All predicates in Prolog fall into one of two categories: static
or dynamic. Dynamic predicates can be modified when a program is
running; in contrast, static predicates can be modified only by
reloading or by
If a predicate is first defined by being loaded from a file, it is
static by default. Sometimes, however, it is necessary to add (assert),
remove (retract), or inspect (using
for a predicate while a program is running. In order to do
that, you must declare the predicate to be dynamic. A predicate can be
made dynamic by specifically declaring it to be so, as described
below, or by using one of the assertion predicates. For a list of the
assertion predicates, and for more information on using them,
refer to ref-mdb-dsp.
To make a predicate dynamic, you insert in the file containing the predicate a line, which declares the predicate to be dynamic. The format of the line is
:- dynamic name/arity.
So, for example, the following declarations make the named predicates dynamic.
:- dynamic exchange_rate/3, spouse_of/2, gravitational_constant/1.
:-symbol must appear at the beginning of any line with a dynamic declaration, as shown above.
dynamic/1cannot be called as a predicate.