# `lgtunit`

The `lgtunit` tool provides testing support for Logtalk. It can also be used for testing plain Prolog code and Prolog module code.

This tool is inspired by the xUnit frameworks architecture and by the works of Joachim Schimpf (ECLiPSe library `test_util`) and Jan Wielemaker (SWI-Prolog `plunit` package).

Tests are defined in objects, which represent a test set or test suite. In simple cases, we usually define a single object containing the tests. But it is also possible to use parametric test objects or multiple objects defining parametrizable tests or test subsets for testing more complex units and facilitating tests maintenance. Parametric test objects are specially useful to test multiple implementations of the same protocol using a single set of tests by passing the implementation object as a parameter value.

## Main files

The `lgtunit.lgt` source file implements a framework for defining and running unit tests in Logtalk. The `lgtunit_messages.lgt` source file defines the default translations for the messages printed when running unit tests. These messages can be intercepted to customize the output, e.g. to make it less verbose or for integration with e.g. GUI IDEs and continuous integration servers.

Other files that are part of this tool provide support for alternative output formats of test results and are discussed below.

## API documentation

This tool API documentation is available at:

../../apis/library_index.html#lgtunit

## Loading

This tool can be loaded using the query:

    | ?- logtalk_load(lgtunit(loader)).

## Testing

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

    | ?- logtalk_load(lgtunit(tester)).

## Writing and running tests

In order to write your own unit tests, define objects extending the `lgtunit` object. You may start by copying the `samples/tests-sample.lgt` file (at the root of the Logtalk distribution) to a `tests.lgt` file in your project directory and editing it to add your tests:

    :- object(tests,
        extends(lgtunit)).

        % test definitions
        ...

    :- end_object.

The section on test dialects below describes in detail how to write tests. See the `tests` top directory for examples of actual unit tests. Other sources of examples are the `library` and `examples` directories.

The tests must be term-expanded by the `lgtunit` object by compiling the source files defining the test objects using the option `hook(lgtunit)`. For example:

    | ?- logtalk_load(lgtunit(loader)),
         logtalk_load(tests, [hook(lgtunit)]).

As the term-expansion mechanism applies to all the contents of a source file, the source files defining the test objects should preferably not contain entities other than the test objects. Additional code necessary for the tests should go to separate files. In general, the tests themselves can be compiled in optimized mode. Assuming that’s the case, also use the `optimize(on)` compiler option for faster execution.

The term-expansion performed by the `lgtunit` object sets the test object `source_data` flag to `on` and the `context_switching_calls` flag to `allow` for code coverage and debugging support. But these settings can always be overridden in the test objects.

The `samples/tester-sample.lgt` file (at the root of the Logtalk distribution) exemplifies how to compile and load `lgtunit` tool, the source code under testing, the unit tests, and how to automatically run all the tests after loading:

    :- initialization((
        % minimize compilation reports to the essential ones (errors and warnings)
        set_logtalk_flag(report, warnings),
        % load any necessary library files for your application; for example
        logtalk_load(basic_types(loader)),
        % load the unit test tool
        logtalk_load(lgtunit(loader)),
        % load your application files (e.g., "source.lgt") enabling support for
        % code coverage, which requires compilation in debug mode and collecting
        % source data information; if code coverage is not required, remove the
        % "debug(on)" option for faster execution
        logtalk_load(source, [source_data(on), debug(on)]),
        % compile the unit tests file expanding it using "lgtunit" as the hook
        % object to preprocess the tests; if you have failing tests, add the
        % option debug(on) to debug them (see "tools/lgtunit/NOTES.md" for
        % debugging advice); tests should be loaded after the code being tested
        % is loaded to avoid warnings such as references to unknown entities
        logtalk_load(tests, [hook(lgtunit)]),
        % run all the unit tests; assuming your tests object is named "tests"
        tests::run
    )).

You may copy this sample file to a `tester.lgt` file in your project directory and edit it to load your project and test files. The `logtalk_tester` testing automation script defaults to looking for test driver files named `tester.lgt` or `tester.logtalk` (if you have work-in-progress test sets that you don’t want to run by default, simply use a different file name such as `tester_wip.lgt`; you can still run them automated by using `logtalk_tester -n tester_wip`).

Debugged test sets should preferably be compiled in optimal mode, specially when containing deterministic tests and when using the utility benchmarking predicates.

Assuming a `tester.lgt` driver file as exemplified above, the tests can be run by simply loading this file:

    | ?- logtalk_load(tester).

Assuming your test object is named `tests`, you can re-run the tests by typing:

    | ?- tests::run.

You can also re-run a single test (or a list of tests) using the `run/1` predicate:

    | ?- tests::run(test_identifier).

When testing complex units, it is often desirable to split the tests between several test objects or use parametric test objects to be able to run the same tests using different parameters (e.g., different data sets or alternative implementations of the same protocol). In this case, you can run all test subsets using the goal:

    | ?- lgtunit::run_test_sets([test_set_1, test_set_2, ...]).

where the `run_test_sets/1` predicate argument is a list of two or more test object identifiers. This predicate makes it possible to get a single code coverage report that takes into account all the tests.

It’s also possible to automatically run loaded tests when using the `make` tool by calling the goal that runs the tests from a definition of the hook predicate `logtalk_make_target_action/1`. For example, by adding to the tests `tester.lgt` driver file the following code:

    % integrate the tests with logtalk_make/1
    :- multifile(logtalk_make_target_action/1).
    :- dynamic(logtalk_make_target_action/1).

    logtalk_make_target_action(check) :-
        tests::run.

Alternatively, you can define the predicate `make/1` inside the test set object. For example:

    :- object(tests, extends(lgtunit)).

        make(check).
        ...

    :- end_object.

This clause will cause all tests to be run when calling the `logtalk_make/1` predicate with the target `check` (or its top-level shortcut, `{?}`). The other possible target is `all` (with top-level shortcut `{*}`).

Note that you can have multiple test driver files. For example, one driver file that runs the tests collecting code coverage data and a quicker driver file that skips code coverage and compiles the code to be tested in optimized mode.

## Automating running tests

You can use the `scripts/logtalk_tester.sh` Bash shell script or the `scripts/logtalk_tester.ps1` PowerShell script for automating running unit tests (e.g., from a CI/CD pipeline). When using one of the Logtalk installers, the `.sh` extension can usually be omitted. For example, assuming your current directory (or sub-directories) contains one or more `tester.lgt` files:

    $ logtalk_tester -p gnu

The only required argument is the identifier of the backend Prolog system. For other options, see the `scripts/NOTES.md` file or type:

    $ logtalk_tester -h

On POSIX systems, you can also access extended documentation by consulting the script man page:

    $ man logtalk_tester

The scripts support the same set of options. But the option for passing additional arguments to the tests uses different syntax. For example:

    $ logtalk_tester -p gnu -- foo bar baz

    PS> logtalk_tester -p gnu -a foo,bar,baz

On POSIX systems, assuming Logtalk was installed using one of the provided installers or installation scripts, there is also a `man` page for the script:

    $ man logtalk_tester

Alternatively, an HTML version of this man page can be found at:

https://logtalk.org/man/logtalk_tester.html

On POSIX systems, the `logtalk_tester.sh` Bash script timeout option requires either a `timeout` or a `gtimeout` command (provided by the GNU coreutils package). The `logtalk_tester.ps1` PowerShell script timeout option requires that Git for Windows is also installed, as it requires the GNU timeout command bundled with it.

In addition to using the `logtalk_tester.ps1` PowerShell script, the Bash shell version of the automation script can also be used in Windows operating-systems with selected backends by using the Bash shell included in the Git for Windows installer. That requires defining a `.profile` file setting the paths to the Logtalk scripts and the Prolog backend executables. For example:

    $ cat ~/.profile
    # YAP
    export PATH="/C/Program Files/Yap64/bin":$PATH
    # GNU Prolog
    export PATH="/C/GNU-Prolog/bin":$PATH
    # SWI/Prolog
    export PATH="/C/Program Files/swipl/bin":$PATH
    # ECLiPSe
    export PATH="/C/Program Files/ECLiPSe 7.0/lib/x86_64_nt":$PATH
    # SICStus Prolog
    export PATH="/C/Program Files/SICStus Prolog VC16 4.6.0/bin":$PATH
    # Logtalk
    export PATH="$LOGTALKHOME/scripts":"$LOGTALKHOME/integration":$PATH

The Git for Windows installer also includes GNU `coreutils` and its `timeout` command, which is used by the `logtalk_tester` script `-t` option.

