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.. rst-class:: align-right

**built-in method**

.. index:: pair: forall/2; Built-in method
.. _methods_forall_2:

``forall/2``
============

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

::

   forall(Generator, Test)

For all solutions of ``Generator``, ``Test`` is true. This meta-predicate
implements a *generate-and-test* loop using a definition equivalent to
``\+ (Generator, \+ Test)``. As a consequence, no variables in the arguments
are bound by a call to this predicate. This predicate often provides a better
alternative to a *failure-driven loop* as an unexpected ``Test`` failure will
not be ignored as it will make the ``forall/2`` call fail.

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

Meta-predicate template
-----------------------

::

   forall(0, 0)

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

::

   forall(@callable, @callable) - zero_or_one

Errors
------

| Either ``Generator`` or ``Test`` is a variable:
|     ``instantiation_error``
| ``Generator`` is neither a variable nor a callable term:
|     ``type_error(callable, Generator)``
| ``Test`` is neither a variable nor a callable term:
|     ``type_error(callable, Test)``

Examples
--------

| To call both goals in the context of the object or category containing the call:
|     ``forall(Generator, Test)``
| To send both goals as messages to :term:`self`:
|     ``forall(::Generator, ::Test)``
| To send both goals as messages to explicit objects:
|     ``forall(Object1::Generator, Object2::Test)``

.. seealso::

   :ref:`methods_bagof_3`,
   :ref:`methods_findall_3`,
   :ref:`methods_findall_4`,
   :ref:`methods_setof_3`
