct program”, then their system would fail as it didn’t adapt to “increasing skill, insight, ambition, and opportunity”.
The programmer who would thrive in this environment is more akin to Ward Cunningham’s opportunistic rewriter, based on his experience of the WyCash Portfolio Management System. That programmer rewrites every module they touch, to ensure that it represents the latest information they have. We recognise the genesis of Ward’s “technical debt” concept in this quote, and also perhaps what we would now call “refactoring”:
Shipping first time code is like going into debt. A little debt speeds development so long as it is paid back promptly with a rewrite. Objects make the cost of this transaction tolerable. The danger occurs when the debt is not repaid. Every minute spent on not-quite-right code counts as interest on that debt. Entire engineering organizations can be brought to a stand-still under the debt load of an unconsolidated implementation, object-oriented or otherwise.
The traditional waterfall development cycle has endeavored to avoid programming catastrophe by working out a program in detail before programming begins. We watch with some interest as the community attempts to apply these techniques to objects. However, using our debt analogy, we recognize this amounts to preserving the concept of payment up-front and in-full. The modularity offered by objects and the practice of consolidation make the alternative, incremental growth, both feasible and desirable in the competitive financial software market.
Ward also doesn’t use go to statements, his programming environment doesn’t supply them. But it is not the ability of his team to avoid incorrect programs by using other control structures that he finds valuable; rather the willingness of his programmers to jettison old code and evolve their system with its context.
Pingback: First, Consider no Harmful. – Structure and Interpretation of Computer Programmers
Pingback: When to “address” “technical debt”? | Structure and Interpretation of Computer Programmers