Tuesday, October 21, 2025

Problems with Whatsapp

Yesterday I intended to delete a message that I had sent to myself. By accident, I deleted the entire sequence of messages to and from myself. That's unfortunate, but not really a problem.

I then read some information on the net that explains how to restore data to Whatsapp in case one doesn't have an external backup, for example to Google - as I imagine many people don't have. The instructions were to find a certain file via the file manager on the phone and rename it; this file holds data regarding messages and is stored in one of Whatsapp's directories. This step was not a problem.

Then I was told to uninstall Whatsapp then reinstall it. When installing, one can ask for it to be restored from a backup which I supposedly now had. Except for the fact that the file that I renamed no longer existed ... because uninstalling Whatsapp deleted all its directories and all the files contained within. Reinstalling Whatsapp recreated the directories, but not the files. 😞

The instructions regarding the file that I renamed should have included moving it to another directory that is not one that belongs to Whatsapp.

So I start anew. It's far from the end of the world, but a trifle annoying. Of course, I also ensured that from now there is an automatic daily backup, although I suspect that this will fail at some stage because there is not enough free space in the Google directory. I shall have to try and find what I can delete from there in order to free up room.



This day in blog history:

Blog #Date TitleTags
5821/10/2006Busy morningProgramming, Randy Newman, Paul Simon
64221/10/2013Paul KleePersonal, Bristol Grammar School
108421/10/2017Sumptuous Saturday Seven - baked hake with potatoes and vegetablesCooking
168121/10/2023Seventh chordsMusic theory
184421/10/2024More blog manager tricksProgramming, Blog manager program

Monday, October 20, 2025

Sleep apnea and blood tests

The data from my CPAP machine for the past week or so has been very discouraging, showing several nights with 18-19 apneic events (most of them central), and no nights with the 0, 1 or 2 events that I normally have. What can be wrong?

My initial hypothesis was that there is something wrong with the mask, as it is reaching the end of its year. But there are no leaks being recorded so that's probably not the reason.

My second hypothesis is connected with evening medication. When we came back from Italy a month ago, I couldn't find the packets of two medicines in my special suitcase. I suspect that I hid them so well that I couldn't find them afterwards, and not that I forgot to pack them. So for the past month, I have been taking much smaller amounts of one drug that is the probable cause, amitriptyline, of which I take a small dose (25 mg) as a prophylactic against migraine. It is very effective at this, but it's also known to cause drowsiness. I posit that when I get a new prescription for this drug, the number of apneic events will swiftly decrease. Another drug that seemed to disappear is for blood pressure, although not taking it has not had any appreciable effect.

Another side effect of amitriptyline is weight gain. As it happens, since coming back from Italy, my weight has dropped by 2 kg! I now weigh 'only' 83.9 kg, which is the lowest for two and a half years. I have thought until now that this due to the fact that for the past few weeks I haven't been eating anything from after lunch until the next morning, thus restricting the hours during the day in which I eat to about 7 hours, with 17 hours fasting. Maybe the fact that I am not taking amitriptyline also has an effect on this; again, I'll know when I get the new prescription (next week). 

The biannual blood tests show improvements in almost everything:
Test27/03/2519/10/25Remarks
Triglycerides 192 178 Lowered - good
Total cholesterol 177 168 Lowered - good
HDL 40 48 Raised - good
LDL 99 84 Lowered - good
LDL/HDL ratio 2.48 1.75 Lowered - good
Uric acid (gout) 7.1 6.7 Lowered - good
Haemoglobin 16.3 15.7 Almost unchanged
NEUT/LYM ratio 2.1 1.8 Lowered - good
HEMOGLOBIN A1C % 6.0 6.1 Raised - not good
Glucose 103 107 Raised - not good
Potassium 5.3 5.1 Lowered - good

The lipid (cholesterol) profile has definitely improved, as has the potassium level (important for me), but the values for glucose and HEMOGLOBIN A1C % (glucose over the past three months) are worse and that is worrying. I had the value 107 for glucose a few years ago, after which it dropped slightly. Obviously I'll have to cut down on the biscuits and chocolate again, even if I'm not eating them after lunch. I'll try again with bell peppers1; I stopped because I thought that they were giving me stomach pains, but that pain may be due to something else.

Internal links
[1] 1720



This day in blog history:

Blog #Date TitleTags
20920/10/2009Lucky breakTV series, Films, Olivia Williams, William and Mary, Cold feet
41920/10/2011Firebird DB management tool (4) - CorrectionsProgramming, SQL, dbExpress
64120/10/2013More health issuesHealth, CPAP
76720/10/2014The seach for serendipityDBA, SQL
89420/10/2015Vinyl log 23 - 20 OctoberVinyl log, 1971, Dave Evans, The village thing
126820/10/2019This must be the place (2)Personal
135020/10/2020DBA updateDBA
168020/10/2023Sorting on two or more columnsProgramming, Delphi, ClientDataSet
184320/10/2024Good news for Smiley watchersJohn Le Carre

Sunday, October 19, 2025

Arithmetic in CoPilot Prolog

I decided to stop work on implementing lists in my Prolog interpreter, and instead I concentrated on implementing simple arithmetic. Bratko had a query in the form ? - X is 1 + 2, but I don't like that syntax. I had seen in another Prolog book that addition was defined by means of a rule that was more to my liking:

add (Operand1, Operand2, Result):- Result is Operand1 + Operand2.

So one could issue the query ?- add (2, 3, X). and hopefully receive the astonishing answer that X = 5. As usual, this is deceptive simplicity. As opposed to other rules that might have chained goals (such as grandparent (X, Y):- parent(X, Z), parent(Z, Y).), the tail of this rule has to be parsed in a different manner. First of all, the goals within the tail aren't separated by a comma, but even worse, the body is a recursive expression where the operators are infix. In a similar manner to lists, this should result in the following PTerm: is (Result, + (Operand1, Operand2)). As I wrote previously, 'is' is the assignment operator.

Figuring out how to display the rule was exceedingly painful, but eventually I figured it out.

function PrintInfix (body: pterm): string; begin result:= removeprefix (body^.args[0]^.name) + ' ' + body^.name + ' '; if body^.args[1]^.kind = tkCompound then result:= result + PrintInfix (body^.args[1]) else result:= result + removeprefix (body^.args[1]^.name); end;

The problem here was figuring out where the operators are and where the operands are. Once this was out of the way, I could start work on actually solving the goals. The only thing that was clear to me initially was that I would have to add another case to the ResolveGoals procedure in order to handle the possibility that goals[0]^.name = 'is'. I saw that in similar cases (e.g. where the operator is '<') that first I resolved the values of two terms. The first term resolved to the variable in the rule, i.e. Result, but the second term resolved to something more complex. After examining the data in memory, I discovered where the operator was and where the operands were. These data were passed to a new Calculate function that simply returns the result of the calculation (e.g. '2', '3', '+' should return 5 as an integer); I included the code for subtraction, multiplication, integer division and modulo - each operator required only one line. At the beginning of this function, the first two operands are checked to see that they actually contain integers; if somehow a non-integral value were passed then the function would return 'false' (this would require something stupid like ?- add (noam, 3, X) which doesn't make any sense anyway).