Note that some tests may give different results when run from within the Bash shell compared with running the tests manually using a Windows GUI version of the Prolog backend. Some backends may also not be usable for automated testing due to the way they are made available as Windows applications.

Additional advice on testing and on automating testing using continuous integration servers can be found at:

https://logtalk.org/testing.html

## Parametric test objects

Parameterized unit tests can be easily defined by using parametric test objects. A typical example is testing multiple implementations of the same protocol. In this case, we can use a parameter to pass the specific implementation being tested. For example, assume that we want to run the same set of tests for the library `random_protocol` protocol. We can write:

    :- object(tests(_RandomObject_),
        extends(lgtunit)).

        :- uses(_RandomObject_, [
            random/1, between/3, member/2,
            ...
        ]).

        test(between_3_in_interval) :-
            between(1, 10, Random),
            1 =< Random, Random =< 10.

        ...

    :- end_object.

We can then test a specific implementation by instantiating the parameter. For example:

    | ?- tests(fast_random)::run.

Or use the `lgtunit::run_test_sets/1` predicate to test all the implementations:

    | ?- lgtunit::run_test_sets([
            tests(backend_random),
            tests(fast_random),
            tests(random)
         ]).

## Test dialects

Multiple test dialects are supported by default. See the next section on how to define your own test dialects. In all dialects, a ground callable term, usually an atom, is used to uniquely identify a test. This simplifies reporting failed tests and running tests selectively. An error message is printed if invalid or duplicated test identifiers are found. These errors must be corrected; otherwise the reported test results can be misleading. Ideally, tests should have descriptive names that clearly state the purpose of the test and what is being tested.

Unit tests can be written using any of the following predefined dialects:

    test(Test) :- Goal.

This is the most simple dialect, allowing the specification of tests that are expected to succeed. The argument of the `test/1` predicate is the test identifier, which must be unique. A more versatile dialect is:

    succeeds(Test) :- Goal.
    deterministic(Test) :- Goal.
    fails(Test) :- Goal.
    throws(Test, Ball) :- Goal.
    throws(Test, Balls) :- Goal.

This is a straightforward dialect. For `succeeds/1` tests, `Goal` is expected to succeed. For `deterministic/1` tests, `Goal` is expected to succeed once without leaving a choice-point. For `fails/1` tests, `Goal` is expected to fail. For `throws/2` tests, `Goal` is expected to throw the exception term `Ball` or one of the exception terms in the list `Balls`. The specified exception must subsume the actual exception for the test to succeed.

An alternative test dialect that can be used with more expressive power is:

    test(Test, Outcome) :- Goal.

The possible values of the outcome argument are:

- `true`
  The test is expected to succeed.
- `true(Assertion)`
  The test is expected to succeed and satisfy the `Assertion` goal.
- `deterministic`
  The test is expected to succeed once without leaving a choice-point.
- `deterministic(Assertion)`
  The test is expected to succeed once without leaving a choice-point and satisfy the `Assertion` goal.
- `subsumes(Expected, Result)`
  The test is expected to succeed by binding `Result` to a term that is subsumed by the `Expected` term.
- `variant(Term1, Term2)`
  The test is expected to succeed by binding `Term1` to a term that is a variant of the `Term2` term.
- `exists(Assertion)`
  A solution exists for the test goal that satisfies the `Assertion` goal.
- `all(Assertion)`
  All test goal solutions satisfy the `Assertion` goal.
- `fail`
  The test is expected to fail.
- `false`
  The test is expected to fail.
- `error(Error)`
  The test is expected to throw the exception term `error(ActualError, _)` where `ActualError` is subsumed `Error`.
- `errors(Errors)`
  The test is expected to throw an exception term `error(ActualError, _)` where `ActualError` is subsumed by an element of the list `Errors`.
- `ball(Ball)`
  The test is expected to throw the exception term `ActualBall` where `ActualBall` is subsumed `Ball`.
- `balls(Balls)`
  The test is expected to throw an exception term `ActualBall` where `ActualBall` is subsumed by an element of the list `Balls`.

In the case of the `true(Assertion)`, `deterministic(Assertion)`, and `all(Assertion)` outcomes, a message that includes the assertion goal is printed for assertion failures and errors to help to debug failed unit tests. Same for the `subsumes(Expected, Result)` and `variant(Term1, Term2)` assertions. Note that this message is only printed when the test goal succeeds, as its failure will prevent the assertion goal from being called. This allows distinguishing between test goal failure and assertion failure.

Note that the `all(Assertion)` outcome simplifies pinpointing which test goal solution failed the assertion. See also the section below on testing non-deterministic predicates.

The `fail` and `false` outcomes are better reserved for cases where there is a single test goal. With multiple test goals, the test will succeed when any of those goals fail.

Some tests may require individual condition, setup, or cleanup goals. In this case, the following alternative test dialect can be used:

    test(Test, Outcome, Options) :- Goal.

The currently supported options are (non-recognized options are ignored):

- `condition(Goal)`
  Condition for deciding if the test should be run or skipped (default goal is `true`).
- `setup(Goal)`
  Setup goal for the test (default goal is `true`).
- `cleanup(Goal)`
  Cleanup goal for the test (default goal is `true`).
- `flaky`
  Declare the test as a flaky test.
- `note(Term)`
  Annotation to print (between parentheses by default) after the test result (default is `''`); the annotation term can share variables with the test goal, which can be used to pass additional information about the test result.

Also supported is QuickCheck testing, where random tests are automatically generated and run given a predicate mode template with type information for each argument (see the section below for more details):

    quick_check(Test, Template, Options).
    quick_check(Test, Template).

The valid options are the same as for the `test/3` dialect plus all the supported QuickCheck specific options (see the QuickCheck section below for details).

For examples of how to write unit tests, check the `tests` folder or the `testing` example in the `examples` folder in the Logtalk distribution. Most of the provided examples also include unit tests, some of them with code coverage.

## User-defined test dialects

Additional test dialects can be easily defined by extending the `lgtunit` object and by term-expanding the new dialect into one of the default dialects. As an example, suppose that you want a dialect where you can simply write a file with tests defined by clauses using the format:

    test_identifier :-
        test_goal.

First, we define an expansion for this file into a test object:

    :- object(simple_dialect,
        implements(expanding)).

        term_expansion(begin_of_file, [(:- object(tests,extends(lgtunit)))]).
        term_expansion((Head :- Body), [test(Head) :- Body]).
        term_expansion(end_of_file, [(:- end_object)]).

    :- end_object.

Then we can use this hook object to expand and run tests written in this dialect by using a `tester.lgt` driver file with contents such as:

    :- initialization((
        set_logtalk_flag(report, warnings),
        logtalk_load(lgtunit(loader)),
        logtalk_load(hook_flows(loader)),
        logtalk_load(simple_dialect),
        logtalk_load(tests, [hook(hook_pipeline([simple_dialect,lgtunit]))]),
        tests::run
    )).

The hook pipeline first applies our `simple_dialect` expansion, followed by the default `lgtunit` expansion. This solution allows other hook objects (e.g., required by the code being tested) to also be used by updating the pipeline.

## QuickCheck

QuickCheck was originally developed for Haskell. Implementations for several other programming languages soon followed. QuickCheck provides support for property-based testing. The idea is to express properties that predicates must comply with and automatically generate tests for those properties. The `lgtunit` tool supports both `quick_check/2-3` test dialects, as described above, and `quick_check/1-3` public predicates for interactive use:

    quick_check(Template, Result, Options).
    quick_check(Template, Options).
    quick_check(Template).

The following options are supported:

- `n/1`: number of random tests that will be generated and run (default is 100).

- `s/1`: maximum number of shrink operations when a counter-example is found (default is 64).

- `ec/1`: boolean option deciding if type edge cases are tested before generating random tests (default is `true`).

- `rs/1`: starting seed to be used when generating the random tests (no default).

- `pc/1`: pre-condition closure for generated tests (extended with the test arguments; no default).

- `l/1`: label closure for classifying the generated tests (extended with the test arguments plus the label argument; no default).

