built-in method

# `asserta/1`

## Description

    asserta(Head)
    asserta((Head:-Body))

Asserts a clause as the first one for an object dynamic predicate.

When the predicate was not previously declared (using a scope directive), a dynamic predicate declaration is added to the object. In this case, the predicate scope depends on how this method is called:

`asserta(Clause)`

  The predicate is dynamically declared as private.

`::asserta(Clause)`

  The predicate is dynamically declared as protected.

`Object::asserta(Clause)`

  The predicate is dynamically declared as public.

Note, however, that dynamically declaring a new predicate requires either a local assert or the dynamic_declarations compiler flag set to `allow` when the object was created or compiled.

When the predicate indicator for `Head` is declared in a uses/2 or use_module/2 directive, the clause is asserted in the referenced object or module. When the backend Prolog compiler supports a module system, the predicate argument can also be module qualified.

This method may be used to assert clauses for predicates that are not declared dynamic for dynamic objects provided that the predicates are declared in this. This allows easy initialization of dynamically created objects when writing constructors.

## Modes and number of proofs

    asserta(+clause) - one

## Errors

`Head` is a variable:
`instantiation_error`
`Head` is a neither a variable nor a callable term:
`type_error(callable, Head)`
`Body` cannot be converted to a goal:
`type_error(callable, Body)`
The predicate indicator of `Head`, `Name/Arity`, is that of a private predicate:
`permission_error(modify, private_predicate, Name/Arity)`
The predicate indicator of `Head`, `Name/Arity`, is that of a protected predicate:
`permission_error(modify, protected_predicate, Name/Arity)`
The predicate indicator of `Head`, `Name/Arity`, is that of a static predicate:
`permission_error(modify, static_predicate, Name/Arity)`
The predicate indicator of `Head`, `Name/Arity`, does not match a declared predicate and the target object was created or compiled with support for dynamic declaration of predicates turned off:
`permission_error(create, predicate_declaration, Name/Arity)`

## Examples

To assert a clause as the first one for a local dynamic predicate or a dynamic predicate in this:
`asserta(Clause)`
To assert a clause as the first one for any public or protected dynamic predicate in self:
`::asserta(Clause)`
To assert a clause as the first one for any public dynamic predicate in an explicit object:
`Object::asserta(Clause)`

An example of asserting clauses in this and in self from a category:

    :- category(attributes,
        implements(attributes_protocol)).

        :- private(attr_/1).
        :- dynamic(attr_/1).

        set_in_this(A, X) :-
           asserta(attr_(A, X)).

        set_in_self(A, X) :-
           ::asserta(attr_(A, X)).

        ...

An example of asserting clauses into another object with the predicates listed using a uses/2 directive (similar when using a use_module/2 directive):

    :- object(reasoner(_KnowledgeBase_)).

        :- uses(_KnowledgeBase_, [
            foo/1, bar/1
        ]).

        baz(X) :-
           % compiled as _KnowledgeBase_::assertz(foo(X))
           asserta(foo(X)).

       foobar(Name, Argument) :-
          Clause =.. [Name, Argument],
          % runtime resolved to _KnowledgeBase_::assertz(Clause)
          % when Name is either foo or bar
          asserta(Clause).

        ...

See also

abolish/1, assertz/1, clause/2, retract/1, retractall/1 dynamic/0, dynamic/1, uses/2, use_module/2