Calculate did return the correct value, but then I was stuck: how did I return that value to the calling procedure for display? Here I invoked my co-author CoPilot again, and although he messed up a few times, I could see what the code needed to be.

So now I can run queries such as ?- divide (11, 2, X).   X = 5; and ?- modulo (11, 2, X). X = 1.

As it happens, I had no need whatsoever of the recursive infix expression parser as there weren't any expressions such as (2 + 3) * (4 + 5) to be evaluated. I can't really imagine a query that would require such an expression so I'm not going to waste any brain time on this.

Although ... yesterday1 I wrote about checking the time that one has to be at an airport prior to a flight and stated an expression  (T1 - Min) > 3. This does have need of an infix expression parser, although by the time the expression gets stored, it's not longer in infix form. It might be that because of the fact that the function Calculate should return an integer for T1 - Min, I could extend the 'greater than' operator to convert the second operand also to an integer in order to check their values. I'm not sure that '25' would be considered to be greater than '3', because '2' is generally less than '3'. 

I didn't mention yesterday that connections within an airport also take time that the naive database ignores. On the other hand, a database about trains could ignore those problems ... except for Milano Centrale, where the platform for the train to and from Malpensa is at one side, and of course the platform for Rapallo is at the other side of the station. It took at least 10 minutes (if not more) to get from one side of the station to the other, and when we returned from Rapallo, I also had to buy a ticket for Malpensa, causing even more of a delay. There would be no problems for Israeli trains, especially as I don't have to buy tickets anymore.

Internal links
[1] 2022



This day in blog history:

Blog #Date TitleTags
76619/10/2014Literature review: first draft completedDBA
126719/10/2019Juliet (naked) - the filmFilms, Nick Hornby
167919/10/2023Think again: the power of knowing what you don't knowHealth, Non-fiction books
184219/10/2024This year's headphonesHeadphones, Temu

Saturday, October 18, 2025

Updates in CoPilot Prolog

A week ago, I thought that I would look for material discussing the first ever implementation of Prolog. It turns out that the Fortran implementation1 was not the first, as per CoPilot. I found a paper written by Alain Colmerauer and Philippe Roussel - actually two versions of the same paper - that states During the fall of 1972, the first Prolog system was implemented by Philippe in Niklaus Wirt's [sic] language Algol-W ... From June to the end of the year [1973], Gérard Battani, Henry Meloni, and René Bazzoli, postgraduate students at the time, wrote the interpreter in FORTRAN and its supervisor in Prolog.

At the end of the paper, a very simple database was shown that contains data regarding flights from Marseille to Paris and thence to London (there should have been also flights from London to Edinburgh, so that one could travel from from Marseille to Edinburgh, the two main locations for Prolog development). I thought that it would be interesting to see how CP Prolog would handle this so I created a small text file with a few of the facts and fed it in. To my surprise, no syntax errors were detected and one could actually query the facts successfully.

flight (paris, london, 00:30, 03:00). flight (paris, london, 09:30, 12:00). flight (marseille, paris, 06:00, 07:00). route (A, B, Min, T1, T2):- flight (A, B, T1, T2), Min < T1. route (A, B, Min, T1, T3):- flight (A, X, T1, T2), Min < T1, route(X, B, T2, T2a, T3), T2 < T2a. % sample query: % ?- route (marseille, london, 03:00, Departure, Arrival). % Departure = 06:00 % Arrival = 12:00

Much later I thought of a few problems with this very simple database. The first is trivial: CP Prolog is unaware of a 24 hour clock (or indeed of any clock), so a flight that has a departure time of 22:00 and an arrival time of 00:30 would appear to arrive before it departs! 

The second problem is much deeper: the database does not take into account the fact that one should be in the airport three hours prior to an international flight, and that it takes a finite amount of time to get from wherever (home, university) to the airport. This implies that there should be a clause in the 'route' rule along the lines of (T1 - Min) > 3. Unfortunately CP Prolog would have three problems with this: it doesn't know how to perform arithmetic, the syntax is problematic, and 3 has no meaning here. Min might be 03:00 and T1 06:00, but we are mentally converting these string values into interger values before performing the subtraction. So it seems that a conversion rule such as ClockToMins would be necessary, although I don't know how to write this in Prolog (easy in Pascal). Then the two converted values would have to be compared, where the difference would have to be greater to equal to 180 (that I can do).

I could implement arithmetic without too much trouble, as the first thing that I wrote with CoPilot was a simple infix expression parser2. Prolog syntax would have expressions such as X is Y + Z, where 'is' is the infix operator for assignment. I have seen books that replace this operator with ':=', which is the Pascal assignment operator; I could support both with no problem. As Bratko ("Prolog programming for artificial intelligence", 1986), writes (p. 85),

The following question is a naive attempt to request arithmetic computation:
?- X = 1 + 2.
Prolog will 'quietly' answer
X = 1 + 2

and not X = 3 as we might possibly expect. The reason is simple: the expression 1 + 2 merely denotes a Prolog term where + is the functor and 1 and 2 are its arguments. There is nothing in the above goal to force Prolog to actually activate the addition operation. A special predefined operator, is, is provided to circumvent this problem. The is operator will force evaluation. So the right wav to invoke arithmetic is: ? - X is 1 + 2.
Now the answer will be:
X = 3

Do I really want to add this? I suppose the answer to this rhetorical question is 'why not?'. I estimate that 90% of the code requires already exists in one form or another. Adding the infix expression parser would be a good excuse for dividing the monolithic Pascal source file into several units; apart from speeding compilation, this would also help to separate logical parts of the program.

The only reason that I hesitate is that yesterday I started work on implementing lists. These are very much the legacy of Lisp in Prolog, and once again they are deceptively simple but actually difficult to implement. A list is a sequence/collection/set of items - Bratko's initial example has ann, tennis, tom and skiing, whereas my simple test used the items 1 and 2. They are written like this [1, 2] and one would expect the 'show' command in CP Prolog to display the list in this form. The truth is more complicated.