- `v/1`: boolean option for verbose reporting of generated random tests (default is `false`).

- `pb/2`: progress bar option for executed random tests when the verbose option is false (first argument is a boolean, default is `false`; second argument is the tick number, a positive integer).

The `quick_check/1` predicate uses the default option values. The `quick_check/1-2` predicates print the test results and are thus better reserved for testing at the top-level interpreter. The `quick_check/3` predicate returns results in reified form:

- `passed(SequenceSeed, Discarded, Labels)`

- `failed(Goal, SequenceSeed, TestSeed)`

- `error(Error, Goal, SequenceSeed, TestSeed)`

- `broken(Why, Culprit)`

The `broken(Why, Culprit)` result only occurs when the user-defined testing setup is broken. For example, a non-callable template (e.g., a non-existing predicate), an invalid option, a problem with the pre-condition closure or with the label closure (e.g., a pre-condition that always fails or a label that fails to classify a generated test), or errors/failures when generating tests (e.g., due to an unknown type being used in the template or a broken custom type arbitrary value generator).

The `Goal` argument is the random test that failed.

The `SequenceSeed` argument is the starting seed used to generate the sequence of random tests. The `TestSeed` is the seed used to generate the test that failed. Both seeds should be regarded as opaque terms. When the test seed is equal to the sequence seed, this means that the failure or error occurred while using only type edge cases. See below how to use the seeds when testing bug fixes.

The `Discarded` argument returns the number of generated tests that were discarded for failing to comply with a pre-condition specified using the `pc/1` option. This option is specially useful when constraining or enforcing a relation between the generated arguments and is often used as an alternative to define a custom type. For example, if we define the following predicate:

    condition(I) :-
        between(0, 127, I).

We can then use it to filter the generated tests:

    | ?- lgtunit::quick_check(integer(+byte), [pc(condition)]).
    % 100 random tests passed, 94 discarded
    % starting seed: seed(416,18610,17023)
    yes

The `Labels` argument returns a list of pairs `Label-N` where `N` is the number of generated tests that are classified as `Label` by a closure specified using the `l/1` option. For example, assuming the following predicate definition:

    label(I, Label) :-
        (   I mod 2 =:= 0 ->
            Label = even
        ;   Label = odd
        ).

We can try:

    | ?- lgtunit::quick_check(integer(+byte), [l(label), n(10000)]).
    % 10000 random tests passed, 0 discarded
    % starting seed: seed(25513,20881,16407)
    % even: 5037/10000 (50.370000%)
    % odd: 4963/10000 (49.630000%)
    yes

The label statistics are key to verifying that the generated tests provide the necessary coverage. The labeling predicates can return a single test label or a list of test labels. Labels should be ground and are typically atoms. To examine the generated tests themselves, you can use the verbose option, `v/1`. For example:

    | ?- lgtunit::quick_check(integer(+integer), [v(true), n(7), pc([I]>>(I>5))]).
    % Discarded: integer(0)
    % Passed:    integer(786)
    % Passed:    integer(590)
    % Passed:    integer(165)
    % Discarded: integer(-412)
    % Passed:    integer(440)
    % Discarded: integer(-199)
    % Passed:    integer(588)
    % Discarded: integer(-852)
    % Discarded: integer(-214)
    % Passed:    integer(196)
    % Passed:    integer(353)
    % 7 random tests passed, 5 discarded
    % starting seed: seed(23671,3853,29824)
    yes

When a counter-example is found, the verbose option also prints the shrink steps. For example:

    | ?- lgtunit::quick_check(atom(+atomic), [v(true), ec(false)]).
    % Passed:    atom('dyO=Xv_MX-3b/U4KH U')
    *     Failure:   atom(-198)
    *     Shrinked:  atom(-99)
    *     Shrinked:  atom(-49)
    *     Shrinked:  atom(-24)
    *     Shrinked:  atom(-12)
    *     Shrinked:  atom(-6)
    *     Shrinked:  atom(-3)
    *     Shrinked:  atom(-1)
    *     Shrinked:  atom(0)
    *     quick check test failure (at test 2 after 8 shrinks):
    *       atom(0)
    *     starting seed: seed(3172,9814,20125)
    *     test seed:     seed(7035,19506,18186)
    no

The template can be a `(::)/2`, `(<<)/2`, or `(:)/2` qualified callable term. When the template is an unqualified callable term, it will be used to construct a goal to be called in the context of the sender using the `(<<)/2` debugging control construct. Another simple example is passing a template that will trigger a failed test (as the `random::random/1` predicate always returns non-negative floats):

    | ?- lgtunit::quick_check(random::random(-negative_float)).
    *     quick check test failure (at test 1 after 0 shrinks):
    *       random::random(0.09230089279334841)
    *     starting seed: seed(3172,9814,20125)
    *     test seed:     seed(3172,9814,20125)
    no

When QuickCheck exposes a bug in the tested code, we can use the reported counter-example to help diagnose it and fix it. As tests are randomly generated, we can use the starting seed reported with the counter-example to confirm the bug fix by calling the `quick_check/2-3` predicates with the `rs(Seed)` option. For example, assume the following broken predicate definition:

    every_other([], []).
    every_other([_, X| L], [X | R]) :-
        every_other(L, R).

The predicate is supposed to construct a list by taking every other element of an input list. Cursory testing may fail to notice the bug:

    | ?- every_other([1,2,3,4,5,6], List).
    List = [2, 4, 6]
    yes

But QuickCheck will report a bug with lists with an odd number of elements with a simple property that verifies that the predicate always succeeds and returns a list of integers:

    | ?- lgtunit::quick_check(every_other(+list(integer), -list(integer))).
    *     quick check test failure (at test 2 after 0 shrinks):
    *       every_other([0],A)
    *     starting seed: seed(3172,9814,20125)
    *     test seed:     seed(3172,9814,20125)
    no

We could fix this particular bug by rewriting the predicate:

    every_other([], []).
    every_other([H| T], L) :-
        every_other(T, H, L).

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

By retesting with the same test seed that uncovered the bug, the same random test that found the bug will be generated and run again:

    | ?- lgtunit::quick_check(
            every_other(+list(integer), -list(integer)),
            [rs(seed(3172,9814,20125))]
         ).
    % 100 random tests passed, 0 discarded
    % starting seed: seed(3172,9814,20125)
    yes

Still, after verifying the bug fix, is also a good idea to re-run the tests using the sequence seed instead, as bug fixes sometimes cause regressions elsewhere.

When retesting using the `logtalk_tester` automation script, the starting seed can be set using the `-r` option. For example:

    $ logtalk_tester -r "seed(3172,9814,20125)"

We could now move to other properties that the predicate should comply with (e.g., all elements in the output list being present in the input list). Often, both traditional unit tests and QuickCheck tests are used, complementing each other to ensure the required code coverage.

Another example using a Prolog module predicate:

    | ?- lgtunit::quick_check(
            pairs:pairs_keys_values(
                +list(pair(atom,integer)),
                -list(atom),
                -list(integer)
            )
         ).
    % 100 random tests passed, 0 discarded
    % starting seed: seed(3172,9814,20125)
    yes

As illustrated by the examples above, properties are expressed using predicates. In the most simple cases, that can be the predicate that we are testing itself. But, in general, it will be an auxiliary predicate calling the predicate or predicates being tested and checking properties that the results must comply with.

The QuickCheck test dialects and predicates take as argument the mode template for a property, generate random values for each input argument based on the type information, and check each output argument. For common types, the implementation tries first (by default) common edge cases (e.g., empty atom, empty list, or zero) before generating arbitrary values. When the output arguments check fails, the QuickCheck implementation tries (by default) up to 64 shrink operations of the counter-example to report a simpler case to help debugging the failed test. Edge cases, generating arbitrary terms, and shrinking terms make use of the library `arbitrary` category via the `type` object (both entities can be extended by the user by defining clauses for multifile predicates).

The mode template syntax is the same as that used in the `info/2` predicate directives with an additional notation, `{}/1`, for passing argument values as-is instead of generating random values for these arguments. For example, assume that we want to verify the `type::valid/2` predicate, which takes as its first argument a type. Randomly generating random types would be cumbersome at best but the main problem is that we need to generate random values for the second argument according to the first argument. Using the `{}/1` notation, we can solve this problem for any specific type, e.g. integer, by writing:

    | ?- lgtunit::quick_check(type::valid({integer}, +integer)).

