If a user defined predicate is dynamic then it can be updated after it is first defined. It can subsequently have clauses removed using retract/1 and retractall/1 or new clauses added using asserta(X), assertz(X) and consult/1.
Examples
?- dynamic(true/0).
Cannot replace already defined built-in predicate: true/0
?- dynamic(is/2).
Cannot replace already defined built-in predicate: is/2
?- dynamic(test/2).
yes
?- write_to_file('dynamic1.tmp', 'test(a,1). test(b,2). test(c,3). test(d,4). test(e,5). testRule(X) :- test(X, Y), Y mod 2 =:= 0.').
yes
?- consult('dynamic1.tmp').
yes
?- testRule(X).
X = b
yes;
X = d
yes;
no
?- write_to_file('dynamic2.tmp', 'test(f,6). test(g,7). test(h,8).').
yes
?- consult('dynamic2.tmp').
yes
?- testRule(X).
X = b
yes;
X = d
yes;
X = f
yes;
X = h
yes
?- assertz(test(x,180)).
yes
?- asserta(test(y,42)).
yes
?- testRule(X).
X = y
yes;
X = b
yes;
X = d
yes;
X = f
yes;
X = h
yes;
X = x
yesNot OK to call dynamic/1 on a predicate that has already been defined and is not marked as dynamic.
?- write_to_file('dynamic3.tmp', 'not_dynamic(1,2,3).').
yes
?- consult('dynamic3.tmp').
yes
?- dynamic(not_dynamic/3).
Predicate has already been defined and is not dynamic: not_dynamic/3OK to call dynamic/1 on a predicate that has already been marked as dynamic.
?- dynamic(test/2).
yes write_to_file(Filename, Contents) :-
open(Filename, write, Z),
set_output(Z),
writef(Contents),
close(Z),
set_output('user_output').