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.. rst-class:: align-right

built-in method

.. index:: pair: goal_expansion/2; Built-in method .. _methods_goal_expansion_2:

goal_expansion/2

Description

::

goal_expansion(Goal, ExpandedGoal)

Defines an expansion for a goal. The first argument is the goal to be expanded. The expanded goal is returned in the second argument. This predicate is called recursively on the expanded goal until a fixed point is reached. Thus, care must be taken to avoid compilation loops. This predicate, when defined and within scope, is automatically called by the :ref:methods_expand_goal_2 method. Use of this predicate by the expand_goal/2 method may be restricted by changing its default public scope.

Goal expansion may be also be applied when compiling source files by defining the object providing access to the goal_expansion/2 clauses as a :term:`hook object`. Clauses for the goal_expansion/2 predicate defined within an object or a category are never used in the compilation of the object or the category itself. Moreover, in this context, goals wrapped using the :ref:control_external_call_1 compiler bypass control construct are not expanded and any expanded goal wrapped in this control construct will not be further expanded.

Objects and categories implementing this predicate should declare that they implement the :ref:`expanding apis:expanding/0` built-in protocol if no ancestor already declares it. This protocol implementation relation can be declared as either :ref:`protected or private <inheritance_scope>` to restrict the scope of this predicate.

Modes and number of proofs

::

goal_expansion(+callable, -callable) - zero_or_one

Errors

(none)

Examples

::

goal_expansion(write(Term), (write_term(Term, []), nl)). goal_expansion(read(Term), (write('Input: '), {read(Term)})).

.. seealso::

:ref:methods_expand_goal_2, :ref:methods_expand_term_2, :ref:methods_term_expansion_2, :ref:predicates_logtalk_load_context_2