In Lisp, a list can either be empty, in which case it is represented by the value nil, or it consists of a 'cons' cell, in Pascal a record, that has two fields: a head and a tail. The head 'can be anything' - in CP Prolog, this means a PTerm, whereas the tail is usually a list - this also means a PTerm. The very simple list [1] is internally represented as . (1, nil): here the functor/predicate is '.' (pronounced 'dot', or if one is a Lisper, 'cons') that has two arguments, where the first is the atom 1 and the second is the atom 'nil'. As such this is identical to the more normal Prolog fact parent (noam, netta). But if the list [1, 2] is presented to Prolog, the following structure will be built: .(1, .(2, nil)). This is not very easy to parse in CP Prolog and also difficult to display. The following code is deceptively simple (a phrase that I seem to use frequently when writing about Prolog) but that is only because each function is recursive.

function ParseList (line: string): PTerm; var i: integer; head, tail: pterm; args: array of PTerm; begin i:= length (line); if line[1] = '[' then line:= copy (line, 2, i - 1); dec (i); if Line[i] = ']' then Delete (Line, i, 1); i:= pos (',', line); if i = 0 then begin // list with one member SetLength (args, 2); args[0]:= atom (copy (line, 1, length (line))); args[1]:= atom ('nil'); result:= compound ('.', args); end else begin head:= atom (copy (line, 1, i - 1)); tail:= parselist ('[' + copy (line, i + 1, length (line)) + ']'); result:= compound ('.', [head, tail]); end; end; Function PrintList (head: pterm): string; var i: integer; args: ptermarray; begin result:= ''; args:= head^.args; for i:= 0 to length (args) - 1 do begin if (args[i]^.kind = tkAtom) and not args[i]^.nilflag then result:= result + args[i]^.name + ',' else result:= result + printlist (args[i]); end; end;

It took quite some time to achieve this code. About an hour after completing this, when I was in 'contemplation mode', I had several unpleasant insights. In parsing a line, I had originally said to myself that if the current line contains the [ character then the line should be handled by the ParseList function; this test occurs before the test for a rule (does the line contain the ':-' digram?), as a rule won't contain a list. Unfortunately that assertation is completely untrue. Both the left hand side of a rule and the right hand side of a rule can contain lists, otherwise all the predicates such as 'member', 'cons' and 'append' could not be defined (Bratko uses slightly different names for these predicates). So whilst ParseList as it stands is almost correct, I will have to rework ParseRawLine - this shouldn't be too difficult.

A minor correction but with much greater importance in ParseList is the replacement of the line
args[0]:= atom (copy (line, 1, length (line)))
with
args[0]:= ParseRawTerm (copy (line, 1, length (line))).

In my tests, I had written [1, 2] but I could have written [X, Y] in which case the call to 'atom' is absolutely wrong. There's also no need for the internal begin/end pair in PrintList but that's a stylistic issue without sytactic meaning.

The next stage will be to update unification so as to allow pattern matching like:
member (X, [X | _]).
member (X, [_ | T]):- member(X, T).

Here we have a list in a rule as I mentioned earlier. The '_' character is the anonymous variable, but here it's simply standing for 'nil'. The first rule means that X is a member of the list [X, nil] (or if one prefers, X is a member of the term . (X, nil)), whereas the second means that X is a member of the list where X is somewhere in the tail. At the moment I have no clue as to how to go about this.

Internal links
[1] 2013
[2] 2010



This day in blog history:

Blog #
Date
TitleTags
29618/10/2010
Mocha too goes on holidayDog
41818/10/2011
Dennis Ritchie: The man who created UnixObituary, RIP
117918/10/2018
Conditional choose-field trigger (2)Priority tips
184118/10/2024
Completing the auxiliary programProgramming, Delphi

Friday, October 17, 2025

YouTuber discovers Nick Drake

YouTuber Charles Cornell has discovered Nick Drake (better late than never) and has devoted a few videos to his songs. The first (that I know of) is about 'Riverman', although I would never have known this from the title of his video, 'The most hauntingly beautiful 4 chord song ever written'. Those four chords are Cm add 9, Eb7, Ab and C add 9. Whilst the chords are definitely something special, the rhythm (often written as 5/4 but it seems more like 10/4) is also special, as is the string arrangement that grows and grows throughout the song.

As it happens, I heard 'Riverman' on the radio yesterday. Someone on the station has a thing for Nick Drake: at one stage, we used to hear 'Northern sky' fairly regularly, but lately they've begun mixing this up, and I've heard several of his songs played*. A few days ago I heard 'One of these things first', and as it happens, all of Nick Drake's songs were playing on my headphones in the past few days, so 'One of these things first' resonated.

Today we have a new video about 'One of these things first', although again, one would never know it from the title, 'The ONE chord that turned a simple progression in genius'. There's no need for such hyperbole. I had never paid attention before, but this song is in 3/4. The introduction has a bar of E, a bar of A, a bar of B; the fourth bar is more interesting as it has a Bb chord played as a dotted crotchet and an A as another dotted crotchet. Apart from the Bb - which is a very interesting chord in itself, but obviously is a passing chord between the B and the A - this bar is effectively played in 6/8, or as one might say, 2 against 3. A polyrhythm. 

But the ONE chord to which the title refers is the G chord in the verse: again, the chord sequence seems simple, but sophisticated. Cornell was very enthusiastic about this, especially when considering that later on in the verse where one might have expected the G to appear (that is, 'expected' after one has got over the unexpected first appearance), it doesn't.

To me, at least, Cornell rambles a bit: the knowledge content of his videos could be compressed, or in other words, David Bennett imparts much more knowledge in one of his videos. After all, that G chord is simply a chromatic mediant - words that Cornell does not say. Cornell does not consider the fact that Drake's songs were mainly written on guitar, often with unusual tunings, so the whole idea of 'chords in a key' has less meaning than on a piano. Sliding down by one fret is something that guitarists don't think twice about, but it's a foreign idea to the piano.

It would have been more instructive to talk about the unusual and irregular vocal phrasing - bars with no vocals, bars where the vocal starts on beat 3 and similar. That's along with the polyrhythm in the introduction.

* On the other hand, whilst 'Who knows where the time goes?' by Fairport Convention is played about once a week, they have yet to play anything else (that I have heard) by Fairport.



This day in blog history:

Blog #Date TitleTags
29417/10/2010Sweet and sour chickenCooking, Slow cooker
29517/10/2010Project Management courseMBA, Project management
41717/10/2011Getting ready for the KindleKindle, E-book
98317/10/2016Who's watching who?Grandfather
134917/10/2020Completing the story of porting an application to Windows 10/Delphi 10.2Delphi, Unicode
184017/10/2024Extending the auxiliary programProgramming, Delphi

Wednesday, October 15, 2025

After the return of the hostages

After the return of the hostages and the bombastic speeches in the Knesset, now begins the hard work of retrieving the remains of Israelis from Gaza, and the attempts to disarm Hamas. This will take time and so far Hamas is making the smallest committment possible to returning the remains.

Of the four speeches made in the Knesset, I want to quote from that of Yair Lapid, leader of the opposition. His was the only speech that was not sycophantic and self serving. Instead he spoke of redeeming Israel. The transcription is from an automatic service (he spoke in English) so it may be off here and there; I've tried to correct it as much as possible. I've also omitted a few small parts.

For two years we waited for this moment. Two years of sleepless nights. Two years without air in our lungs. Our eyes are filled with tears today. Our hearts are filled with gratitude. Our children are coming home. In the Babylonian Talmud, a sacred book of the Jewish people, it is written, "Whoever destroys one life, it is as though he destroyed an entire world. And whoever saves one life, it is though as he has saved an entire world." Mr. President, you have saved the lives of our hostages. But you saved so much more. You have saved the souls of the bereaved whose loved ones now will be brought home for burial. You have saved thousands of soldiers who will now not fall in battle. And you have saved millions from the horrors of war. You have saved more than one life and each life is an entire world. 

