Dijkstra versus Perlis (updated)

Here’s Dijkstra

He [Perlis] published a very obnoxious paper arguing against a mathematical approach to programming cite

Here’s the paper by De Millo, Lipton and Perlis. It starts as follows:

Many people have argued that computer programming should strive to become more like mathematics. Maybe so, but not in the way they seem to think. The aim of program verification, an attempt to make programming more mathematics-like, is to increase dramatically one’s confidence in the correct functioning of a piece of software, and the device that verifiers use to achieve this goal is a long chain of formal, deductive logic. In mathematics, the aim is to increase one’s confidence in the correctness of a theorem, and it’s true that one of the devices mathematicians could in theory use to achieve this goal is a long chain of formal logic. But in fact they don’t. What they use is a proof, a very different animal. Nor does the proof settle the matter; contrary to what its name suggests, a proof is only one step in the direction of confidence. We believe that, in the end, it is a social process that determines whether mathematicians feel confident about a theorem–and we believe that, because no comparable social process can take place among program verifiers, program verification is bound to fail.

To me, the problem with Dijkstra is that he was so sharp and such a good writer that he was able to make persuasive cases out of wrong ideas. Dijkstra wanted to be a scientist in the model of theoretical physics, not an engineer. I’m pretty confident that Dijkstra was wrong: programming is engineering – in fact, physics is not as far from engineering as some people would like to believe. I’m not a huge fan of the engineering discipline as it exists in the USA. It has all sorts of negative aspects – including those Dijkstra railed against. But the vision of a programmer as, not a mathematician, but a formal logician flying far above the grubby compromises and trade-offs of mere engineering in a platonic bubble of pure reasoning is wrongheaded.

Dijkstra published some criticism of the Demillo paper at the time and in their response the authors stated something that, as far as I know, remains true

We must begin by refusing to concede that our confidence in a piece of
real software has ever been increased by a proof of its correctness

When I was in graduate school, a famous formal methods scholar came for a talk and explained to us that formal methods were needed if we were ever going to develop fault tolerant software. I pointed out that, for example, the Tandem Software worked pretty well in practice. “It cannot”, retorted the famous scholar.

So there.

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3 thoughts on “Dijkstra versus Perlis (updated)”

  1. “We must begin by refusing to concede that our confidence in a piece of real software has ever been increased by a proof of its correctness” – sounds very defensive, childish almost

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