• specifying some facts about objects and their relationships
• defining some rules about objects and their relationships
• asking questions about objects and their relationships

  likes(john, mary).    /* John likes Mary */
  likes(mary, john).   /* not necessarily the case */

  valuable(gold).	/* Gold is valuable */
  female(jane).	        /* Jane is a female */
  owns(jane, gold).     /* Jane owns gold */
  father(john, mary).   /* John is the father of Mary */
  gives(john, book, mary)  /* Johns gives the book to Mary */

  play(john, mary, football).
  play(jane, jim, badminton).
  king(john, france).

  likes(joe,fish).
  likes(joe,mary).
  likes(mary,book).
  likes(john,book).
  likes(john,france).
  
  human(socrates).
  human(aristotle).
  athenian(socrates).
  
 

   ?-likes(joe,money).
   no
   ?-likes(mary,joe).
   no
   ?-likes(mary,book).
   yes
 
   ?-athenian(socrates).
   yes
   ?-athenian(aristotle).
   no

   ?-likes(john,X).
   X=mary;
   X=book

   ?-likes(john,WhatdoesJohnLike).
   WhatdoesJohnLike=mary

   A CONJUNCTION: ','
   ?-likes(mary,X), like(john,X).
 

   likes(john,X) :- likes(X,book).

   likes(john,X) :- likes(X, football), 
                    likes(X, badminton).
 
   sister_of(X,Y) :-
        female(X),
	parents(X, M, F),
	parents(Y, M, F).

   may_steal(P,T) :- thief(P), likes(P,T).
   
 

 test(X,Y) :- Y is X - 10.

 cube(X,Y) :- Y is X*X*X.

 volume(SphereRadius, V) :- cube(SphereRadius, C), 
                            V is (4/3) * 3.14159 * C. 
 /*
?- test(22,Y).   
Y = 12 
Yes

?- volume(10,X).
X = 4188.79 
Yes

?- cube(10,X).
X = 1000 
Yes

*/
 

 first([Head|_], Head).   /* The underscore, '_', is an anonymous variable in Prolog */
                         /* This means that you don't care about this variable's value */

rest([_|RestofList], RestofList).

parts([Head|Tail], Head, Tail).

/*

?- first([a,c,s,d,e], X).
X = a 
Yes

?- rest([a,c,s,d,e], X).
X = [c, s, d, e] 
Yes

?- parts([a,c,s,d,e], X,Y).
X = a
Y = [c, s, d, e] 
Yes

*/
 

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

/*
member(X, [X|_]).  This means "X is a member of the list with X as the first member.

member(X, [_|RestofList) :- member(X, RestofList).
             This is checking recursively whether X is a member of the RestofList, if X was not the first element.

?- consult(test).
% test compiled 0.01 sec, 92 bytes
Yes

?- member(a,[b,e,r,a,c,g]).
Yes

?- member(a,[b,e,r,c,g]).
No
*/
 

• swiprolog starts SWI Prolog
• ?-consult(first). Loads first.pl
• or you can put single quotes around the name of the file to load, such as ?-consult('first.pl').
• halt. exits SWI Prolog.