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.. rst-class:: align-right

**built-in method**

.. index:: pair: coinductive_success_hook/1-2; Built-in method
.. _methods_coinductive_success_hook_1_2:

``coinductive_success_hook/1-2``
================================

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

::

   coinductive_success_hook(Head, Hypothesis)
   coinductive_success_hook(Head)

User-defined hook predicates that are automatically called in case of
coinductive success when proving a query for a coinductive predicates.
The hook predicates are called with the head of the coinductive
predicate on coinductive success and, optionally, with the hypothesis
that has used to reach coinductive success.

When both hook predicates are defined, the
``coinductive_success_hook/1`` clauses are only used if no
``coinductive_success_hook/2`` clause applies. The compiler ensures zero
performance penalties when defining coinductive predicates without a
corresponding definition for the coinductive success hook predicates.

The compiler assumes that these hook predicates are defined as static
predicates in order to optimize their use.

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

::

   coinductive_success_hook(+callable, +callable) - zero_or_one
   coinductive_success_hook(+callable) - zero_or_one

Errors
------

(none)

Examples
--------

::

   % Are there "occurrences" of arg1 in arg2?
   :- public(member/2).
   :- coinductive(member/2).

   member(X, [X| _]).
   member(X, [_| T]) :-
       member(X, T).

   % Are there infinitely many "occurrences" of arg1 in arg2?
   :- public(comember/2).
   :- coinductive(comember/2).
   comember(X, [_| T]) :-
       comember(X, T).

   coinductive_success_hook(member(_, _)) :-
       fail.
   coinductive_success_hook(comember(X, L)) :-
       member(X, L).

.. seealso::

   :ref:`directives_coinductive_1`
