ISO directive: initialization. This should work. :- initialization main. main :- write(‘Hello World\n’). edit sorry, I skipped over most interesting details. Here is a sample script, let’s say saved in ~/test/main.pl #!/home/carlo/bin/swipl -f -q :- initialization main. main :- current_prolog_flag(argv, Argv), format(‘Hello World, argv:~w\n’, [Argv]), halt(0). and made executable with chmod +x ~/test/main.pl then I … Read more
The key to understanding difference lists is understanding what they are on the level of the nested compound term that represents lists. Normally, we see a list like that: [a, b, c] This is now a list with three elements. The exactly same nested term using the dot as the list functor, ./2, and the … Read more
Besides asserting data as you go, you can also use an extra-logical predicate such as nb_setarg/3. Then once a parent is found, you fail back past nb_setarg and find another parent. All previously found solutions should stay in the term you did nb_setarg on, then after all results are exhausted, the nb_setarg term is the … Read more
It sometimes really makes sense to introduce green cuts — even into append/3, but care must be taken that such a cut remains a green cut. That is, a cut that does improve efficiency (on a certain level) and does not affect answers. There is a very simple rule-of-thumb for introducing green cuts: If you … Read more
Short answer: Tail recursion is desirable, but don’t over-emphasize it. Your original program is as tail recursive as you can get in Prolog. But there are more important issues: Correctness and termination. In fact, many implementations are more than willing to sacrifice tail-recursiveness for other properties they consider more important. For example steadfastness. But your … Read more
Let us first get used to a declarative reading of logic programs. Declaratively, a Prolog program states what is true. For example natural_number(0). natural_number(s(X)) :- natural_number(X). The first clause states: 0 is a natural number. The second clause states: If X is a natural number, then s(X) is a natural number. Let us now consider … Read more
Here is a pure version using dif/2 which implements sound inequality. dif/2 is offered by B-Prolog, YAP-Prolog, SICStus-Prolog and SWI-Prolog. firstdup(E, [E|L]) :- member(E, L). firstdup(E, [N|L]) :- non_member(N, L), firstdup(E, L). member(E, [E|_L]). member(E, [_X|L]) :- member(E, L). non_member(_E, []). non_member(E, [F|Fs]) :- dif(E, F), non_member(E, Fs). The advantages are that it can also … Read more
Yes, there is such a control construct in ISO Prolog, called ->. You use it like this: ( condition -> then_clause ; else_clause ) Here is an example that uses a chain of else-if-clauses: ( X < 0 -> writeln(‘X is negative. That’s weird! Failing now.’), fail ; X =:= 0 -> writeln(‘X is zero.’) … Read more
Adventure in Prolog Dennis Merritt | Springer Published in 1990, 186 pages An Introduction to Logic Programming through Prolog Michael Spivey | Prentice Hall Published in 2008, 258 pages Applications of Prolog Attila Csenki | BookBoon Published in 2009, 203 pages Artificial Intelligence through Prolog Neil C. Rowe | Prentice-Hall Published in 1988, 481 pages … Read more
TL;DR: Don’t. The cut prunes Prolog’s search tree. That is, given a pure Prolog program without cut and the same program with cuts the only difference is that the program with cuts might spend less time in fruitless branches, and thus is more efficient ; might have fewer answers ; it might also terminate whereas … Read more