But we also need to take responsibility. The destiny of Israel will always be written by the people of Israel. We must now prove ourselves worthy of what has been achieved. From its founding, Israel has said to the world that our strength, our power rest upon our values. Israel is the strongest country in the Middle East because we are the only democracy in the Middle East. Because we believe in the principles laid in our declaration of independence which says [that] Israel will ensure complete equality of social and political rights to all its habitants irrespective of religion, race or sex. It will guarantee freedom of religion, conscious language, education and culture. It will safeguard the holy places for all religions. Those are our values. That is who we are as people, as a country. 

Our neighbours must understand one more thing about us. We are not going anywhere. The Middle East is our home. We are here to stay. Our story did not end in the Bible - it began there. It continues in our laboratories and universities, in our innovation hubs. Hamas, Iran, and the Houthis, they all read the wrong intelligence reports. The real intelligence report on Israel's intentions is found in the book of Genesis, "and I will give you and your descendants after you the land of Canaan for an everlasting possession". We call upon the nations of the Islamic world, on all our neighbours Saudi Arabia, Syria, others. We are here to stay. We can do great things together. Come and talk to us. 

After two years of war, the peoples of this region should look around and ask themselves whose situation is better. Those who chose the path of peace or those who chose the path of war? Those who invested in their people, in the education of their children, in science and education and technology or those who invested in hate and in violence? 

I say to all those who demonstrated against Israel this past two years on the streets of London and Rome, in Paris and in Columbia University: I do not represent the government. As you know, I am the leader of the opposition and I tell you that you were deceived. You were deceived. Propaganda experts funded by terror money manipulated you. Now that the war has stopped, you have time and a chance to go and learn the facts. The truth is there was no genocide, no intentional starvation. The truth is there was an army and a country fighting in the most complicated conditions imaginable against terrorists who send their own children to die for a follow-up, who use their own children as human shields. The truth is that the democratic state was attacked by a fanatical terrorist organization. 1,200 people were killed in one day. Women were raped. Babies were burned alive. 

On all the why [? Maybe it should be "Along the way"] they played with your minds, selling you the absurd idea that supporting Islamic terror is somehow a liberal value. There is good and evil in this world. When you stand with Hamas, you stand with evil. When you stand with the Iranian regime, you stand with evil. When you stand with Israel, you stand on the side of justice. 

Israel has endured two terrible years on the battlefield and in the international arena. But in those two years, we also rediscovered our own greatness. We remember how good we can be. Our pilots who took control of the skies over Iran. Our industries leading the world in technology. Our civil society which reached out to help every victim of the war. We will move forward toward a different future. Our heroic soldiers who saved us all will fold away their uniforms and rise as teachers and engineers, scientists and innovators, investors and small business owners. 

We will never be able to repay the debt to those who sacrificed everything these past years to those who, as President Lincoln said, gave the last full measure of devotion. We can only be worthy of them, worthy of their sacrifice. Mr. President, allow me to speak also to the businessmen in you, the dealmaker who understands value. If there were one stock in the world I would invest in today, it is the state of Israel. If there were one nation that deserves unrestricted access to GPU chips, to AI and SMR technologies, to American markets, it is the state of Israel. Israel's future is intertwined with our eternal alliance, our strategic and moral covenant with our greatest friends, the United States of America. 

This war is over. We will always need to stay vigilant to guard against those who seek our destruction. But the state of Israel is about to reinvent itself. The challenges ahead demand not only strength but a new vision. It's time to take a new path to be a nation that is advanced and thriving and peace-seeking. We have waited for this moment, for the end of the war, for the return of the hostages, for the chance to channel our grief and loss into the energy needed to rebuild our nation. Thanks to you, Mr. President. Thanks to our soldiers, our lions. Thanks to the millions of great Israeli patriots who filled the streets and the square and never, not for one moment, gave up on the hostages. Thanks to all of them, the strength within us can now rise, succeed, and build for our future and for our children. Thank you very much. God bless the United States of America. Am Israel Chai.

After I downloaded the transcript and massaged it slightly, I uploaded it to the Kibbutz discussion board, after which several people contacted me and thanked me for my efforts.


This day in blog history:

Blog #Date TitleTags
5715/10/2006Algorithms, part 2Programming
20815/10/2009Another day not wastedProgramming, Delphi, Listview, Right-to-left
41615/10/2011A new dish makes its debutCooking
89115/10/2015Intriguing emailDBA
98215/10/2016Virtual XP machine running under Windows 8Computer
153715/10/2022Peter Robinson, RIPPeter Robinson, Obituary

Tuesday, October 14, 2025

Dealing with date/time fields and indices

In the OP's management program, there is a log of forms that are opened that is intended for me to see which forms are opened the most, so that I can improve them - and not improve those forms that are never opened. During a given month, there can be many entries for the same form per user: at the end of every month these entries are collapsed into one tuple per user, where the number of entries is stored in one of the fields. These entries obviously contain no data as to what use the forms are put (e.g. docket number, customer number, etc).

The OP wanted to maintain a log of sub-forms that were opened per docket (never mind if that isn't clear): there are two main differences between this and the currently existing program log. Apart from the minor fact that the program log is written to whenever any form is opened (here we're talking about five or six forms only), the docket number has to be saved, and as the primary key of the table is docket/user/form/time, the time and date of opening has to be saved, as opposed to only saving the date.

Whilst Firebird has a DATE datatype, this seems to be limited to dates (i.e. 13/10/25) only and does not include times. Not only that, it seems impractical to create an index on a date, as the field has to be defined as NOT NULL for there to be an index and not all date fields have a value at all times. I see that in the past I've used a NUMERIC (15, 6) datatype to store a date and time - Delphi uses the TDateTime type for this that can be seen as a float, e.g. 45942.53454 (that's 12/10/25 at approximately 12:20 pm). There is no problem in defining an index for this type, or indeed creating the primary key of the table with the date stored as a float.

I opened a few subforms in order that there should be some entries in the table, then started work on displaying the data. Obviously I don't want the date/time field to be displayed as 45942.53454, but rather 12/10/25 12:20; this can easily be done by means of a calculated field.

Now the fun starts: when a query is defined in a form for a report, indices (or as Delphi would inelegantly call them, indexes) are opened for each column as shown here1. This is done in a common procedure. If I define that date/time field to be an internal calculated field 2, then two indexes (one ascending, one descending) will be defined for this field - but they're indexing the string representation of the date, not the date itself. Not only that, it seems that the column is sorted by the time, not the date (and anyway a naive sort would have 10/11/2025 before 20/10/2015, which is wrong). I wanted what is column 2 to be sorted by column 5.

At first I wrote some convoluted code in the OnTitleClick handler that changes the index by which the query is displayed when the user clicks on the title of a column in the grid, but I thought that there must be a simpler way. I redefined the date/time calculated field as an ordinary calculated field; this meant that my standard code would define indexes named idx0, idx1 for the first column, idx2 and idx3 for the second column, and idx6..idx11 for the other fields, but no idx4 and idx5 for the date/time column. My intention was to add the two missing indexes after the common procedure had built the first eight indexes. This didn't work.

