assert(+Clause)

+Clause

Atom or compound term

Description

The procedure must be declared to be dynamic using the dynamic/1 built-in. If the procedure is undefined an exception is raised. However, the default error handler for this exception simply declares the procedure dynamic and asserts the clause.

The asserted clause is NOT removed from the database on backtracking through the call to assert/1.

assert/1 satisfies the logical update semantics. Asserting a clause to a procedure will not, in any way, affect previous calls to it when backtracking.

Fail Conditions

Resatisfiable

Exceptions

(4) instantiation fault

Clause is a free variable.

(4) instantiation fault

The head of Clause is a free variable.

(5) type error

Clause is not a valid Prolog clause.

(63) procedure not dynamic

The procedure is not dynamic.

(70) accessing an undefined dynamic procedure

The procedure is undefined. However, the default error handler for this exception simply declares the procedure dynamic and asserts the clause, if the error was caused by an assert.

Examples

   Success:
    assert(city(munich)).
    assert((likes(X,Y):-man(X),woman(Y)).

   Error:
    assert(X).                        - gives error 4.
    assert("the man").                - gives error 5.
    assert(1).                        - gives error 5.
    assert((my_static(X):-write(X))). - gives error 63.
                                        if my_static/1 is not
                                        dynamic.
   Logical semantics :

   If the following clauses are in the database :
p :- assert(p), fail.
p :- fail.

q :- fail.
q :- assert(q), fail.

   The queries p.  and q.  will both fail.

See Also