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   Copyright 1998-2022 Paulo Moura <pmoura@logtalk.org>
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.. index:: pair: bagof/3; Built-in method
.. _methods_bagof_3:

``bagof/3``
===========

Description
-----------

::

   bagof(Template, Goal, List)

Collects a bag of solutions for the goal for each set of instantiations
of the free variables in the goal. The order of the elements in the bag
follows the order of the goal solutions. The free variables in the goal
are the variables that occur in the goal but not in the template. Free
variables can be ignored, however, by using the ``^/2`` existential
qualifier. For example, if ``T`` is term containing all the free
variables that we want to ignore, we can write ``T^Goal``. Note that the
term ``T`` can be written as ``V1^V2^...``.

When there are free variables, this method is re-executable on
backtracking. This method fails when there are no solutions, never
returning an empty list.

This built-in meta-predicate is declared as a private method and thus
cannot be used as a message to an object.

Modes and number of proofs
--------------------------

::

   bagof(@term, +callable, -list) - zero_or_more

Errors
------

| ``Goal`` is a variable:
|     ``instantiation_error``
| ``Goal`` is neither a variable nor a callable term:
|     ``type_error(callable, Goal)``
| ``Goal`` is a call to a non-existing predicate:
|     ``existence_error(procedure, Predicate)``

Examples
--------

| To find a bag of solutions in the context of the object or category containing the call:
|     ``bagof(Template, Goal, List)``
| To find a bag of solutions of sending a message to :term:`self`:
|     ``bagof(Template, ::Message, List)``
| To find a bag of solutions of sending a message to an explicit object:
|     ``bagof(Template, Object::Message, List)``

.. seealso::

   :ref:`methods_findall_3`,
   :ref:`methods_findall_4`,
   :ref:`methods_forall_2`,
   :ref:`methods_setof_3`