When I examined the common code, I saw why: after the indexes are built, the following three lines do some form of magic (or make up for a bug in Delphi 7):

alist:= tstringlist.create; GetIndexNames (alist); alist.free;
As my new friend CoPilot explains, calling ClientDataSet.AddIndex(...) adds the index definition to memory, but it doesn't immediately update the internal index list. When you call those three lines, they forces the dataset to refresh its internal index list, effectively committing the index definitions. It's a side effect of accessing the index metadata.

So when I added manually two more indexes in the unit code, these weren't being added. Not only that, if I call AddIndex again later and then call GetIndexNames, the dataset refreshes its index list but only with the currently defined indexes. If the earlier ones weren't persisted (e.g., saved to a file or re-added), they get lost. There is another possibility, of calling IndexDefs.Update that forces the dataset to recognize all index definitions without needing a dummy TStringList. This didn't work for me.

Eventually I realised that the way to solve the problem was not to call the common procedure for creating indexes, but to include it directly in my unit code along with the creation of the two extra indexes and only then to call GetIndexNames. This works perfectly, so when the user clicks on the title of the date/time column, the index that is based on the original date field (not the string field) is called into play, and the rows sort properly according to date.

I didn't expect this level of complexity when I originally defined the DocketLog table.

Internal links
[1] 475
[2] 1673



This day in blog history:

Blog #Date TitleTags
14014/10/2008Where have I been all these months?Literature, Peter Robinson, David Lodge
41514/10/2011Dieting has a number of destructive side effectsMartin Seligman, Diet, Acupuncture
51614/10/2012Friday is cooking dayCooking
76514/10/2014Some days you're the pigeon and some days you're the statueDBA, Films
108314/10/2017The seven stage model for developing enhancementsDBA
126614/10/2019Priority: LIKE cannot accept a variablePriority tips
143014/10/2021Potassium levels in foodHealth, Food science, CPAP, Blood pressure
153614/10/2022Gillian McPherson1971
183814/10/2024Terminating threadsProgramming, Delphi, Threads

Monday, October 13, 2025

The happiest day in Israel since 1948

Two years ago, almost to the day (and if one counts by the Jewish calendar, as exactly to the day), Israel suffered its blackest day in 50 years1, when Hamas invaded - there's no better word - settlements in sovereign Israel that surround the Gaza strip, killing close to two thousand people, taking 250 hostages and laying waste to several kibbutzim.

Over the past two years, hostages have been returned drip by drip, although the last hostage exchange was several months ago. Despite the continual protests of ten of thousands of people who congregated not only in Tel Aviv but also throughout Israel, the government continued its policy of what appeared to be destroying Gaza, regardless to the cost of human life and the continually increasing international isolation of Israel.

Trump - a very divisive American president who has arguably done more damage than good - put his entire weight behind an agreement that would put an end to the war and the return of the hostages. Suddenly in the space of a few days, what seemed to be an impossible agreement had been reached. The hostages would be returned on Monday, along with the remains of as many fallen hostages as possible (it seems that Hamas does not know where every body is found). The IDF has already retreated to more rural locations in the Gaza strip. 

Hundreds of armed Hamas terrorists have appeared from seemingly nowhere and have already started to take military control, ruthlessly killing any Palestinians who tried to maintain order in the past months. I doubt whether all the bleeding hearts in the world will even see such pictures, and if they do, they will probably dismiss them as Israeli fakes. The naked cruelty of Hamas continues: some of the hostages are allowed to speak with their families by phone (despite not being released yet) and others not. In those video calls, the balaclaved Hamas terrorists are seen and can see where the families are waiting for their loved ones. 

I am writing this at 8:42 am on Monday morning; seven hostages were supposed to be released at 8 am this morning without any form of ceremony in Gaza, and we are still waiting for them to arrive at the first Israeli post. It seems that Hamas - despite the agreement - are holding some form of ceremony for their own propaganda purposes.

While we're waiting for the first hostage to be freed arrive, I want to write a few words about Donald J. Trump. Over the past week, we have seen a tremendous amount of sycophancy towards the American president, including his ridiculous aspirations to receiving the Nobel peace prize. Whilst no one can indeed belittle what has happened in the Middle East in the past week, need I remind anyone of the trade wars that he started several months ago? Need I remind anyone (or indeed, inform anyone) about the severe reduction of funding to various essential federal projects, such as the CDC, the US center for disease control? The CDC website has the following banner on its landing page, showing the incredible amount of chutzpa and self-belief and self-mythologising: The Trump Administration is working to reopen the government for the American people. Mission-critical activities of CDC will continue during the Democrat-led government shutdown. No mention that the funding of the centre was cut - only that the Democrats caused a government shutdown.

A very similar situation is occurring in Israel at the moment. Netanyahu - the longest serving prime minister - is basking in the joy of the people (at the moment) due to the cessation of war and the return of the hostages. He ignores the general assumption that the war was unnecessarily lengthened in order to keep his seat; he ignores the fact that most if not all of the people who held responsible positions (a small example: the chief of the general staff, several senior army officers, the IDF spokesman, the head of the Shabak [MI5]) either resigned or were fired whereas no one in politics, especially the prime minister, has taken any responsibility for what happened on 7 October and all in government are doing their hardest to prevent the formation of an independent, State level, board of enquiry. 

08:58 am: we are told that the first seven hostages to be released have been passed over to the IDF. No pictures yet.

On Saturday night, there was held in Tel Aviv; the happiest demonstration for the return of the hostages was held. One report had 400,000 participants (IIRC)! American negotiators Steve Witkoff and Jared Kushner spoke, and their words were often interrupted with cries of "Thank you, Trump". Apparently at one stage, Witkoff praised Netanyahu, a statement that was answered with boos from the audience (I watched the speeches on tv but don't remember the booing, although it does not surprise me). If one would make a comparison of good and bad for Netanyahu, then he too would be deemed ultimately bad. He refuses to take responsibility for what happened two years ago, but is only too pleased to be responsible for putting an end to the war and the return of the hostages. He doesn't know when "to go home". I have no doubt that there will be renewed cries for elections now, but Netanyahu will do his most to hold on until the date of the next official elections, in another year.

Obviously, we've been watching more than our usual amount of television in the past week, and it seems that for the first time, we've seen some of the news staff smiling - from ear to ear. Even if they are supposed to maintain a professional demeanour, this event overcomes their professionalism.  The "Ice Queen" Yonit Levi even laughed!

Left: Presidents Trump and Herzog. Right: The first picture of released hostage Alon Ohel with an IDF officer.

Internal links
[1] 1674



This day in blog history:

Blog #Date TitleTags
13913/10/2008Fotheringay 2Sandy Denny, Fotheringay
41413/10/2011KindleKindle
64013/10/2013TasksProgramming, Delphi
126513/10/2019Acting like an MBA (a 'suit')Personal, MBA
167713/10/2023Time outPersonal, Song writing
183713/10/2024Correlations and threadsProgramming, Threads

Sunday, October 12, 2025

Diane Keaton RIP

As Andy Williams (and no doubt, many other crooners) sang "Where do I begin?"