We can also test all (ground, i.e. non-parametric) types with arbitrary value generators by writing:

    | ?- forall(
            (type::type(Type), ground(Type), type::arbitrary(Type)),
            lgtunit::quick_check(type::valid({Type}, +Type))
         ).

You can find the list of the basic supported types for use in the template in the API documentation for the library entities `type` and `arbitrary`. Note that other library entities, including third-party or your own, can contribute with additional type definitions, as both `type` and `arbitrary` entities are user-extensible by defining clauses for their multifile predicates.

The user can define new types to use in the property mode templates to use with its QuickCheck tests by defining clauses for the `type` library object and the `arbitrary` library category multifile predicates. QuickCheck will use the later to generate arbitrary input arguments and the former to verify output arguments. As a toy example, assume that the property mode template has an argument of type `bit` with possible values `0` and `1`. We would then need to define:

    :- multifile(type::type/1).
    type::type(bit).

    :- multifile(type::check/2).
    type::check(bit, Term) :-
        once((Term == 0; Term == 1)).

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

    :- multifile(arbitrary::arbitrary/2).
    arbitrary::arbitrary(bit, Arbitrary) :-
        random::member(Arbitrary, [0, 1]).

## Skipping tests

A test object can define the `condition/0` predicate (which defaults to `true`) to test if some necessary condition for running the tests holds. The tests are skipped if the call to this predicate fails or generates an error.

Individual tests that for some reason should be unconditionally skipped can have the test clause head prefixed with the `(-)/1` operator. For example:

    - test(not_yet_ready) :-
        ...

In this case, it’s a good idea to use the `test/3` dialect with a `note/1` option that briefly explains why the test is being skipped. For example:

    - test(xyz_reset, true, [note('Feature xyz reset not yet implemented')]) :-
        ...

The number of skipped tests is reported together with the numbers of passed and failed tests. To skip a test depending on some condition, use the `test/3` dialect and the `condition/1` option. For example:

    test(test_id, true, [condition(current_prolog_flag(bounded,true))) :-
        ...

The test is skipped if the condition goal fails or generates an error. The conditional compilation directives can also be used in alternative, but note that in this case there will be no report on the number of skipped tests.

## Selecting tests

While debugging an application, we often want to temporarily run just a selection of relevant tests. This is specially useful when running all the tests slows down and distracts from testing fixes for a specific issue. This can be accomplished by prefixing the clause heads of the selected tests with the `(+)/1` operator. For example:

    :- object(tests,
        extends(lgtunit)).

        cover(ack).

        test(ack_1, true(Result == 11)) :-
            ack::ack(2, 4, Result).

        + test(ack_2, true(Result == 61)) :-
            ack::ack(3, 3, Result).

        test(ack_3, true(Result == 125)) :-
            ack::ack(3, 4, Result).

    :- end_object.

In this case, only the `ack_2` would run. Just be careful to remove all `(+)/1` test prefixes when done debugging the issue that prompted you to run just the selected tests. After, be sure to run all the tests to ensure there are no regressions introduced by your fixes.

## Checking test goal results

Checking test goal results can be performed using the `test/2-3` supported outcomes such as `true(Assertion)` and `deterministic(Assertion)`. For example:

    test(compare_3_order_less, deterministic(Order == (<))) :-
        compare(Order, 1, 2).

For the other test dialects, checking test goal results can be performed by calling the `assertion/1-2` utility predicates or by writing the checking goals directly in the test body. For example:

    test(compare_3_order_less) :-
        compare(Order, 1, 2),
        ^^assertion(Order == (<)).

or:

    succeeds(compare_3_order_less) :-
        compare(Order, 1, 2),
        Order == (<).

Using assertions is, however, preferable to directly checking test results in the test body as it facilitates debugging by printing the unexpected results when the assertions fail.

The `assertion/1-2` utility predicates are also useful for the `test/2-3` dialects when we want to check multiple assertions in the same test. For example:

    test(dictionary_clone_4_01, true) :-
        as_dictionary([], Dictionary),
        clone(Dictionary, DictionaryPairs, Clone, ClonePairs),
        empty(Clone),
        ^^assertion(original_pairs, DictionaryPairs == []),
        ^^assertion(clone_pairs, ClonePairs == []).

Ground results can be compared using the standard `==/2` term equality built-in predicate. Non-ground results can be compared using the `variant/2` predicate provided by `lgtunit`. The standard `subsumes_term/2` built-in predicate can be used when testing a compound term structure while abstracting some of its arguments. Floating-point numbers can be compared using the `=~=/2`, `approximately_equal/3`, `essentially_equal/3`, and `tolerance_equal/4` predicates provided by `lgtunit`. Using the `=/2` term unification built-in predicate is almost always an error, as it would mask test goals failing to bind output arguments. The `lgtunit` tool implements a linter check for the use of unification goals in test outcome assertions. In the rare cases that a unification goal is intended, wrapping the `(=)/2` goal using the `{}/1` control construct avoids the linter warning.

When the meta-argument of the `assertion/1-2` predicates is call to a local predicate (in the tests object), you need to call them using the `(::)/2` message-sending control construct instead of the `(^^)/2` super call control construct. This is necessary as super calls preserve the sender, and the tests are implicitly run by the `lgtunit` object sending a message to the tests object. For example:

    :- uses(lgtunit, [
        assertion/1
    ]).

    test(my_test_id, true) :-
        foo(X, Y),
        assertion(consistent(X, Y)).

    consistent(X, Y) :-
        ...

In this case, the sender is the tests object, and the `assertion/1` meta-predicate will call the local `consistent/2` predicate in the expected context.

## Testing local predicates

The `(<<)/2` debugging control construct can be used to access and test object local predicates (i.e., predicates without a scope directive). In this case, make sure that the `context_switching_calls` compiler flag is set to `allow` for those objects. This is seldom required, however, as local predicates are usually auxiliary predicates called by public predicates and thus tested when testing those public predicates. The code coverage support can pinpoint any local predicate clause that is not being exercised by the tests.

## Testing non-deterministic predicates

For testing non-deterministic predicates (with a finite and manageable number of solutions), you can wrap the test goal using the standard `findall/3` predicate to collect all solutions and check against the list of expected solutions. When the expected solutions are a set, use in alternative the standard `setof/3` predicate.

If you want to check that all solutions of a non-deterministic predicate satisfy an assertion, use the `test/2` or `test/3` test dialect with the `all(Assertion)` outcome. For example:

    test(atom_list, all(atom(Item))) :-
        member(Item, [a, b, c]).

See also the next section on testing generators.

If you want to check that a solution exists for a non-deterministic predicate that satisfies an assertion, use the `test/2` or `test/3` test dialect with the `exists(Assertion)` outcome. For example:

    test(at_least_one_atom, exists(atom(Item))) :-
        member(Item, [1, foo(2), 3.14, abc, 42]).

## Testing generators

To test all solutions of a predicate that acts as a generator, we can use either the `all/1` outcome or the `forall/2` predicate as the test goal with the `assertion/2` predicate called to report details on any solution that fails the test. For example:

    test(test_solution_generator, all(test(X,Y,Z))) :-
        generator(X, Y, Z).

or:

    :- uses(lgtunit, [assertion/2]).
    ...

    test(test_solution_generator_2) :-
        forall(
            generator(X, Y, Z),
            assertion(generator(X), test(X,Y,Z))
        ).

While using the `all/1` outcome results in a more compact test definition, using the `forall/2` predicate allows customizing the assertion description. In the example above, we use the `generator(X)` description instead of the `test(X,Y,Z)` description implicit when we use the `all/1` outcome.

## Testing input/output predicates

Extensive support for testing input/output predicates is provided, based on similar support found on the Prolog conformance testing framework written by Péter Szabó and Péter Szeredi.

Two sets of predicates are provided, one for testing text input/output and one for testing binary input/output. In both cases, temporary files (possibly referenced by a user-defined alias) are used. The predicates allow setting, checking, and cleaning text/binary input/output. These predicates are declared as protected and thus called using the `(^^/1)` control construct.

