It's the 80s all over again

In the 1980s I was very interested in programming languages and artificial intelligence. I had begun programming on a PDP/11 which was situated about 20 km from where I lived at the time; we had a telephone line connecting the computer to a keyboard/printer, and when I wasn't accounting, I was programming. In 1986, if I remember correctly, I was one of the first people I knew to have a PC at home - some kind of IBM clone. This was to be the first of many computers that I would own. I programmed in Turbo Pascal although I as I say, I was interested in other languages and had several programming language diskettes.

It was the August 1985 edition of Byte magazine that captured my attention; this had a picture of a robot arm emerging from an egg on the cover. I began a subscription to the magazine and shortly learned about a fascinating language called Prolog. I bought a book about the language and had to suffice with this as I had no implementation. At the same time, Byte was running a series of articles by someone called Jonathan Amsterdam (again, if I remember correctly) who was writing about implementing a Modula-2 like language.

At the time I didn't know enough to know better and so I thought that I would be able to use some of Amsterdam's code and write an interpreter for Prolog, as the book I had made it seem simple. Little did I know. I never did succeed in writing a real pico-Prolog at the time, although in the early 2000s I found the source code to a pico-Prolog and finally implemented it in Delphi.

I did try my hand, though, at writing other kinds of inference engines and completed something called 'The Amateur Reasoner', which was distributed via a shareware library with whom I had contacts at the time. Someone in America sent me his knowledge base for determining which catalyst to use in which petroleum refining process, and I incorporated it along with the several other knowledge bases that I had cobbled together.

This program might well have been inspired by a book which I owned called 'Writing Expert Systems in Pascal" - or maybe it was in Basic. Another source of inspiration would have been an article in Byte called "Inside an expert system: from index cards to PASCAL program", about which I had completely forgotten until today.

In the last few months, the itch of trying to implement Lisp in Pascal has re-awakened in me; unfortunately I have the time, but I don't know what I intend to do with this even if I do succeed. As a way of relieving the itch, I often scan the Internet looking for suitable programs. Today I persevered more than I normally do and eventually stumbled upon a treasure trove. I immediately went to the AI languages section and found several interesting programs, including the source to a "very tiny Prolog" implementation in Pascal, written by Bill and Bev Thompson. They were the authors of the "Inside an expert system" article to which I referred in the previous paragraph.

The source code states that there was a pair of articles in "AI Expert" magazine which presumably explained how the program worked, but so far I have been unable to locate those magazines which were published 25 years ago. Whilst looking through the treasure trove site, in an attempt to see whether maybe the articles were there in another guise, I came across a program which seemed terribly familiar - yes, it was my Amateur Reasoner program!

I had to download this, primarily to see whether there was anything cringe-worthy there. Apart from a regrettable tendency to mix display code (including inline assembly and a few BIOS tricks) along with the inference engine code, it's actually not bad at all. The code is full of linked lists, which is what we had in the mid-80s, in the absence of anything of a higher order. I don't know whether I would re-implement this today with a series of types based on TObject and put them in lists or rather keep the low level code.

It's fascinating reading the code. I see how I kept the syntax as simple as possible so that I wouldn't need to include a recursive descent parser. I probably knew about such things then but did not necessarily understand them, which is one reason why my Prolog interpreter never got very far.

I accidentally double clicked on the executable file and I'm pleased to note that the program works fine in a DOS box under Windows. Maybe I should re-implement it in Delphi - only 21 years after it was originally written.