British propaganda poster from 1939.


Of course, I am hoping that readers will mistake Prolog for Prologue = an introduction to something. In addition, I have a further hope that the poster, displayed above, will induce a feeling of calmness in these same readers, so that they will be able to approach the real content of this weblog post with detachment, but not indifference. The main problem with the poster, is that almost everything about it, apart from its wording, and especially its signal red background, but also large sans-serif white lettering and the British crown, reinforce a feeling of danger!

Wikipedia tells us, Keep Calm and Carry On was a propaganda poster produced by the British government in 1939 in preparation for World War II. The poster was intended to raise the morale of the British public. Although 2.45 million copies were printed, the poster was only rarely publicly displayed and was little known until a copy was rediscovered in 2000 at Barter Books, a bookshop in Alnwick, a market town in Northumberland, in north-east England.

Some topics, toothaches in particular, or dentistry more generally, do not induce calmness. Instead, they increase the flow of adrenaline, and other forms of psychomotor agitation, resulting in psychological and physical restlessness. Thus, before confessing what this topic is really about, I want to reassure readers that it is a topic that can be fun, if approached correctly. Initially, I had thought of dividing the topic into multiple parts and publishing them at the rate of one part a day, over more than a week. The parts are still subdivided, but each reader will have to determine her/ his/ its etc. own consumption rate.


I am used to dealing with actors, people pretending to be someone else. In the process, these people have helped me developed my own acting talents. Some of the actors I had to deal with, had failed their auditions, often called court appearances or trials. One of the consequences of such a failure, could be imprisonment at the Norwegian low security prison where I was assigned as their teacher.

Other actors were youth in the final years of their compulsory education, at senior secondary school. They had to attend school, but some of them were better than others at presenting themselves in a positive light. Not that everyone sought positivity. In a Media and Communication English class, I once asked the pupils to write about something they wanted to accomplish in the future, and why they wanted to do so. The reply that created the most work, not just for myself, but for the student, the school principal, the school psychologist and others, was an essay that detailed how this person wanted to become a mass murderer. Afterwards, he claimed that this was a work of fiction.

I have experienced a lot of acting performances by students. The most problematic actors are those who pretend they understand a topic, when they have absolutely no idea about it. The role of the teacher is to channel student activity so that the student finds a route that suits her/ his personality, and is effective at helping the student learn new sets of knowledge and develop new skills. This route-finding skill is the primary talent needed to teach.


This weblog post’s topic is programming, in a specific language. While numbers vary with the situation, perhaps ten percent of actors will delight in learning the programming language they are confronted with. A similar number, give or take, will not master anything. Those remaining in the middle will accept programming languages as a necessary evil in this internet age. Stated another way, a small percentage will find their own route without assistance, another small percentage will never find a route, while most people in the middle will struggle to varying degrees, but ultimately find a route, hopefully one that suits their personality.

The main difficulty in terms of learning to program, is that schools begin computer science studies assuming that students will want to learn to program the particular language being offered. Admittedly, some languages are fairly general, including some that are designed more for teaching/ learning, than for any more practical applications. Pascal, is probably the best example of such a language. However, my contention is that the first computing course a student takes should look at programming principles.

I was fortunate to read Bruce J. MacLennan’s, Principles of Programming Languages: Design, Evaluation and Implementation (1983). A second edition was published in 1987, and a third in 1999. There is not much difference between the three editions, and the same languages are discussed in all three: pseudo-code interpreters, Fortran, Algol-60, Pascal, Ada, Lisp, Smalltalk and Prolog. All the editions of this book explain that computer languages can have different purposes, and asks readers to examine the purpose of each programming language. Not everyone should learn the same one. Before they decide to learn programming, people should know what they want to do with that language, after they have learned its basics. Much of the time the answer is, learn a more appropriate language.


Books can have multiple uses.

The Prolog in the title of this post refers to the Prolog programming language. Fifty years ago, in 1972, Prolog was created by Alain Colmerauer (1941 – 2017), a French computer scientist, at Aix-Marseille University, based on the work of Robert Kowalski (1941 – ), an American-British computer scientist, at the time at the University of Edinburgh.

Prolog is a logic programming language associated with artificial intelligence and computational linguistics. That doesn’t say much. It might be more understandable to say that students typically learn Prolog by creating a program/ system that shows social relationships between people. Despite their reputation as rather awkward social creatures, even computer scientists have the capability of understanding some social markers: mother, father, daughter, son, at a minimum. Thus, even computer scientists can construct a system that will determine then show relationships between any two people. The system can be constructed slowly, so that initially only, say, four relationships are allowed. Outside of those four choices, people will be labelled as having no relationship. However, in subsequent iterations, the number of relationships can be expanded, almost indefinitely.

Prolog consists of three main components: 1) a knowledge base = a collection of facts and rules fully describing knowledge in the problem domain; 2) an interface engine, that chooses which facts and rules to apply when attempting to solve a user query; 3) a user interface, that takes in the user’s query in a readable form and passes it to the interface engine. Afterwards, it displays results to the user.