As an example of testing an input predicate, consider the standard `get_char/1` predicate. This predicate reads a single character (atom) from the current input stream. Some test for basic functionality could be:

    test(get_char_1_01, true(Char == 'q')) :-
        ^^set_text_input('qwerty'),
        get_char(Char).

    test(get_char_1_02, true(Assertion)) :-
        ^^set_text_input('qwerty'),
        get_char(_Char),
        ^^text_input_assertion('werty', Assertion).

As you can see in the above example, the testing pattern consists on setting the input for the predicate being tested, calling it, and then checking the results. It is also possible to work with streams other than the current input/output streams by using the `lgtunit` predicate variants that take a stream alias as argument. For example, when testing the standard `get_char/2` predicate, we could write:

    test(get_char_2_01, true(Char == 'q')) :-
        ^^set_text_input(in, 'qwerty'),
        get_char(in, Char).

    test(get_char_2_02, true(Assertion)) :-
        ^^set_text_input(in, 'qwerty'),
        get_char(in, _Char),
        ^^text_input_assertion(in, 'werty', Assertion).

Testing output predicates follows a similar pattern by using instead the `set_text_output/1-2` and `text_output_assertion/2-3` predicates. For example:

    test(put_char_2_02, true(Assertion)) :-
        ^^set_text_output(out, 'qwert'),
        put_char(out, y),
        ^^text_output_assertion(out, 'qwerty', Assertion).

The `set_text_output/1` predicate diverts only the standard output stream (to a temporary file) using the standard `set_output/1` predicate. Most backend Prolog systems also support writing to the de facto standard error stream. But there’s no standard solution to divert this stream. However, several systems provide a `set_stream/2` or similar predicate that can be used for stream redirection. For example, assume that you wanted to test a backend Prolog system warning when an `initialization/1` directive fails that is written to `user_error`. An hypothetical test could be:

    test(singletons_warning, true(Assertion)) :-
        ^^set_text_output(''),
        current_output(Stream),
        set_stream(Stream, alias(user_error)),
        consult(broken_file),
        ^^text_output_assertion('WARNING: initialization/1 directive failed', Assertion).

For testing binary input/output predicates, equivalent testing predicates are provided. There is also a small set of helper predicates for dealing with stream handles and stream positions. For testing with files instead of streams, testing predicates are provided that allow creating text and binary files with given contents and check text and binary files for expected contents.

For more practical examples, check the included tests for Prolog standard conformance of built-in input/output predicates.

## Suppressing tested predicates output

Sometimes predicates being tested output text or binary data that, at best, clutters testing logs and, at worst, can interfere with parsing of test logs. If that output itself is not under testing, you can suppress it by using the goals `^^suppress_text_output` or `^^suppress_binary_output` at the beginning of the tests. For example:

    test(proxies_04, true(Color == yellow)) :-
        ^^suppress_text_output,
        {circle('#2', Color)}::print.

The `suppress_text_output/0` and `suppress_binary_output/0` predicates work by redirecting standard output to the operating-system null device. But the application may also output to e.g. `user_error` and other streams. If this output must also be suppressed, several alternatives are described next.

Output of expected warnings can be suppressed by turning off the corresponding linter flags. In this case, it is advisable to restrict the scope of the flag value changes as much as possible.

Output of expected compiler errors can be suppressed by defining suitable clauses for the `logtalk::message_hook/4` hook predicate. For example:

    :- multifile(logtalk::message_hook/4).
    :- dynamic(logtalk::message_hook/4).

    % ignore expected domain error
    logtalk::message_hook(compiler_error(_,_,error(domain_error(foo,bar),_)), error, core, _).

In this case, it is advisable to restrict the scope of the clauses as much as possible to exact exception terms. For the exact message terms, see the `core_messages` category source file. Defining this hook predicate can also be used to suppress all messages from a given component. For example:

    :- multifile(logtalk::message_hook/4).
    :- dynamic(logtalk::message_hook/4).

    logtalk::message_hook(_Message, _Kind, code_metrics, _Tokens).

Note that there’s no portable solution to suppress all output. However, several systems provide a `set_stream/2` or similar predicate that can be used for stream redirection. Check the documentation of the backend Prolog systems you’re using for details.

## Tests with timeout limits

There’s no portable way to call a goal with a timeout limit. However, some backend Prolog compilers provide this functionality:

- B-Prolog: `time_out/3` built-in predicate

- ECLiPSe: `timeout/3` and `timeout/7` library predicates

- XVM: `call_with_timeout/2-3` built-in predicates

- SICStus Prolog: `time_out/3` library predicate

- SWI-Prolog: `call_with_time_limit/2` library predicate

- Trealla Prolog: `call_with_time_limit/2` and `time_out/3` library predicates

- XSB: `timed_call/2` built-in predicate

- YAP: `time_out/3` library predicate

Logtalk provides a `timeout` portability library implementing a simple abstraction for those backend Prolog compilers.

The `logtalk_tester` automation script accepts a timeout option that can be used to set a limit per-test set.

## Setup and cleanup goals

A test object can define `setup/0` and `cleanup/0` goals. The `setup/0` predicate is called, when defined, before running the object unit tests. The `cleanup/0` predicate is called, when defined, after running all the object unit tests. The tests are skipped when the setup goal fails or throws an error. For example:

    cleanup :-
        this(This),
        object_property(This, file(_,Directory)),
        atom_concat(Directory, serialized_objects, File),
        catch(ignore(os::delete_file(File)), _, true).

Per test setup and cleanup goals can be defined using the `test/3` dialect and the `setup/1` and `cleanup/1` options. The test is skipped when the setup goal fails or throws an error. Note that a broken test cleanup goal doesn’t affect the test but may adversely affect any following tests. Variables in the setup and cleanup goals are shared with the test body.

## Test annotations

It’s possible to define per-unit and per-test annotations to be printed after the test results or when tests are skipped. This is particularly useful when some units or some unit tests may be run while still being developed. Annotations can be used to pass additional information to a user reviewing test results. By intercepting the unit test framework message printing calls (using the `message_hook/4` hook predicate), test automation scripts and integrating tools can also access these annotations.

Units can define a global annotation using the predicate `note/1`. To define per-test annotations, use the `test/3` dialect and the `note/1` option. For example, you can inform why a test is being skipped by writing:

    - test(foo_1, true, [note('Waiting for Deep Thought answer')]) :-
        ...

Another common use is to return the execution time of one of the test sub-goals. For example:

    test(foobar, true, [note(bar(seconds-Time))]) :-
        foo(...),
        benchmark(bar(...), Time).

Annotations are written, by default, between parentheses after and in the same line as the test results.

## Test execution times and memory usage

Individual test CPU and wall execution times (in seconds) are reported by default when running the tests. Total CPU and wall execution times for passed and failed tests are reported after the tests complete. Starting and ending date and time when running a set of tests is also reported by default. The `lgtunit` object also provides several public benchmarking predicates that can be useful for e.g. reporting test sub-goals execution times using either CPU or wall clocks. When running multi-threaded code, the CPU time may or may not include all threads CPU time depending on the backend.

Be aware that the accuracy of CPU and wall time depends of the backend. Accuracy can also be different between CPU and wall time (e.g. CPU time can have nanosecond accuracy with wall time only having millisecond accuracy).

Test memory usage is not reported by default due to the lack of a portable solution to access memory data. However, several backend Prolog systems provide a `statistics/2` or similar predicate that can be used for a custom solution. Depending on the system, individual keys may be provided for each memory area (heap, trail, atom table, …). Aggregating keys may also be provided. As a hypothetical example, assume you’re running Logtalk with a backend providing a `statistics/2` predicate with a `memory_used` key:

    test(ack_3, true(Result == 125), [note(memory-Memory)]) :-
        statistics(memory_used, Memory0),
        ack::ack(3, 4, Result),
        statistics(memory_used, Memory1),
        Memory is Memory1 - Memory0.

Consult the documentation of the backend Prolog systems for actual details.

## Working with test data files

Frequently, tests make use of test data files that are usually stored in the test set directory or in sub-directories. These data files are referenced using their relative paths. But to allow the tests to run independently of the Logtalk process current directory, the relative paths often must be expanded into an absolute path before being passed to the predicates being tested. The `file_path/2` protected predicate can be used in the test definitions to expand the relative paths. For example:

    % check that the encoding/1 option is accepted
    test(lgt_unicode_open_4_01, true) :-
        ^^file_path(sample_utf_8, Path),
        open(Path, write, Stream, [encoding('UTF-8')]),
        close(Stream).

