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.. rst-class:: align-right

control construct

.. index:: pair: {}/1; Control construct .. _control_external_call_1:

{}/1

Description

::

{Goal} {Closure} {Term}

This control construct allows the programmer to bypass the Logtalk compiler (including its linter but not its optimizer) in multiple contexts:

• Calling a goal as-is (from within an object or category) in the context of the :ref:`user <objects_user>` pseudo-object.
• Extending a closure as-is with the remaining arguments of a :ref:`call/2-N <methods_call_N>` call in order to construct a goal that will be called within the context of the user pseudo-object.
• Wrapping a source file term (either a clause or a directive) or a source file goal to bypass the :ref:`term-expansion mechanism <expansion_expansion>`.
• Using it in place of an object identifier when sending a message. In this case, its argument is proved as a goal within the context of the user pseudo-object with the resulting term being used as an object identifier in the message sending goal. This feature is mainly used with :term:`parametric objects <parametric object>` when their identifiers correspond to predicates defined in user.
• Using it as a message to an object. This is mainly useful when the message is e.g. a :ref:`conjunction of messages <messages_broadcasting>`, some of which being calls to Prolog built-in predicates.

  .. note::

  This control construct is opaque to cuts when used to wrap a goal (thus ensuring the same semantics independently of the argument being bound at compile time or at runtime).

Modes and number of proofs

::

{+callable} - zero_or_more

Errors

| Goal is a variable: | instantiation_error | Goal is neither a variable nor a callable term: | type_error(callable, Goal) | | Closure is a variable: | instantiation_error | Closure is neither a variable nor a callable term: | type_error(callable, Closure) | | Term is a variable: | instantiation_error | Term is neither a variable nor a callable term: | type_error(callable, Term)

Examples

::

% overload the standard (<)/2 operator by % calling its standard built-in definition: N1/D1 < N2/D2 :- {N1*D2 < N2*D1}.

% call a closure in the context of "user": call_in_user(F, X, Y, Z) :- call({F}, X, Y, Z).

% bypass the compiler for a proprietary backend directive: {:- load_foreign_resource(file)}.

% use parametric object proxies: | ?- {circle(Id, Radius, Color)}::area(Area). ...

% use Prolog built-in predicates as messages: | ?- logtalk::{write('hello world!'), nl}. hello world! yes