# `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 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 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 {}/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 expanding built-in protocol if no ancestor already declares it. This protocol implementation relation can be declared as either protected or private 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)})).

See also

expand_goal/2, expand_term/2, term_expansion/2, logtalk_load_context/2