The Essence of Algol

November 12, 2016

Elsewhere [10] I considered ways in which programming languages could be different. One of these ways is expressed by asking the question Does the language have an essence? The possibility of an affirmative answer is suggested by the title of John Reynolds’s paper: “The essence of Algol” [7]. Reynolds used what he perceived to be the essence of Algol to make distinctions among members of what is usually considered to be a single family. Thus he argues that Algol 60 is a carrier of the essence, whereas this is not the case for other members of that family: Algol W, Euler, Algol 68, and Pascal.

What is judged to be essence is in the eye of the beholder. I am more interested in what the members of the Algol family have in common and perhaps even with languages not usually considered as members. For example, Prolog. Soon after completion of this language in Marseille, Robert Kowalski wrote a paper (published as [6]) that established a procedural interpretation of a form of first-order predicate logic. Let us examine this interpretation to see whether this gives a hint concerning the essence of Algol as a procedural language.

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Kinds of Programming Languages

October 21, 2016

I find the number of programming languages mind-boggling. For example, Jean Sammet in her “Roster of programming languages for 1976-1977” [12] lists 156 programming languages in 24 categories. This includes only languages in active use within the USA. The opposite extreme on the spectrum of permissiveness is represented by the “Online historical encyclopedia of programming languages” [7], which claims 8945 “programming languages since the 18th century”.

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Alan Robinson

September 16, 2016

On August 5, 2016 John Alan Robinson died. In him, a great scientist departed, and I mourn a dear friend. His great discovery was the resolution principle in mathematical logic, a discovery that capped two decades of development. In its turn, it spawned a plethora of new developments in computer programming. It became important enough in Artificial Intelligence to become controversial. Read the rest of this entry »

Wildcat Crypto

May 13, 2016

With his “Pretty Good Privacy” (PGP), Phil Zimmermann launched the era of what I call Wildcat Crypto. Up to that point discussions on cryptology had focussed on whether DES, the US government standard, was secure and whether government involvement in its development might have jeopardized its security. To avoid government involvement, PGP used IDEA as alternative to DES for its bulk encryption. Yet IDEA is a close relative of DES and shares its weakness: small, constant-size blocks. True Wildcat Crypto calls for a radical departure in the form of blocks that are much larger, and moreover, vary in length under control of the key. In this article I trace the relevant history and outline my implementation of such a radical alternative.

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Some Books on C

March 26, 2016

I have gathered such introductory books on the C programming language as I own or could borrow. The result is an eight-storey tower. A quick scan shows that I could easily buy another half dozen, but I doubt whether that would yield any new insights.

For most teachers “an introductory programming book with C” is an oxymoron. The extreme wing in this school of thought consider only designedly friendly languages suitable for an introduction to programming. BASIC is an early example. My current favourite friendly language is Python [1]. But the mainstream of teachers of introductory programming has settled on Java as a compromise between friendliness and attractiveness to prospective employers.

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Why does C not have an exponentiation operator?

March 15, 2016

The most authoritative source for an answer to the question in the title would be Dennis Ritchie. Next best is Bjarne Stroustrup:

The semantics of C operators should be simple to the point where each corresponds to a machine instruction on a typical computer. An exponentiation operator doesn’t meet this criterion. [1], page 247

Stroustrup’s criterion needs to be taken with a grain of salt. For example, the assignment operator does not meet the criterion: a = b takes a LOAD and a STORE. In this case the criterion translates to: “should correspond to no more than a LOAD and a STORE. But Stroustrup was onto something, because we can tweak his answer to:

The semantics of C operators should be simple to the point where they each compile to code that is as fast as what one can write in assembler.

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IA or AI?

October 9, 2015

I’ve been reading a paper [1] written two decades ago, which itself is an account of events two decades before that. It is Alan Kay nominally writing about the history of a programming language called Smalltalk. I say “nominally” because already by Smalltalk 72, Kay was losing interest in language matters and wanted concentrate on what the language, as part of the computer-as-medium, could do for thought. Along with various other sources of inspiration that led to Smalltalk (Sutherland’s Sketchpad, the Burroughs B220, the Burroughs B5000, …) Kay mentions attending in 1968 the conference presentation by Douglas Engelbart [2] that has since become known as the “Mother of All Demos”.

In 1968, when timesharing by users behind teletype terminals was regarded as avant-garde, Engelbart gave a demo that featured a number of firsts: a screen display for both and text and graphics, interactive text-editing, a mouse. All that was integrated into a fluidly handled medium. Kay gives testimony to the huge impression this made on him: “Engelbart was a prophet of Biblical proportions”.

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Recursion versus iteration

July 30, 2014

The first programming language to allow functions to be recursively defined was McCarthy’s LISP in 1959. Its introduction was not controversial: nobody but John McCarthy had any say in what the language was going to be. In addition to his work on LISP, McCarthy was on the committee finalizing the Algol language in 1959 and 1960. In spite of the fact that a majority was opposed to it, the definition of Algol 60 ended up allowing recursively defined procedures. In [1] I gave an acount of how this happened.

Why was recursion such a big deal? For us this is hard to understand: for decades the programming language C, not exactly a paradigm of avant-garde, has allowed recursion. Still, remnants of unease remain. Still, in some introductory courses, recursion plays the role of a pons asinorum. Have instructors been traumatized by a sarcastic teacher finding, oh horrors, a circular definition in their high-school essay? In this essay I’m going to explore recursion by contrasting it to its opposite, iteration.

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How recursion got into programming: a tale of intrigue, betrayal, and advanced programming-language semantics

June 18, 2014

By now it is difficult to imagine that once there was a time when the utility, and even the possibility, of recursion in programming was in doubt. Yet that was true of the programming community around 1960. Even the committee that was to create Algol 60 was divided on the issue. How recursion got into the language is a story of intrigue and misunderstandings. I came across this story for the first time when reading Gauthier van den Hove’s excellent MSc thesis [11]. It is also the subject of Chapter 3 in [12].

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Dijkstra, Blaauw, and the origin of computer architecture

June 14, 2014

E.W. Dijkstra is known for several important contributions. It does not seem to be widely known that he played a role in the origin of computer architecture as a concept. In arguing that this is the case I draw attention to the passage in his 1972 Turing lecture where he recounts that the darkest week in his professional life was when he studied the specifications of a newly announced line of computers that he does not further specify than being “of the third generation”.

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