Programming in Prolog, written by William F. Clocksin (1955 – ) & Christopher S. Mellish (1954 – ), is the most popular textbook about the language. Originally published in 1981, a revised (read: readable) second edition appeared in 1984. My copy has my name printed on the colophon page in capital letters in blue ink, considerably faded now, along with the date, 12 iii 1985.

It is not the only book about Prolog in my library. Among the thirteen others are: Dennis Merritt, Building Expert Systems in Prolog (1989); Kenneth Bowen, Prolog and Expert Systems (1991); Alain Colmerauer & Philippe Roussel, The Birth of Prolog (1992); Krzysztof R. Apt, From Logic Programming to Prolog (1997) and even an updated 5th edition of Clocksin and Mellish, subtitled Using the ISO Standard, (2003). Part of the reason for this large number, was my using of Prolog to teach expert systems.


Expert systems are not particularly popular, now. In artificial intelligence, popularity contests are being won by machine learning tools. Yet, some people don’t have to be at either the height of fashion or the cutting edge of technological advances, and can appreciate older approaches.

Edward Feigenbaum (1936 – ) constructed some of the first expert systems. He established the Knowledge Systems Laboratory (KSL) at Stanford University. Long words are often strung together to describe his work. A favourite phrase is, “Knowledge representation for shareable engineering knowledge bases and systems.” This was often coded into the phrase expert system. He used it mainly in different fields of science, medicine and engineering. KSL was one of several related organizations at Stanford. Others were: Stanford Medical Informatics (SMI), the Stanford Artificial Intelligence Lab (SAIL), the Stanford Formal Reasoning Group (SFRG), the Stanford Logic Group, and the Stanford Center for Design Research (CDR). KSL ceased to exist in 2007.

The focus of Feigenbaum, and American institutions more generally, was on rules-based systems: Typically, these found their way into shells = computer programs that expose an underlying program’s (including operating system) services to a human user, produced by for-profit corporations, that would sit on top of Lisp, one of the programming languages commented on in two chapters of MacLennan’s book, and used extensively for artificial intelligence applications. Feigenbaum and his colleagures worked with several of these expert systems, including: ACME = Automated Classification of Medical Entities, that automates underlying cause-of-death coding rules; Dendral = a study of scientific hypothesis formation generally, but resulting in an expert system to help organic chemists identify unknown organic molecules, by analyzing their mass spectra, and combining this with an existing but growing chemical knowledgebase; and, Mycin = an early backward chaining expert system that identified infection related bacteria, recommend specific antibiotic treatments, with dosage proposals adjusted for patient’s mass/ weight. He also worked with SUMEX = Stanford University Medical Experimental Computer. Feigenbaum was a co-founder of two shell producing companies: IntelliCorp and Teknowledge. Shells are often used by experts lacking programming skills, but fully capable of constructing if-then rules.


Prolog is frequently contrasted with Lisp, and offers a different approach for developing expert systems. Some users are fond of saying that Prolog has a focus on first-order logic. First-order is most appropriately translated as low-level, or even simple. The most important difference between the two languages, is that anyone with average intelligence should be able to understand, and work with Prolog. Much of the work done with Lisp involves higher-orders of logic, often requiring the insights of real logicians, with advanced mathematics in their backgrounds. An introductory logic course, gives sufficient insight for anyone to work with Prolog.

Prolog is also claimed to be a more European approach. This probably has something to do with the way teaching is organized. In Norway, for example, a (Danish) Royal decree from 1675 and still valid today, required all university students to undertake an Examen philosophicum, devised by advisor Peder Griffenfeld (from griffin, the legendary creature, plus field, but originally, Schumacher = shoemaker, 1635 – 1699). Under the Danish King, Christian V (1646 – 1699), he became the king’s foremost adviser and in reality Denmark’s (and Norway’s) actual ruler. In 1676 he fell into disfavour and was imprisoned. He was sentenced to death, for treason, but the sentence was commuted to life imprisonment. He was a prisoner on Munkholmen, outside Trondheim, and about 55 km directly south-east of Cliff Cottage, for 18 years (1680–1698), and was released after 22 years of captivity.

Until the end of the 1980s, this exam involved an obligatory course in logic, including mathematical logic, along with other subjects. This means that almost every university student (at that time), no matter what they studied, had the necessary prerequisites to work with Prolog.


Expert systems often involve heuristics, an approach to problem solving using methods that are not expected to be optimal, perfect, or rational, but good enough/ satisfactory for reaching an approximate, immediate or short-term goal. George Pólya (1887-1985), who worked at Stanford 1940 – 1953, and beyond, took up this subject in How to Solve It (1945). He advised: 1) draw a picture, if one has difficulty understanding a problem; 2) work backwards, if one can’t find a solution, assuming there is one, and see what can be derived from it; 3) develop a concrete example, from an abstract problem; 4) solving a more general problem first – this involves the inventor’s paradox where a more ambitious plan may have a greater chance of success.

