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.