.. _library_arbitrary:

``arbitrary``
=============

The ``arbitrary`` library defines an ``arbitrary`` category providing
predicates for generating random values for selected types to the
``type`` object, complementing its type checking predicates. Both the
object and the category predicates can be extended by the user with
definitions for new types by defining clauses for multifile predicates.
This library is notably used in the QuickCheck implementation by the
``lgtunit`` tool.

API documentation
-----------------

Open the
`../../docs/library_index.html#arbitrary <../../docs/library_index.html#arbitrary>`__
link in a web browser.

Loading
-------

To load all entities in this library, load the ``loader.lgt`` file:

::

   | ?- logtalk_load(arbitrary(loader)).

Testing
-------

To test this library predicates, load the ``tester.lgt`` file:

::

   | ?- logtalk_load(arbitrary(tester)).

Usage
-----

The ``arbitrary`` category complements the ``type`` object and thus its
predicates are accessed via this object. For example:

::

   | ?- type::arbitrary(integer, Arbitrary).
   Arbitrary = -816
   yes

To define a generator of arbitrary values for a type, define a clause
for the ``arbitrary::arbitrary/1`` multifile predicate specifying the
type and a clause for the ``arbitrary::arbitrary/2`` multifile predicate
generating an arbitrary term of the specified type. For example:

::

   :- multifile(arbitrary::arbitrary/1).
   arbitrary::arbitrary(foo).

   :- multifile(arbitrary::arbitrary/2).
   arbitrary::arbitrary(foo, Arbitrary) :-
       ...

Optionally, define a clause for the ``arbitrary::shrinker/1`` multifile
predicate to declare a new shrinker and a ``arbitrary::shrink/3``
multifile predicate for shrinking arbitrary values for QuickCheck usage.
For example:

::

   :- multifile(arbitrary::shrinker/1).
   arbitrary::shrinker(foo).

   :- multifile(arbitrary::shrink/3).
   arbitrary::shrink(foo, Large, Small) :-
       ...

The ``shrink/3`` should either succeed or fail but never throw an
exception.

It is also possible to define edge cases for a given type for use with
QuickCheck implementations. For example:

::

   :- multifile(arbitrary::edge_case/2).
   arbitrary::edge_case(cost, 0).

Edge cases are usually tried before resorting to generating arbitrary
values for a type.

The ``arbitrary`` category also provides access to the pseudo-random
generator it uses via the ``get_seed/1`` and ``set_seed/1``. This allows
sequences of arbitrary values to be reproduced. For example:

::

   | ?- type::get_seed(Seed).
   Seed = seed(3172, 9814, 20125)
   yes

   | ?- type::arbitrary(integer, Arbitrary).
   Arbitrary = -816
   yes

   | ?- type::arbitrary(integer, Arbitrary).
   Arbitrary = -113
   yes

   | ?- type::arbitrary(integer, Arbitrary).
   Arbitrary = 446

   | ?- type::set_seed(seed(3172, 9814, 20125)).
   yes

   | ?- type::arbitrary(integer, Arbitrary).
   Arbitrary = -816
   yes

   | ?- type::arbitrary(integer, Arbitrary).
   Arbitrary = -113
   yes

   | ?- type::arbitrary(integer, Arbitrary).
   Arbitrary = 446
   yes

The seed should be regarded as an opaque term and handled using the
``get_seed/1`` and ``set_seed/1`` predicates. These predicates are
notably used in the QuickCheck implementation provided by the
``lgtunit`` tool.

Examples
--------

See the implementation of the ``optionals`` and ``expecteds`` libraries.
See also the ``test_files/custom.lgt`` source file for an example of
defining custom arbitrary term generators.

Known issues
------------

Some Prolog systems either don't support the null character or provide
buggy results when calling ``char_code/2`` with a code of zero. When
that's the case, the null character is excluded when generating
arbitrary characters or character codes.

Generating arbitrary Unicode characters (instead of Unicode codepoints)
is inherently problematic as the process first generates codepoints and
then tries to use the standard ``char_code/2`` to convert them to
characters. But, depending on the backend Prolog system and its internal
(if any) Unicode normalization, it may not be possible to convert a
codepoint to a single character.
