Archive for the ‘Programming’ Category

Children of the Miracle: from Algol to Prolog

March 18, 2017

The appearance of Fortran inaugurated a fruitful period in programming languages that was to last until the early 1970s. When, in 1999, E.W. Dijkstra gave the keynote address at the ACM Symposium on Applied Computing in San Antonio, Texas, he gave an overview of what he saw as the large-scale trends in the preceding half century. I quote:

And then the 60s started with an absolute miracle, viz. ALGOL 60. This was a miracle because on the one hand this programming language had been designed by a committee, while on the other hand its qualities were so outstanding that in retrospect it has been characterized as “a major improvement on most of its successors” (C.A.R. Hoare).

Several friends of mine, when asked to suggest a date of birth for Computing Science, came up with January 1960, precisely because it was ALGOL 60 that showed the first ways in which automatic computing could and should and did become a topic of academic concern. [1]).

Algol was a miracle as a language. It was short-lived, but it left a momentous legacy that acted in two ways: in the way the Revised Report on Algol 60 describes the language and in the way subsequent language designers were influenced by being shown what a programming language could be. In celebration of Algol 60 I refer to these designers as “Children of the Miracle”.

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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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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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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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Python slams into its exponential wall

April 6, 2014

My first Python sighting was around 1999, in the part of the building where the hackers hang out. Somebody had a poster on the door saying something like If you would have used Python, then you would have been done by now.

Next stop 2005. Since 1986 I had been a fan of “Structure and Interpretation of Computer Programs” by Harold Abelson and Gerald Sussman. At the University of Waterloo it was only in a fourth-year topics course that this book could make a brief appearance. I was envious of MIT, where masses of first-year students took EECS 6.001, where Abelson and Sussman was used from day one. That was class. In 2005 I heard the sad news that, after two glorious decades, EECS 6.001 was closed down, replaced by a course where the text was … a book written for high-school students. Maybe EECS wouldn’t have picked that book if its language would have been BASIC. Perhaps it had to do with the book’s choice of Python. It was from this news item that I learned that Python is a programming language.

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Boltzmann’s Brood

March 8, 2014

When Ludwig Boltzmann (physicist, 1844–1906) was criticized for his ugly brute-force calculations, his defence was: “Elegance should be the concern of shoemakers and tailors”. The criticism was of course not about elegance in the sense of fashion. It was about the fact that an elegant structure would have had the power to convince the reader of whatever the calculations were intended to demonstrate.

I think this episode has something to do with the fact that much of what we do on computers depends on software that, among other weaknesses, is in need of frequent and urgent “security updates”. Those who criticize this state of affairs are dismissed as unrealistic. To be realistic is to understand that any useful piece of software consists of millions of lines of code written by many programmers, mostly no longer traceable and certainly not accountable. No one has a clear view of the system as a whole; there is no description that enables anyone to gain such a view. The realists of today, who accept this state of affairs as inevitable, are of Boltzmann’s brood.

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