/* Facts about countries and their capitals */
capital(paris, france).
capital(berlin, germany).
capital(london, uk).
capital(rome, italy).
capital(madrid, spain).

/* Rules to find the country for a given capital */
country(Capital, Country) :- capital(Capital, Country).

/* Sample Queries */
sample_queries :-
    country(paris, Country1), write('The capital of '), write(Country1), write(' is Paris.'), nl,
    country(Capital2, italy), write('Italy\'s capital is '), write(Capital2), write('.'), nl,
    capital(Capital3, germany), write('The capital of Germany is '), write(Capital3), write('.'), nl.

/* Main predicate */
main :-
    sample_queries,
    halt.

/* Initialization */
:- initialization(main).
by

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About Prolog

Prolog (Programming in Logic) is a logic programming language associated with artificial intelligence and computational linguistics. It works through facts, rules, and queries, using a form of symbolic reasoning known as backward chaining. Prolog is declarative, meaning you describe what you want instead of how to compute it.

Sample Code

The following is a simple Prolog program that prints a greeting:

:- initialization(main).

main :-
    write('Hello, World!').

Syntax Basics

Facts

Facts represent basic assertions about the world.

likes(alice, pizza).
likes(bob, pasta).

Rules

Rules define logical relationships using facts.

friends(X, Y) :- likes(X, Z), likes(Y, Z).

Queries

Queries are used to find information based on facts and rules.

?- likes(alice, What).

Operators

OperatorDescription
:-Rule definition
,Logical AND
;Logical OR
=Unification

Lists

member(X, [X|_]).
member(X, [_|T]) :- member(X, T).

Recursion

Prolog heavily relies on recursion.

factorial(0, 1).
factorial(N, F) :-
  N > 0,
  N1 is N - 1,
  factorial(N1, F1),
  F is N * F1.

This guide provides a quick reference to Prolog programming syntax and features. Start writing Prolog code using OneCompiler’s Prolog online compiler today!