One list of areas where expert systems can be used, involve system control, in particular: 1) interpretation, making high-level conclusions/ descriptions based on raw data content; 2) prediction, proposing probable future consequences of given situations; 3) diagnosis, determining the origins/ consequences of events, especially in complex situations based on observable data/ symptoms; 4) design, configuring components to meet/ enhance performance goals, while meeting/ satisfying design constraints; 5) planning, sequencing actions to achieve a set of goals with start and run-time constraints; 6) monitoring, comparing observed with expected behaviour, and issuing warnings if excessive variations occur; 7) repair, prescribing and implementing remedies when expected values are exceeded.

Sometimes one comes across Prolog tutorials that begin with subjective knowledge/ considerations. Music is a good example. Unfortunately, it is not always easy to remember if one has labelled something as trash metal or punk, and this may have operational consequences. It is much easier to confirm that person X is person Y’s granddaughter, and that person Y is person X’s grandfather, especially if persons X and Y are members of your own family.

It is always hard to know which Prolog expert system implementation will impress readers most. Here are some choices: Bitcoinolog = configures bitcoin mining rigs for an optimal return on investment; CEED = Cloud-assisted Electronic Eye Doctor, for screening glaucoma (2019); Sudoku = solves sudoku problems; an unnamed system constructed by Singla to diagnose 32 different types of lung disease (2013), another for diabetes (2019); an unnamed system by Iqbal, Maniak, Doctor and Karyotis for automated fault detection and isolation in industrial processes (2019); an unnamed system by Eteng and Udese to diagnose Candidiasis (2020). These are just some of hundreds, if not thousands, many open source.


One of the challenges/ problems with expert systems is that the scope of its domain can be unknown. In other words, when a person starts using an implemented expert system, it can be unknown just how big or little the range of problems is that can be used successfully with it. There can also be challenges with system feedback. What looks like an answer, may be a default because the system has insufficient insights (read: rules) to process information. Expert systems do not rely on common sense, only on rules and logic. Systems are not always up to date, and do not learn from experience. This means that real living experts are needed to initiate and maintain systems. Frequently, an old system is an out of date system, that may do more harm than good.

This begs a question of responsibility/ liability in case the advice provided by a system is wrong. Consider the following choices: The user, the domain expert, the knowledge engineer, the programmer of the expert system or its shell, the company selling the software or providing it as an open-source product.


Just before publication, I learned of the death of crime novelist Susie Steiner (1971 – 2022). I decided to mention her in this weblog post, when I read in her obituary that she had spotted a Keep Calm poster on the kitchen wall at a writing retreat in Devon. She was cheered by its message of stoicism and patience.

Speaking of kitchens, at one point my intention was to use Prolog to develop a nutritional expert system, that will ensure a balanced diet over a week long time frame, along with a varied menu for three meals a day. I still think that this would be a useful system. Unfortunately, I do not think that I am the right person to implement it, lacking both stoicism and patience, to complete the undertaking.

Reflecting on Susie, I am certain that a Prolog system could be made to help writers construct their novels, especially crime fiction. A knowledge base could keep track of the facts, as well as red herrings and other fish introduced to confuse the reader, and prevent them from solving the crime. Conversely, a Prolog system could also be built that would help readers deconstruct these works, and help them solve the crime and find textual inconsistencies.


  1. Readers should be delighted to hear that while writing this post I used my original Clocksin and Mellish book on a daily basis! Yes, it held my laptop open at an angle of about 145°, about 10° further open than without it. When writing on other topics, I also use other books for the same purpose. Note to self: ensure that your next laptop opens at least 180 degrees!
  2. The writer should be dismayed about the length of this post. Patricia reminds me, repeatedly, that shorter is better. She felt last week’s post on Transition One was a more appropriate length. Transition One was written in the course of an hour, with a couple of additional proof-reading sessions. Writing Prolog took more than a year, with multiple writing sessions, each adding several paragraphs.

3 Replies to “Prolog”

  1. Excellent feint, if that’s the correct term for the way you slid into the real topic of this piece Brock. The introduction was captivating. I only stopped reading after I realized you were talking about a programming language, about a quarter of the way through..

    1. I’m glad that you allowed yourself to be entertained, Charles. My experience is that real programmers (those who use C, Java and other more conventional programming languages) usually dislike Prolog. Back when I was teaching it in the late 1980s – early 1990s, I would initially ask students to write statements on sheets of paper, rather than put it into a machine. Many of those most unfamiliar with programming languages did not realize that they were actually writing code! They were programming without being aware that they were doing it.

  2. Dear Brock,
    As one who began programming in the 1960s and still has to do it at times today … well, if you count things like macros in spreadsheets, then I do a moderate amount … as I read, I kept asking, “Where is he going with this?”
    I never came up with an answer, but did enjoy the trip.
    Then it struck me … maybe that WAS the point … the good feeling of covering familiar ground. After all, writing is an artform and so is music and when you finish listening to a piece, you are unlikely to ask, “OK, what was the point?”
    However, as usual, I still learned something. I hit “colophon page” and thought, “What is that?”
    Asking my wife, she answered immediately. Those darn journalism types!


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