All clauses that X matches are removed from the knowledge base. X must be suitably instantiated that the predicate of the clause can be determined.
Examples
?- assertz(x(1,a)).
yes
?- assertz(x(2,b)).
yes
?- assertz(x(3,a)).
yes
?- assertz(y(1,a)).
yes
?- x(X,Y).
X = 1
Y = a
yes;
X = 2
Y = b
yes;
X = 3
Y = a
yes
?- y(X,Y).
X = 1
Y = a
yes
?- retractall(x(_,a)).
yes
?- x(X,Y).
X = 2
Y = b
yes
?- y(X,Y).
X = 1
Y = a
yes
?- retractall(x(_,_)).
yes
?- x(X,Y).
noSucceeds even if there are no facts to remove
?- retractall(x(_,_)).
yes
?- retractall(xyz(_)).
yesArgument must be suitably instantiated that the predicate of the clause can be determined.
?- retractall(X).
Expected an atom or a predicate but got a VARIABLE with value: X
?- retractall(true).
Cannot inspect clauses of built-in predicate: true/0
?- retractall(is(1,2)).
Cannot inspect clauses of built-in predicate: is/2 non_dynamic_predicate(1,2,3).
?- retractall(non_dynamic_predicate(1,2,3)).
Cannot retract clause from user defined predicate as it is not dynamic: non_dynamic_predicate/3
?- retractall(non_dynamic_predicate(_,_,_)).
Cannot retract clause from user defined predicate as it is not dynamic: non_dynamic_predicate/3
?- retractall(non_dynamic_predicate(4,5,6)).
yes