clause(X,Y) causes X and Y to be matched to the head and body of an existing clause. If no clauses are found for the predicate represented by X then the goal fails. If there are more than one that matches, the clauses will be matched one at a time as the goal is re-satisfied. X must be suitably instantiated that the predicate of the clause can be determined.
retract(X) - remove clauses from the knowledge base. The first clause that X matches is removed from the knowledge base. When an attempt is made to re-satisfy the goal, the next clause that X matches is removed. X must be suitably instantiated that the predicate of the clause can be determined.
Examples
Examples of using "clause":
test(a,b) :- true.
test(1,2) :- true.
test(A,B,C) :- true.
test([_|Y],p(1)) :- true.
?- clause(test(X,Y), Z).
X = a
Y = b
Z = true
yes;
X = 1
Y = 2
Z = true
yes;
X = [_|Y]
Y = p(1)
Z = true
yes
?- clause(test(a,b,c), true).
yes
?- clause(test(1,2,3), true).
yes
?- clause(tset(1,2,3), true).
noExamples of using "retract":
?- assertz(p(a,b,c)).
yes
?- assertz(p(1,2,3)).
yes
?- assertz(p(a,c,e)).
yes
?- retract(p(x,y,z)).
no
?- retract(p(a,b,e)).
no
?- p(X,Y,Z).
X = a
Y = b
Z = c
yes;
X = 1
Y = 2
Z = 3
yes;
X = a
Y = c
Z = e
yes
?- retract(p(a,Y,Z)).
Y = b
Z = c
yes;
Y = c
Z = e
yes
?- p(X,Y,Z).
X = 1
Y = 2
Z = 3
yes
?- retract(p(X,Y,Z)).
X = 1
Y = 2
Z = 3
yes
?- p(X,Y,Z).
noretract and clause will fail if predicate does not exist
?- retract(unknown_predicate(1,2,3)).
no
?- clause(unknown_predicate(1,2,3),X).
noArgument must be suitably instantiated that the predicate of the clause can be determined.
?- retract(X).
Expected an atom or a predicate but got a VARIABLE with value: X
?- clause(X,Y).
Expected an atom or a predicate but got a VARIABLE with value: X
?- retract(true).
Cannot inspect clauses of built-in predicate: true/0
?- clause(true,X).
Cannot inspect clauses of built-in predicate: true/0
?- retract(is(1,2)).
Cannot inspect clauses of built-in predicate: is/2
?- clause(is(1,2),X).
Cannot inspect clauses of built-in predicate: is/2 non_dynamic_predicate(1,2,3).
?- retract(non_dynamic_predicate(1,2,3)).
Cannot retract clause from user defined predicate as it is not dynamic: non_dynamic_predicate/3
?- retract(non_dynamic_predicate(_,_,_)).
Cannot retract clause from user defined predicate as it is not dynamic: non_dynamic_predicate/3
?- retract(non_dynamic_predicate(4,5,6)).
no