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.. rst-class:: align-right

control construct

.. index:: pair: (<<)/2; Control construct .. _control_context_switch_2:

(<<)/2

Description

::

Object<<Goal {Proxy}<<Goal

Debugging control construct. Calls a goal within the context of the specified object. The goal is called with the execution context (:term:sender, :term:this, and :term:self) set to the object. The goal may need to be written between parenthesis to avoid parsing errors due to operator conflicts. This control construct should only be used for debugging or for writing unit tests. This control construct can only be used for objects compiled with the :ref:`context_switching_calls <flag_context_switching_calls>` compiler flag set to allow. Set this compiler flag to deny to disable this control construct and thus preventing using it to break encapsulation.

The {Proxy}<<Goal syntax allows simplified access to :term:`parametric object proxies <parametric object proxy>`. Its operational semantics is equivalent to the goal conjunction (call(Proxy), Proxy<<Goal). I.e. Proxy is proved within the context of the pseudo-object :ref:`user <objects_user>` and, if successful, the goal term is used as a parametric object identifier. Exceptions thrown when proving Proxy are handled by the (<<)/2 control construct. This syntax construct supports backtracking over the {Proxy} goal.

Caveat: although the goal argument is fully compiled before calling, some necessary information for the second compiler pass may not be available at runtime.

Modes and number of proofs

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+object_identifier<<+callable - zero_or_more {+object_identifier}<<+callable - zero_or_more

Errors

| Object is a variable: | instantiation_error | Object is neither a variable nor a valid object identifier: | type_error(object_identifier, Object) | Object does not contain a local definition for the Goal predicate: | existence_error(procedure, Goal) | Object does not exist: | existence_error(object, Object) | Object was created/compiled with support for context switching calls turned off: | permission_error(access, database, Goal) | | Proxy is a variable: | instantiation_error | Proxy is neither a variable nor an object identifier: | type_error(object_identifier, Proxy) | The predicate Proxy does not exist in the user pseudo-object: | existence_error(procedure, ProxyFunctor/ProxyArity) | | Goal is a variable: | instantiation_error | Goal is neither a variable nor a callable term: | type_error(callable, Goal)

Examples

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% call the member/2 predicate in the % context of the "list" object: test(member) :- list << member(1, [1]).