# `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 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.