X;Y specifies a disjunction of goals. X;Y succeeds if either X succeeds or Y succeeds. If X fails then an attempt is made to satisfy Y. If Y fails the entire disjunction fails.
Note: The behaviour of this predicate changes when its first argument is of the form ->/2, i.e. the "if/then" predicate. When a ->/2 predicate is the first argument of a ;/2 predicate then the resulting behaviour is a "if/then/else" statement of the form ((if->then);else).
Examples
?- true; true.
yes;
yes
?- true; fail.
yes;
no
?- fail; true.
yes
?- fail; fail.
no
?- true; true; true.
yes;
yes;
yes
?- true; fail; fail.
yes;
no
?- fail; true; fail.
yes;
no
?- fail; fail; true.
yes
?- true; true; fail.
yes;
yes;
no
?- true; fail; true.
yes;
yes
?- fail; true; true.
yes;
yes
?- fail; fail; fail.
no a :- true.
b :- true.
c :- true.
d :- true.
?- a;b;c.
yes;
yes;
yes
?- a;b;z.
yes;
yes;
no
?- a;y;c.
yes;
yes
?- a;y;z.
yes;
no
?- x;b;c.
yes;
yes
?- x;b;z.
yes;
no
?- x;y;c.
yes
?- x;y;z.
no p2(1) :- true.
p2(2) :- true.
p2(3) :- true.
p3(a) :- true.
p3(b) :- true.
p3(c) :- true.
p4(1, b, [a,b,c]) :- true.
p4(3, c, [1,2,3]) :- true.
p4(X, Y, [q,w,e,r,t,y]) :- true.
p1(X, Y, Z) :- p2(X); p3(Y); p4(X,Y,Z).
?- p1(X, Y, Z).
X = 1
Y = UNINSTANTIATED VARIABLE
Z = UNINSTANTIATED VARIABLE
yes;
X = 2
Y = UNINSTANTIATED VARIABLE
Z = UNINSTANTIATED VARIABLE
yes;
X = 3
Y = UNINSTANTIATED VARIABLE
Z = UNINSTANTIATED VARIABLE
yes;
X = UNINSTANTIATED VARIABLE
Y = a
Z = UNINSTANTIATED VARIABLE
yes;
X = UNINSTANTIATED VARIABLE
Y = b
Z = UNINSTANTIATED VARIABLE
yes;
X = UNINSTANTIATED VARIABLE
Y = c
Z = UNINSTANTIATED VARIABLE
yes;
X = 1
Y = b
Z = [a,b,c]
yes;
X = 3
Y = c
Z = [1,2,3]
yes;
X = UNINSTANTIATED VARIABLE
Y = UNINSTANTIATED VARIABLE
Z = [q,w,e,r,t,y]
yes
?- p2(X); p2(X); p2(X).
X = 1
yes;
X = 2
yes;
X = 3
yes;
X = 1
yes;
X = 2
yes;
X = 3
yes;
X = 1
yes;
X = 2
yes;
X = 3
yes
?- p2(X); p3(X); p2(X).
X = 1
yes;
X = 2
yes;
X = 3
yes;
X = a
yes;
X = b
yes;
X = c
yes;
X = 1
yes;
X = 2
yes;
X = 3
yes
?- X=12; X=27; X=56.
X = 12
yes;
X = 27
yes;
X = 56
yes
?- p2(X); X=12; p3(X); X=27; p2(X).
X = 1
yes;
X = 2
yes;
X = 3
yes;
X = 12
yes;
X = a
yes;
X = b
yes;
X = c
yes;
X = 27
yes;
X = 1
yes;
X = 2
yes;
X = 3
yes