The absolute path is computed relative to the path of self, i.e. relative to the path of the test object that received the message that runs the tests.

It’s also common for tests to create temporary files and directories that should be deleted after the tests completion. The `clean_file/1` and `clean_directory/1` protected predicates can be used for this purpose. For example, assuming that the tests create a `foo.txt` text file and a `tmp` directory in the same directory as the tests object:

    cleanup :-
        ^^clean_file('foo.txt'),
        ^^clean_directory('tmp').

Similar to the `file_path/2` predicate, relative paths are interpreted as relative to the path of the test object. This predicate also closes any open stream connected to the file before deleting it.

## Flaky tests

Flaky tests are tests that pass or fail non-deterministically, usually due to external conditions (e.g., computer or network load). Thus, flaky tests often don’t result from bugs in the code being tested itself but from test execution conditions that are not predictable. The `flaky/0` test option declares a test to be flaky. For example:

    test(foo, true, [flaky]) :-
        ...

For backwards compatibility, the `note/1` annotation can also be used to alert that a test failure is for a flaky test when its argument is an atom containing the sub-atom `flaky`.

The testing automation support outputs the text `[flaky]` when reporting failed flaky tests. Moreover, the `logtalk_tester` automation script will ignore failed flaky tests when setting its exit status.

## Mocking

Sometimes the code being tested performs complex tasks that are not feasible or desirable when running tests. For example, the code may perform a login operation requiring the user to provide a username and a password using some GUI widget. In this case, the tests may require the login operation to still be performed but using canned data (also simplifying testing automation). I.e. we want to mock (as in imitate) the login procedure. Ideally, this should be accomplished without requiring any changes to the code being tested. Logtalk provides two solutions that can be used for mocking: term-expansion and hot patching. A third solution is possible if the code we want to mock uses the message printing mechanism.

Using the term-expansion mechanism, we would define a hook object that expands the login predicate into a fact:

    :- object(mock_login,
        implements(expanding)).

        term_expansion((login(_, _) :- _), login(jdoe, test123)).

    :- end_object.

The tests driver file would then load the application object responsible for user management using this hook object:

    :- initialization((
        ...,
        logtalk_load(mock_login),
        logtalk_load(user_management, [hook(mock_login)]),
        ...
    )).

Using hot patching, we would define a complementing category patching the object that defines the login predicate:

    :- category(mock_login,
        complements(user_management)).

        login(jdoe, test123).

    :- end_category.

The tests driver file would then set the `complements` flag to `allow` and load the patch after loading application code:

    :- initialization((
        ...,
        set_logtalk_flag(complements, allow),
        logtalk_load(application),
        logtalk_load(mock_login),
        ...
    )).

There are pros and cons for each solution. Term-expansion works by defining hook objects that are used at compile-time, while hot patching happens at runtime. Complementing categories can also be dynamically created, stacked, and abolished. Hot patching disables static binding optimizations, but that’s usually not a problem as the code being tested is often compiled in debug mode to collect code coverage data. Two advantages of the term-expansion solution are that it allows defining conditions for expanding terms and goals and can replace both predicate definitions and predicate calls. Limitations in the current Prolog standards prevent patching callers to local predicates being patched. But often both solutions can be used, with the choice depending on code clarity and user preference. See the Handbook sections on term-expansion and hot patching for more details on these mechanisms.

In those cases where the code we want to mock uses the message printing mechanism, the solution is to intercept and rewrite the messages being printed and/or the questions being asked using the `logtalk::message_hook/4` and `logtalk::question_hook/6` hook predicates.

## Debugging messages in tests

Sometimes it is useful to write debugging or logging messages from tests when running them manually. But those messages are better suppressed when running the tests automated. A common solution is to use debug meta-messages. For example:

    :- uses(logtalk, [
        print_message(debug, my_app, Message) as dbg(Message)
    ]).

    test(some_test_id, ...) :-
        ...,
        dbg('Some intermediate value'-Value),
        ...,
        dbg([Stream]>>custom_print_goal(Stream, ...)),
        ...

The messages are only printed (and the user-defined printing goals are only called) when the `debug` flag is turned on. Note that this doesn’t require compiling the tests in debug mode: you simply toggle the flag to toggle the debug messages. Also note that the `print_message/3` goals are suppressed by the compiler when compiling with the `optimize` flag turned on.

## Debugging failed tests

Debugging of failed unit tests is simplified by using test assertions as the reason for the assertion failures is printed out. Thus, use preferably the `test/2-3` dialects with `true(Assertion)`, `deterministic(Assertion)`, `subsumes(Expected, Result)`, or `variant(Term1, Term2)` outcomes. If a test checks multiple assertions, you can use the predicate `assertion/2` in the test body. In the case of QuickCheck tests, the `v(true)` verbose option can be used to print the generated test case that failed if necessary.

If the assertion failures don’t provide enough information, you can use the `debugger` tool to debug failed unit tests. Start by compiling the unit test objects and the code being tested in debug mode. Load the debugger and trace the test that you want to debug. For example, assuming your tests are defined in a `tests` object and that the identifier of the test to be debugged is `test_foo`:

    | ?- logtalk_load(debugger(loader)).
    ...

    | ?- debugger::trace.
    ...

    | ?- tests::run(test_foo).
    ...

You can also compile the code and the tests in debug mode but without using the `hook/1` compiler option for the tests compilation. Assuming that the `context_switching_calls` flag is set to `allow`, you can then use the `(<<)/2` debugging control construct to debug the tests. For example, assuming that the identifier of the test to be debugged is `test_foo` and that you used the `test/1` dialect:

    | ?- logtalk_load(debugger(loader)).
    ...

    | ?- debugger::trace.
    ...

    | ?- tests<<test(test_foo).
    ...

In the more complicated cases, it may be worth defining `loader_debug.lgt` and `tester_debug.lgt` driver files that load code and tests in debug mode and also load the debugger.

When using the `logtalk_tester` automation script, some test sets can be reported as broken or crashed. The script creates a logs directory, by default named `logtalk_tester_logs`, in the same directory where the script was called. The log files, notably the `*.errors` files, often contain valuable information for debugging broken and crashed test sets.

## Code coverage

If you want entity predicate clause coverage information to be collected and printed, you will need to compile the entities that you’re testing using the flags `debug(on)` and `source_data(on)`. Be aware, however, that compiling in debug mode results in a performance penalty.

A single test object may include tests for one or more entities (objects, protocols, and categories). The entities being tested by a unit test object for which code coverage information should be collected must be declared using the `cover/1` predicate. For example, to collect code coverage data for the objects `foo` and `bar` include in the tests object the two clauses:

    cover(foo).
    cover(bar).

Code coverage is listed using the predicates clause indexes (counting from one). For example, using the `points` example in the Logtalk distribution:

    % point: default_init_option/1 - 2/2 - (all)
    % point: instance_base_name/1 - 1/1 - (all)
    % point: move/2 - 1/1 - (all)
    % point: position/2 - 1/1 - (all)
    % point: print/0 - 1/1 - (all)
    % point: process_init_option/1 - 1/2 - [1]
    % point: position_/2 - 0/0 - (all)
    % point: 7 out of 8 clauses covered, 87.500000% coverage

The numbers after the predicate indicators represent the clauses covered and the total number of clauses. E.g. for the `process_init_option/1` predicate, the tests cover 1 out of 2 clauses. After these numbers, we either get `(all)` telling us that all clauses are covered or a list of indexes for the covered clauses. E.g. only the first clause for the `process_init_option/1` predicate, `[1]`. Summary clause coverage numbers are also printed for entities and for clauses across all entities.

In the printed predicate clause coverage information, you may get a total number of clauses smaller than the covered clauses. This results from the use of dynamic predicates with clauses asserted at runtime. You may easily identify dynamic predicates in the results as their clauses often have an initial count equal to zero.