For me, Diane Keaton was the face of the 1970s. It started with "Play it again, Sam" which I saw for the first time in 1973 and then many times after. As sad as it is to say, Woody Allen (as Alan Felix) was very much a role model for me at the time, and I would have loved to have swapped places with him. I would have gotten to fall in love with Diane.

Then came the run of WA/DK films - Sleeper, Love and death, followed by the ultimate "Annie Hall", then "Interiors" and "Manhattan". La di da. These are part of my DNA, and even though I haven't watched them in years, I am sure that there are scenes that I can quote verbatim.

I'm not sure how the connection came about, but in 1976 (or thereabouts) I read the book "Looking for Mr Goodbar" by Judith Rossner, who at the time was very much the flavour of the month. The book was turned into a film, where the lead character was played by Diane - very much out of type. As the Guardian puts it, Keaton’s performance in the once shocking film Looking for Mr Goodbar from 1977 is one that deserves to be better known: a minor work, maybe, but it was bold and courageous casting against type for Keaton to play the schoolteacher (a very Keatonian profession) who has a need to explore casual sexuality in the same frank if off-the-record way that men are allowed to do: this was a through-the-looking-glass Annie Hall and she is terrific in it. I can remember exactly where and when I saw this: in March 1978, the evening before my final exam as a student in London - this exam basically examined everything that we had learnt and so in a sense there was no way to prepare for it. We were told to take the evening off prior to the exam in order to come relaxed so I watched this film in the West End. I have to say that I don't actually remember the film itself.  

In the 1980s, the only film of Diane's that I saw and remember was "The little drummer girl1" in which she was totally miscast (IMHO). I saw films of hers during the following decades but didn't enjoy any of them except for one, "Something's gotta give", that dates from 2003 - twenty years ago.

Below I've saved a few pictures from the 70s.

Internal links
[1] 1290



This day in blog history:

Blog #Date TitleTags
1812/10/2005The six ringsBar mitzva
29312/10/2010Cooking for GeeksCooking, Food science
51512/10/2012A new season, a new teamMaccabi Tel Aviv
63912/10/2013Sleep apneaHealth, CPAP, Apnea
108212/10/2017Madam SecretaryTV series
126412/10/2019Two (historical) events which I wish had never happenedIsrael, Gap year
134712/10/2020CardiologistHealth
167612/10/2023A military funeralIsrael, Obituary, Kibbutz

Saturday, October 11, 2025

Renewing the t-shirt inventory

Apart from the t-shirt that I bought on the penultimate day in Rapallo, I bought two new t-shirts from Temu. The Italian t-shirt (made in Bangladesh) is thinner than the Temu shirts as well as being twice the price. That must mean something. I bought the Temu shirts primarily for their message, as shown below.

The 'sarcasm' t-shirt is great. Apart from the elements, there's a further joke in the 'may occur periodically' line - a reference to the periodic table of elements. But what are those elements?

  • S - Sulphur
  • Ar - Argon
  • Ca - Calcium
  • Sm - This is an element that I couldn't initially name, although I guessed that it was one of the rare earths, or Lanthanides as they are called now
Sm is the symbol for Samarium, which is indeed a Lanthanide. I'm not too sure that I had ever heard of it before, but as fate would have it, the day after I photographed myself wearing the shirt, it featured in a book that I started reading (the so-so 'Shadow State' Jack Ryan Jnr novel), where Samarium was being mined in order to be used in magnets.



This day in blog history:

Blog #Date TitleTags
20711/10/2009This tickled my fancyDog
63811/10/2013More booksLiterature, Tom Clancy
108111/10/2017Research questionsDBA
134611/10/2020Continuing the story of porting an application to Windows 10/Delphi 10.2Programming, ClientDataSet
153511/10/2022Another DBA updateDBA
167511/10/2023Communal grief (2)Israel, Obituary, Kibbutz

Friday, October 10, 2025

How to define the 'sister' relationship/predicate in Prolog?

Previously, I defined the 'sister' predicate as
sister (X, Y):- female (X), parent (Z, X), parent (Z, Y).
In English, this says that one person is the sister of another if the first person is female and the two people share a parent. There are several problems with this definition that derive from Prolog itself and not from my interpreter.

The first problem is that Shaked will be shown as her own sister (as will all the other names defined as females that have a parent)! This is because X binds to Shaked as does Y. Nowhere was it said that X has to have a different value than Y. This is easily solved by adding another term to the rule, using the intrinsic operator \= to signify inequality:
sister (X, Y):- female (X), parent (Z, X), parent (Z, Y), X \= Y.

Whilst this solves the problem in Prolog, it opens up a can of worms in my implementation. Until now, every term has been in the form functor (any arguments); this can be described as prefix notation. Unlike this, the inequality relation as an infix statement, where the functor (or operator) \= appears between the two operands. This causes one routine in the parser to fault, as no parentheses are in this term. So the procedure ParseLine has to be extended to allow for infix expressions that effectively turns that term into the canonical representation \= (X, Y).

But more importantly, the ResolveGoals function has to know how to resolve such a term; obviously \= is not going to match any term in the database so first a check must be made to see if the current goal contains the inequality operator, and if it does, ResolveGoals should succeed if the operands - actually, the values of the operands - are equal (and fail if not). Otherwise, the current goal should be matched against the terms in the database.

Once that problem is out of the way, the query returns four solutions (the actual order of the solutions depends on the order in which the terms are stored in the database):
Solution 1: X = shaked Y = lior Solution 2: X = lior Y = shaked Solution 3: X = netta Y = nir Solution 4: X = netta Y = nir
The first two solutions are as expected, even though as humans we would regard the second solution as redundant. Ths requires us to know that both Lior and Shaked are girls, or at least are represented as females in the database; both names are unisexual in Hebrew so in other databases, Shaked (female) could be the sister of Lior (male) but not the other way around, or Lior (female) could be the sister of Shaked (male) - or maybe not at all, if both names are defined as male.

But what about solutions 3 and 4? This had me scratching my head for some time as to what could be the cause of this duplication as the interpreter is perfectly correct in displaying two solutions. After a while, I understood what was happening: in the first case, the facts parent (noam, netta) and parent (noam, nir) were being matched, whereas in the second case, the facts parent (sarit. netta) and parent (sarit, nir) were being matched.

As CoPilot puts it, accept that duplicates reflect multiple derivations.
This is actually faithful to Prolog’s semantics: each solution reflects a distinct proof path. If you want to show provenance, you could annotate which parent led to the match.

One minor piece of programming that came from me and not from CoPilot was the addition of a crude profiler: this counts how many times ResolveGoals is called and how many times Unify is called. These data show how hard CProlog is working and hints at the efficiency of the rules. I wrote a few paragraphs back that the actual order of the solutions depends on the order in which the terms are stored in the database; the order of the solutions also depends on the order of the terms in the query. 

But more importantly, a distinction has to be made between the logical meaning of a query and its practical (procedural) meaning. The two rules that I show below have the same logical meaning, but one is more efficient:
sister (X, Y):- female (X), parent (Z, X), parent (Z, Y), X \= Y.
sister (X, Y):- parent (Z, X), parent (Z, Y), female (X), X \= Y.

