Edsger W. Dijkstra: Difference between revisions

There is no evidence that Dijkstra was ever a user of APL, and several of his remarks suggest he would refuse any opportunity to use or learn it. His familiarity with the language comes from attending presentations by [[Ken Iverson]] and others, and friendship with [[Alan Perlis]], a fan of the language.

{{quote|Dijkstra: How would you represent a more complex operation, for example, the sum of all elements of a matrix M which are equal to the sum of the corresponding row and column indices?<br/>

* A typical characteristic of the APL devotee is, for instance, his closeness to an implementation of it. I know of a visiting professor at an American University [sic] who, trying to teach APL, bitterly complained about the absence of APL terminals. He was clearly unable to teach it without them. And you, too, write to me that you would like to meet me in your part of the world, so that you can “demonstrate APL” to me. This is in sharp contrast to people who prefer programming languages that can be adadequately [sic] “demonstrated”—i.e. shown, taught and discussed—with pencil and paper.

{{Quote|What [Fritz Bauer] saw or heard was Ken’s remark that APL is an extremely appropriate language for teaching algebra, and he muttered under his breath to me, in words I will never forget, “As long as I am alive, APL will never be used in Munich.” And Dijkstra, who was sitting on my other side, leaned toward Bauer and said, “Nor in Holland.” The three of us were listening to the same lecture, but we obviously heard different things.}}

* [https://www.jsoftware.com/papers/TAOaxioms.htm "TAO Axioms"]

One specific complaint that Dijkstra levels at APL is its use of an expanded set of operations relative to the typical set used in computer science research. It's true that APL dedicates much of its functionality to working with arrays, but this shouldn't be taken as a theoretical weakness. In fact, APLs array operators are much like structured programming, which eliminates a single construct, Go to, in favor of several branching and looping constructs like ''if'' and ''while''. Moving from scalar processing to array processing requires new operations in exchange for, not at the expense of, improved theoretical properties. Nonetheless, language designers such as [[Arthur Whitney]] put considerable effort into reducing the number of symbols needed in the language in order to express ideas. Indeed, [[Fred Brooks]] praised [[Ken Iverson|Iverson]] for a "fierce determination not to invent any new constructs, until you have to."<ref>Brooks, Fred. ''A Celebration of Kenneth Iverson'' ([https://www.jsoftware.com/papers/KEIQA.htm#touchstone excerpt])</ref>

Dijkstra's paper "Go To Statement Considered Harmful" was an important factor in the introduction of structured programming, which in APL has led to the addition of [[control structure]]s and encouraging their use in favor of the [[branch]] command. Papers on [https://link.springer.com/chapter/10.1007/3-540-07131-8_25 APLGOL] and [https://www.researchgate.net/publication/262210729_Design_of_a_structured_APL SAPL], two experiments in bringing structured programming to APL in the 1970s, both reference Dijkstra's work.

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