The list of indexes of the covered predicate clauses can be quite long. Some backend Prolog compilers provide a flag or a predicate to control the depth of printed terms that can be useful:

- CxProlog: `write_depth/2` predicate

- ECLiPSe: `print_depth` flag

- SICStus Prolog: `toplevel_print_options` flag

- SWI-Prolog 7.1.10 or earlier: `toplevel_print_options` flag

- SWI-Prolog 7.1.11 or later: `answer_write_options` flag

- Trealla Prolog: `answer_write_options` flag

- XSB: `set_file_write_depth/1` predicate

- XVM 3.2.0 or later: `answer_write_options` flag

- YAP: `write_depth/2-3` predicates

Code coverage is only available when testing Logtalk code. But Prolog modules can often be compiled as Logtalk objects and plain Prolog code may be wrapped in a Logtalk object. For example, assuming a `module.pl` module file, we can compile and load the module as an object by simply calling:

    | ?- logtalk_load(module).
    ...

The module exported predicates become object public predicates. For a plain Prolog file, say `plain.pl`, we can define a Logtalk object that wraps the code using an `include/1` directive:

    :- object(plain).

        :- include('plain.pl').

    :- end_object.

The object can also declare as public the top Prolog predicates to simplify writing the tests. In alternative, we can use the `object_wrapper_hook` provided by the `hook_objects` library:

    | ?- logtalk_load(hook_objects(loader)).
    ...

    | ?- logtalk_load(plain, [hook(object_wrapper_hook)]).
    ...

These workarounds may thus allow generating code coverage data also for Prolog code by defining tests that use the `(<<)/2` debugging control construct to call the Prolog predicates.

See also the section below on exporting code coverage results to XML files, which can be easily converted and published as e.g. HTML reports.

## Utility predicates

The `lgtunit` tool provides several public utility predicates to simplify writing unit tests and for general use:

- `variant(Term1, Term2)`
  To check when two terms are a variant of each other (e.g., to check expected test results against actual results when they contain variables).
- `assertion(Goal)`
  To generate an exception in case the goal argument fails or throws an error.
- `assertion(Description, Goal)`
  To generate an exception in case the goal argument fails or throws an error (the first argument allows assertion failures to be distinguished when using multiple assertions).
- `approximately_equal(Number1, Number2)`
  For number approximate equality using the `epsilon` arithmetic constant value.
- `approximately_equal(Number1, Number2, Epsilon)`
  For number approximate equality. Weaker equality than essential equality.
- `essentially_equal(Number1, Number2, Epsilon)`
  For number essential equality. Stronger equality than approximate equality.
- `tolerance_equal(Number1, Number2, RelativeTolerance, AbsoluteTolerance)`
  For number equality within tolerances.
- `Number1 =~= Number2`
  For number (or list of numbers) close equality (usually floating-point numbers).
- `benchmark(Goal, Time)`
  For timing a goal.
- `benchmark_reified(Goal, Time, Result)`
  Reified version of `benchmark/2`.
- `benchmark(Goal, Repetitions, Time)`
  For finding the average time to prove a goal.
- `benchmark(Goal, Repetitions, Clock, Time)`
  For finding the average time to prove a goal using a `cpu` or a `wall` clock.
- `deterministic(Goal)`
  For checking that a predicate succeeds without leaving a choice-point.
- `deterministic(Goal, Deterministic)`
  Reified version of the `deterministic/1` predicate.

The `assertion/1-2` predicates can be used in the body of tests where using two or more assertions is convenient or in the body of tests written using the `test/1`, `succeeds/1`, and `deterministic/1` dialects to help differentiate between the test goal and checking the test goal results and to provide more informative test failure messages.

When the assertion, benchmarking, and deterministic meta-predicates call a local predicate of the tests object, you must call them using an implicit or explicit message instead of using a super call. For example, to use an implicit message to call the `assertion/1-2` meta-predicates, add the following directive to the tests object:

    :- uses(lgtunit, [assertion/1, assertion/2]).

The reason this is required is that meta-predicates goals arguments are always called in the context of the sender, which would be the `lgtunit` object in the case of a `(^^)/2` call (as it preserves both self and sender and the tests are internally run by a message sent from the `lgtunit` object to the tests object).

As the `benchmark/2-4` predicates are meta-predicates, turning on the `optimize` compiler flag is advised to avoid runtime compilation of the meta-argument, which would add an overhead to the timing results. But this advice conflicts with collecting code coverage data, which requires compilation in debug mode. The solution is to use separate test objects for benchmarking and for code coverage. Note that the CPU and wall execution times (in seconds) for each individual test are reported by default when running the tests.

The `(=~=)/2` predicate is typically used by adding the following directive to the object (or category) calling it:

    :- uses(lgtunit, [
        op(700, xfx, =~=), (=~=)/2
    ]).

Consult the `lgtunit` object API documentation for more details on these predicates.

## Exporting test results

### xUnit XML format

To output test results in the xUnit XML format (from JUnit; see e.g. https://github.com/windyroad/JUnit-Schema or https://llg.cubic.org/docs/junit/), simply load the `xunit_output.lgt` file before running the tests. This file defines an object, `xunit_output`, that intercepts and rewrites unit test execution messages, converting them to the xUnit XML format.

To export the test results to a file using the xUnit XML format, simply load the `xunit_report.lgt` file before running the tests. A file named `xunit_report.xml` will be created in the same directory as the object defining the tests. When running a set of test suites as a single unified suite (using the `run_test_sets/1` predicate), the single xUnit report is created in the directory of the first test suite object in the set.

To use the xUnit.net v2 XML format (https://xunit.net/docs/format-xml-v2), load either the `xunit_net_v2_output.lgt` file or the `xunit_net_v2_report.lgt` file.

When using the `logtalk_tester` automation script, use either the `-f xunit` option or the `-f xunit_net_v2` option to generate the `xunit_report.xml` files in the test set directories.

There are several third-party xUnit report converters that can generate HTML files for easy browsing. For example:

- https://allurereport.org/docs/ (supports multiple reports)

- https://github.com/Zir0-93/xunit-to-html (supports multiple test sets in a single report)

- https://www.npmjs.com/package/xunit-viewer

- https://github.com/JatechUK/NUnit-HTML-Report-Generator

- https://plugins.jenkins.io/xunit

### TAP output format

To output test results in the TAP (Test Anything Protocol) format, simply load the `tap_output.lgt` file before running the tests. This file defines an object, `tap_output`, that intercepts and rewrites unit test execution messages, converting them to the TAP output format.

To export the test results to a file using the TAP (Test Anything Protocol) output format, load instead the `tap_report.lgt` file before running the tests. A file named `tap_report.txt` will be created in the same directory as the object defining the tests.

When using the `logtalk_tester` automation script, use the `-f tap` option to generate the `tap_report.xml` files in the test set directories.

When using the `test/3` dialect with the TAP format, a `note/1` option whose argument is an atom starting with a `TODO` or `todo` word results in a test report with a TAP TODO directive.

When running a set of test suites as a single unified suite, the single TAP report is created in the directory of the first test suite object in the set.

There are several third-party TAP report converters that can generate HTML files for easy browsing. For example:

- https://github.com/Quobject/tap-to-html

- https://plugins.jenkins.io/tap/

### CTRF JSON format

To output test results in the CTRF (Common Test Report Format) JSON format, simply load the `ctrf_output.lgt` file before running the tests. This file defines an object, `ctrf_output`, that intercepts and rewrites unit test execution messages, converting them to the CTRF JSON format.

To export the test results to a file using the CTRF JSON format, load instead the `ctrf_report.lgt` file before running the tests. A file named `ctrf_report.json` will be created in the same directory as the object defining the tests.

When using the `logtalk_tester` automation script, use the `-f ctrf` option to generate the `ctrf_report.json` files in the test set directories.

When running a set of test suites as a single unified suite, the single CTRF report is created in the directory of the first test suite object in the set.

### Subunit formats

To output test results in the Subunit v1 text streaming format, simply load the `subunit_v1_output.lgt` file before running the tests. This file defines an object, `subunit_v1_output`, that intercepts and rewrites unit test execution messages, converting them to a Subunit text stream.

To output test results in the Subunit v2 binary streaming format, load the `subunit_v2_output.lgt` file before running the tests. This file defines an object, `subunit_v2_output`, that intercepts and rewrites unit test execution messages, converting them to Subunit v2 binary packets.

These adapters write directly to the current output stream and are intended for piping test execution output to tools that parse Subunit streams.