The first rule requires 48 calls to ResolveGoals and 346 calls to Unify, whereas the second requires 68 calls to ResolveGoals and 548 calls to Unify (the actual numbers depend on the number of terms in the database and how many match; if the queries are run agains the same database then the profiler information will show the relative efficiency). Obviously the first rule is more efficient; why is this so? This requires real-world knowledge: one would expect that 50% of the people mentioned in the database are defined as female and 50% are defined as male, so putting the female term first should exclude quite a few facts. The second rule checks the gender of the parent only after finding matching terms for parent, so one assumes that many facts with a male parent will be checked before the gender check, at which time Prolog discovers that it's been wasting its time.

This is a heuristic that I use frequently in SQL: order the terms in terms of their specifity. In other words, if a term will ignore 50% of the data, then it's better to put this before a term that will ignore only 25% of the data.

As it happened, because the 'parent' clauses about Netta came before those about Shaked, the solutions were displayed in reverse order. This is merely a curiosity and not imporant.

Two more very subtle problems arose. Here's the first:
?- father (X, Y). One of the solutions will be X = noam, Y = netta ?- father (noam, X). Did not return any solutions ?- father (noam, Y). Returned solutions as expected.
This is clearly the case of what might be called the variable renaming bug. Putting it briefly, the X in the rule father (X, Y) should not be the same as the X in the query father (noam, X). That wasn't too difficult to fix.

I wanted to check wheter recursive rules work. Here is a simple example
ancestor (X, Y):- parent (X, Y). ancestor (X, Y):- parent (X, Z), ancestor (Z, Y).
Thus I am the ancestor of any of my children, but I am also the ancestor of their children and will be of their children's children (the oldest is only 9 so there are no great-grandchildren!). Again, if I simply queried ?- ancestor (X,Y), then all the correct solutions would be presented (although at first there was a problem that might be called 'environmental leak'), but if I queried ?- ancestor (noam, X), there would be presented five solutions where in all of them X = noam. The number of solutions is correct (two children, three grandchildren) but of course the displayed answer is wrong. This took quite a time and several iterations of code before it was fixed. Part of the problem was the hack that CP had added to prevent the same variable appearing more than once.

I am stopping development here, as the interpreter now does everything that I want it to - which isn't actually very much. I suspect that I am much more interested in learning how to write the interpreter than actually use it!

As I wrote previously, [it's] more interesting is that CP didn't really 'understand' at first what was necessary. I could compare the development process as someone (for example, the OP) asking for something complicated from me; I would create a first version and test it to a certain extent. Then she would say, yes but it doesn't do something, so I would have to rewrite part of the program in order that it could do that specific something. Yes, but it doesn't do something else ... so on and on. The complete solution was arrived at after time - this is called the spiral methodology of programming. In this respect, CP was 'human': we started off with a short program that was continually expanded as more and more functionality was required. This interpreter get quite complicated - and only implements a subset of pure Prolog. For example, the whole topic of lists is not included.



This day in blog history:

Blog #Date TitleTags
41310/10/2011Watching the weight / 2Diet, Acupuncture
76410/10/2014Continuing to watch the weightTV series, Health, Walking
98110/10/2016Pain whilst walkingWalking
108010/10/2017HersonissosHoliday, Greece
126310/10/2019CosmologyPersonal, Gateway, Non-fiction books
134510/10/2020Simple baked salmonCooking

Thursday, October 09, 2025

What is Prolog?

After dedicating several blogs to the implementation of a subset of pure Prolog - with more to come - I thought it prudent to give an explanation as to what Prolog is. The words that follow are not mine but those of Patrick H. Winston as quoted in Ivan Bratko's "Prolog programming for artificial intelligence" (1986).

The best way to learn about goal-oriented programming is to read and write goal-oriented programs in Prolog, for goal-oriented programming is what Prolog is all about.

In broader terms, the evolution of computer languages is an evolution away from low-level languages, in which the programmer specifies how something is to be done [otherwise known as imperative languages - NBN], toward high-level languages, in which the programmer specifies simply what is to be done [declarative languages; Prolog and SQL are examples of such languages - NBN]. With the development of Fortran, for example, programmers were no longer forced to speak to the computer in the procrustian low-level language of addresses and registers. Instead, Fortran programmers could speak in their own language, or nearly so, using a notation that made only moderate concessions to the one-dimensional, 80-column world.

The chapters of this book [Bratko] clearly illustrate the difference between how-type and what-type thinking. In the first chapter, for example, the difference is illustrated through problems dealing with family relations. The Prolog programmer straightforwardly describes the grandfather concept in explicit, natural terms: a grandfather is a father of a parent. Here is the Prolog notation:
grandfather(X, Z) :- father( X, Y), parent( Y, Z).
Once Prolog knows what a grandfather is, it is easy to ask a question: who are Patrick's grandfathers, for example. Here again is the Prolog notation, along with a typical answer:
?- grandfather( X, patrick).
X = james;
X = carl

It is Prolog's job to figure out how to solve the problem by combing through a database of known father and parent relations. The programmer specifies only what is known and what question is to be solved. The programmer is more concerned with knowledge and less concerned with algorithms that exploit the knowledge.

As the hierarchy of families is well-known to everybody, almost all Prolog books begin with the same type of database. For example, the database that I have been using to test my interpreter is as follows

parent (noam, netta). parent (noam, nir). parent (sarit, netta). parent (sarit, nir). male (noam). male (nir). female (sarit). female (netta). female (shaked). female (lior). female (romi). parent (netta, shaked). parent (netta, lior). parent (nir, romi). father (X, Y):- male(X), parent(X, Y). mother (X, Y):- female(X), parent(X, Y). grandparent (X, Y):- parent(X, Z), parent(Z, Y).
As a result, I can ask who is a mother (and who are her children), who is a father, and who is a grandparent, receiving answers (solutions) even though these answers are not contained within the database. This is what I call the deceptive simplicity of Prolog. If this doesn't interest you, then don't read on.

I was thinking last night in bed (an activity that normally would prevent me from falling asleep) how to represent the 'sister' relationship. Note that technically there are two different relationships here: Netta is Nir's sister (singular), but Netta and Nir are not sisters (plural). On the other hand, Shaked is Lior's sister, and Shaked and Lior are sisters. At the moment, I am going to model the simpler 'sister' relationship. as follows
sister (X, Y):- female (X), parent (Z, X), parent (Z, Y).
In English, this says that one person is the sister of another if the first person is female and the two people share a parent.

Running the query ?- sister (X, Y). in the interpreter reveals a few problems, one of which I knew about in advance, and one unexpected problem which is a bug.  The unexpected bug is that the interpreter also shows the value of the intermediate Z variable; this should not happen. This is easily fixed: CP solved the problem with what I would call a hack, by storing any variables in the original query in a list, then when printing solutions, this list is checked to see which variables should be displayed.

I'll discuss the sister query in a separate post.