To export test results to files using Subunit streaming formats, load instead the `subunit_v1_report.lgt` file (Subunit text stream) or the `subunit_v2_report.lgt` file (Subunit v2 binary stream) before running the tests. Files named `subunit_v1_report.txt` and `subunit_v2_report.bin` are created in the same directory as the object defining the tests.

## Generating Allure reports

A `logtalk_allure_report.pl` Bash shell script and a `logtalk_allure_report.ps1` PowerShell script are provided for generating Allure reports (version 2.26.0 or later required). This requires exporting test results in xUnit XML format. A simple usage example (assuming a current directory containing tests):

    $ logtalk_tester -p gnu -f xunit
    $ logtalk_allure_report
    $ allure open

The `allure open` command accepts `--host` and `--port` arguments in case their default values are not suitable (e.g., when running Logtalk in a remote host over a SSH connection).

The `logtalk_allure_report` script supports command-line options to pass the tests directory (i.e., the directory where the `logtalk_tester` script was run), the directory where to collect all the xUnit report files for generating the report, the directory where the report is to be saved, and the report title (see the script man page or type `logtalk_allure_report -h`). The script also supports saving the history of past test runs. In this case, a persistent location for both the results and report directories must be used.

It’s also possible to use the script just to collect the xUnit report files generated by `lgtunit` and delegate the actual generation of the report to e.g. an Allure Docker container or to a Jenkins plug-in. Two examples are:

- https://github.com/fescobar/allure-docker-service

- https://plugins.jenkins.io/allure-jenkins-plugin/

In this case, we would use the `logtalk_allure_report` script option to only perform the preprocessing step:

    $ logtalk_allure_report -p

The script also supports passing environment pairs, which are displayed in the generated Allure reports in the environment pane. This feature can be used to pass e.g. the backend name and the backend version or git commit hash. The option syntax differs, however, between the two scripts. For example, using the Bash script:

    $ logtalk_allure_report -- Backend='GNU Prolog' Version=1.5.0

Or:

    $ logtalk_allure_report -- Project='Deep Thought' Commit=`git rev-parse --short HEAD`

In the case of the PowerShell script, the pairs are passed comma separated inside a string:

    PS> logtalk_allure_report -e "Backend='GNU Prolog',Version=1.5.0"

Or:

    PS> logtalk_allure_report -e "Project='Deep Thought',Commit=bf166b6"

To show test run trends in the report (e.g., when running the tests for each application source code commit), save the processed test results and the report data to permanent directories. For example:

    $ logtalk_allure_report \
      -i "$HOME/my_project/allure-results" \
      -o "$HOME/my_project/allure-report"
    $ allure open "$HOME/my_project/allure-report"

Note that (in Allure 2.x) the single file option is not compatible with run trends.

The generated reports can include links to the tests source code. This requires using the `logtalk_tester` shell script option that allows passing the base URL for those links. This option needs to be used together with the option to suppress the tests directory prefix so that the links can be constructed by appending the tests file relative path to the base URL. For example, assuming that you want to generate a report for the tests included in the Logtalk distribution when using the GNU Prolog backend:

    $ cd $LOGTALKUSER
    $ logtalk_tester \
      -p gnu \
      -f xunit \
      -s "$LOGTALKUSER" \
      -u "https://github.com/LogtalkDotOrg/logtalk3/tree/3e4ea295986fb09d0d4aade1f3b4968e29ef594e"

The use of a git hash in the base URL ensures that the generated links will always show the exact versions of the tests that were run. The links include the line number for the tests in the test files (assuming that the git repo is stored in a BitBucket, GitHub, or GitLab server). But note that not all supported backends provide accurate line numbers.

It’s also possible to generate single-file reports. For example:

    $ logtalk_allure_report -s -t "My Amazing Tests Report"

There are some caveats when generating Allure reports that users must be aware of. First, Allure expects test names to be unique across different tests sets. If there are two tests with the same name in two different test sets, only one of them will be reported. Second, when using the `xunit` format, dates are reported as MM/DD/YYYY. Finally, when using the `xunit_net_v2` format, tests are reported in a random order instead of their run order, and dates are displayed as “unknown” in the overview page.

## Exporting code coverage results

The `lgtunit` tool can export code coverage stats in three formats by loading the corresponding report object before running tests:

- `coverage_report.lgt` for a simple XML report (`coverage_report.xml`)

- `cobertura_report.lgt` for a Cobertura XML report (`cobertura.xml`)

- `lcov_report.lgt` for an LCOV report (`lcov.info`)

When using the `logtalk_tester` automation scripts, select the report format using the `-c` option with one of the values `xml`, `cobertura`, or `lcov` (or `none` to disable code coverage reports).

### Simple XML format

To export code coverage results in simple XML format, load the `coverage_report.lgt` file before running the tests. A file named `coverage_report.xml` will be created in the same directory as the object defining the tests.

The XML file can be opened in most web browsers (with the notorious exception of Google Chrome) by copying to the same directory the `coverage_report.dtd` and `coverage_report.xsl` files found in the `tools/lgtunit` directory (when using the `logtalk_tester` script, these two files are copied automatically). In alternative, an XSLT processor can be used to generate an XHTML file instead of relying on a web browser for the transformation. For example, using the popular `xsltproc` processor:

    $ xsltproc -o coverage_report.html coverage_report.xml

On Windows operating-systems, this processor can be installed using e.g. Chocolatey. On a POSIX operating-systems (e.g., Linux, macOS, …) use the system package manager to install it if necessary.

The coverage report can include links to the source code when hosted on Bitbucket, GitHub, or GitLab. This requires passing the base URL as the value for the `url` XSLT parameter. The exact syntax depends on the XSLT processor, however. For example:

    $ xsltproc \
      --stringparam url https://github.com/LogtalkDotOrg/logtalk3/blob/master \
      -o coverage_report.html coverage_report.xml

Note that the base URL should preferably be a permanent link (i.e., it should include the commit SHA1) so that the links to source code files and lines remain valid if the source code is later updated. It’s also necessary to suppress the local path prefix in the generated `coverage_report.xml` file. For example:

    $ logtalk_tester -c xml -s $HOME/logtalk/

Alternatively, you can pass the local path prefix to be suppressed to the XSLT processor (note that the `logtalk_tester` script suppresses the `$HOME` prefix by default):

    $ xsltproc \
      --stringparam prefix logtalk/ \
      --stringparam url https://github.com/LogtalkDotOrg/logtalk3/blob/master \
      -o coverage_report.html coverage_report.xml

If you are using Bitbucket, GitHub, or GitLab hosted on your own servers, the `url` parameter may not contain a `bitbucket`, `github`, or `gitlab` string. In this case, you can use the XSLT parameter `host` to indicate which service you are running.

### Cobertura XML format

To export code coverage results in Cobertura XML format, load the `cobertura_report.lgt` file before running the tests. A file named `cobertura.xml` will be created in the same directory as the object defining the tests.

When using the `logtalk_tester` automation scripts, use the option:

    $ logtalk_tester -c cobertura

### LCOV format

To export code coverage results in LCOV format, load the `lcov_report.lgt` file before running the tests. A file named `lcov.info` will be created in the same directory as the object defining the tests.

When using the `logtalk_tester` automation scripts, use the option:

    $ logtalk_tester -c lcov

## Automatically creating bug reports at issue trackers

To automatically create bug report issues for failed tests in GitHub or GitLab servers, see the `issue_creator` tool.

## Minimizing test results output

To minimize the test results output, simply load the `minimal_output.lgt` file before running the tests. This file defines an object, `minimal_output`, that intercepts and summarizes the unit test execution messages.

## Help with warnings

Load the `tutor` tool to get help with selected warnings printed by the `lgtunit` tool.

## Known issues

Deterministic unit tests are currently not available when using Quintus Prolog as it lacks built-in support that cannot be sensibly defined in Prolog.

Parameter variables (`_VariableName_`) cannot currently be used in the definition of the `condition/1`, `setup/1`, and `cleanup/1` test options when using the `test/3` dialect. For example, the following condition will not work:

    test(some_id, true, [condition(_ParVar_ == 42)]) :-
        ...

The workaround is to define an auxiliary predicate called from those options. For example:

    test(check_xyz, true, [condition(xyz_condition)]) :-
        ...

    xyz_condition :-
        _ParVar_ == 42.