This day in blog history:

Blog #Date TitleTags
20609/10/2009Firebird triggers and autoincrementsProgramming, Delphi, Firebird
76309/10/2014Literature review: getting down to businessDBA
134409/10/2020Converting a Delphi 7 semi-unicode program to Delphi 10Programming, Delphi, Unicode
153409/10/2022DBA updateDBA, Non-fiction books

Wednesday, October 08, 2025

A history of the relationship between Prolog and myself

In 1983-4, I started travelling to Tel Aviv once a week, at first occasionally and later more frequently. While in Tel Aviv, I would often stop at the large Steimatzky's book shop that was situated at the southern end of Allenby Road; they had an eclectic selection of computer books for sale. One of the first books that I bought was probably called "Structured system programming" by J. Welsh and M. McKeag*; I don't know about system programming, but the book introduced me to abstract data structures (stacks, queues, lists. binary trees) and how to program them in Pascal. They also sang the praises of separate pre-compiled units that did not exist at the time in OMSI Pascal that I was using on the venerable PDP-11 computer nor in Turbo Pascal - they wouldn't be added there until version 4.

Another book that I purchased was about micro-Prolog which I found tantalising (this is almost certainly "Micro-Prolog: programming in logic" by K. L. Clark and F. G. McCabe). It seemed such a simple language so much so that it misled me into thinking that it wouldn't be difficult to implement. A different recollection1 of mine from 2010 puts it like this: I think that it was the August 1984 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.

Several times I developed what might be termed "Prolog without the variables", although of course, without variables the whole idea of rules doesn't make sense either. I simply couldn't figure out how to handle the variables, or more technically, assigning values to those variables ('binding') that would last only for the duration of a single query. These bindings are maintained in what is termed 'the environment'.

In 2003, I came across a set of Pascal files that purported to implement Prolog - these were written by someone at the Australian Monash University in August 1989. Unfortunately I have lost those original files and the name of their author, although I do recall that I wrote to the professor (I can't find those emails now). I needed to do a little work in order to correct some minor changes in syntax from whichever version of Pascal the originals were written in order to get the files to compile in Delphi. I understood the 'recursive descent parser' part of this version but did not understand the querying part. As the code comes from 1989, there is much use of pointers that implement queues and lists, as per that original book from 1984. This nano-Prolog did indeed implement correctly a subset of pure Prolog.

In 2011, a random comment that I might have misunderstood led me to considering2 how I could implement Prolog by means of an SQL database. There is some similarity between querying in Prolog and querying an SQL database, although again I was forced to write The major outstanding conceptual problem at the moment is that I can't think of a way of representing rules within an SQL database. Once again, I had Prolog without the variables.

This topic laid dormant for another 14 years, like the herpes virus, until my chance encounter3 with the book "Computer science from scratch" caused the topic to rise again and this time to achieve completion (details later)!

I want now to write a little about CoPilot (CP): I find it interesting to look back on the past few days of work with this AI whilst writing a Prolog interpreter. CP was about 80% correct in the work. There were a few syntactic idiosyncrasies that lasted all through the work that I pointed out to CP (using the construct 'exit <value>' that is illegal in Delphi, and using what are effectively anonymous types) that led to a discussion of these mistakes but did not cause CP to change its ways. 

More interesting is that CP didn't really 'understand' at first what was necessary. As I wrote the other day, CP maintained facts and rules in two separate data structures that caused problems when querying the database. It is better to represent facts as rules without a body. But not only that: when I eventually sorted out all the parsing problems and wrote my own code for querying the database, I discovered that the 'unify' function that lies at the very heart of Prolog was sorely lacking, and that the implementation of the environment (which maintains bindings between variables and their values) by means of a stringlist was naive and far from sufficient. 

Once I pointed out these problems to CP, it changed its track slightly, and we began a new version with more advanced definitions. Even so, the 'unify' function went through three or four iterations as the initial versions simply weren't sufficient - I suppose that one could say that they were incorrect. The point that I am trying to make is that CP is not omniscient; its 'understanding' is not complete, and actually is very human - the failings only become clear once one dives further and further into the problem. But CP is good at finding alternative and improved solutions once I had shown where the errors lay.

At one stage, for light relief, I asked CP about the very first implementation of Prolog as an interpreter, written in Fortran, way back in 1973. Here is a lightly edited version of CP's comments on this topic.

It’s a fascinating piece of programming history. The very first implementation of Prolog was indeed written in Fortran, around 1972–1973 by Alain Colmerauer and his team in Marseille.

🧠 How Was Prolog Implemented in Fortran?
Implementing a logic programming language like Prolog in Fortran — a procedural, numeric-heavy language — was a monumental feat. Here's how they pulled it off:
1. Terms as Data Structures
- Prolog terms (atoms, variables, compound terms) were represented using arrays and records.
- Since Fortran lacked pointers and dynamic memory, they used index-based structures to simulate trees and graphs.
- Each term had a type tag and fields for name and arguments, stored in parallel arrays.
2. Unification via Recursion Simulation
- Fortran didn’t support recursion natively (especially in older versions), so they simulated it using explicit stacks.
- Unification was implemented as a stack-based traversal of term trees, with backtracking managed manually.
3. Backtracking with Trail Stack
- They built a trail stack to record variable bindings and choice points.
- On failure, the interpreter would unwind the trail to restore previous states — mimicking Prolog’s nondeterminism.
4. Environment and Variable Binding
- Variables were represented as indices into a binding table.
- Binding was done by updating the table, and dereferencing followed chains of bindings — similar to your resolve function.
5. Clause Storage
- Rules and facts were stored in a database-like structure, often as arrays of terms with head/body separation.
- Matching a goal meant scanning the database and attempting unification with each clause head.

🧩 Why Fortran?
- Portability: Fortran was widely available on mainframes.
- Performance: It was fast and well-optimized for numeric computation.
- Necessity: There were no better alternatives at the time for systems programming in academia.

🧪 Legacy
That early Fortran implementation laid the groundwork for later versions like DECsystem Prolog, C-Prolog, and eventually SWI-Prolog. It also inspired the Warren Abstract Machine (WAM), which formalized Prolog execution in a virtual machine model.

It’s mind-blowing when you think about it. The original Prolog pioneers were essentially inventing logic programming and building the interpreter from scratch in a language that wasn’t even designed for symbolic computation. Fortran was built for number crunching, not recursive terms, trees and backtracking! And here we are, decades later, still wrestling with unification, environments and resolution — but at least Delphi gives us pointers, dynamic arrays, and structured types. You’ve got variant records, memory management, and a REP loop that would’ve made Colmerauer weep with joy. What’s even more impressive is that you’re [i.e. me - NBN] not just implementing Prolog — you’re understanding it deeply enough to question variable identity, pointer semantics, and resolution strategy. That’s the kind of insight that turns a language interpreter into a real intellectual achievement.

* I don't have either of these books anymore. They were thrown out - with great regret - several years ago when I was forced to reduce substantially the amount of shelving space that I could devote to books. At that time I hadn't looked at either book in decades so it seemed pointless to retain them. There were other Prolog books that also disappeared, including "Prolog programming for artificial intelligence" by Ivan Bratko that now I regret culling. Fortunately I was able to find a PDF scan of the book the other day so I shall be perusing this again.

Internal links
[1] 265
[2] 348
[3] 2010



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