Prolog Unit Tests
Jan Wielemaker
University of Amsterdam
VU University Amsterdam
The Netherlands
E-mail: jan@swi-prolog.org
Abstract
This document describes a Prolog unit-test framework. This framework was initially developed for SWI-Prolog. The current version also runs on SICStus Prolog, providing a portable testing framework. See section 9.1.

# Table of Contents

## 1 Introduction

There is really no excuse not to write tests!

Automatic testing of software during development is probably the most important Quality Assurance measure. Tests can validate the final system, which is nice for your users. However, most (Prolog) developers forget that it is not just a burden during development.

- Tests document how the code is supposed to be used.
- Tests can validate claims you make on the Prolog implementation. Writing a test makes the claim explicit.
- Tests avoid big applications saying‘No' after modifications. This saves time during development, and it saves a lot of time if you must return to the application a few years later or you must modify and debug someone else's application.

## 2 A Unit Test box

Tests are written in pure Prolog and enclosed within the directives begin_tests/1,2 and end_tests/1. They can be embedded inside a normal source module, or be placed in a separate test-file that loads the files to be tested. Code inside a test box is normal Prolog code. The entry points are defined by rules using the head `test(Name)` or `test(Name, Options)`, where Name is a ground term and Options is a list describing additional properties of the test. Here is a very simple example:

    :- begin_tests(lists).
    :- use_module(library(lists)).

    test(reverse) :-
            reverse([a,b], [b,a]).

    :- end_tests(lists).

The optional second argument of the test-head defines additional processing options. Defined options are:

blocked(+Reason:atom)
  The test is currently disabled. Tests are flagged as blocked if they cannot be run for some reason. E.g. they crash Prolog, they rely on some service that is not available, they take too much resources, etc. Tests that fail but do not crash, etc. should be flagged using `fixme(Fixme)`.
fixme(+Reason:atom)
  Similar to `blocked(Reason)`, but the test it executed anyway. If it fails, a ``-`` is printed instead of the ``.`` character. If it passes a ``+`` and if it passes with a choicepoint, ``!``. A summary is printed at the end of the test run and the goal `test_report(fixme)` can be used to get details.
condition(:Goal)
  Pre-condition for running the test. If the condition fails the test is skipped. The condition can be used as an alternative to the `setup` option. The only difference is that failure of a condition skips the test and is considered an error when using the `setup` option.
cleanup(:Goal)
  Goal is always called after completion of the test-body, regardless of whether it fails, succeeds or throws an exception. This option or call_cleanup/2 must be used by tests that require side-effects that must be reverted after the test completes. Goal may share variables with the test body.

      create_file(Tmp) :-
              tmp_file(plunit, Tmp),
              open(Tmp, write, Out),
              write(Out, 'hello(World).\n'),
              close(Out).

      test(read, [ setup(create_file(Tmp)),
                   cleanup(delete_file(Tmp))
                 ]) :-
              read_file_to_terms(Tmp, Terms, []),
              Term = hello(_).

setup(:Goal)
  Goal is run before the test-body. Typically used together with the `cleanup` option to create and destroy the required execution environment.
forall(:Generator)
  Run the same test for each solution of Generator. Each run invokes the setup and cleanup handlers. This can be used to run the same test with different inputs. If an error occurs, the test is reported as `name (forall bindings =` <vars> `)`, where <vars> indicates the bindings of variables in Generator.
true(AnswerTerm Cmp Value)
  Body should succeed deterministically. If a choicepoint is left open, a warning is printed to STDERR ("Test succeeded with choicepoint"). That warning can be suppressed by adding the `nondet(k)`eyword. AnswerTerm is compared to Value using the comparison operator Cmp. Cmp is typically one of =/2, ==/2, =:=/2 or =@=/2,1The =@= predicate (denoted structural equivalence) is the same as variant/2 in SICStus. but any test can be used. This is the same as inserting the test at the end of the conjunction, but it allows the test engine to distinguish between failure of copy_term/2 and producing the wrong value. Multiple variables must be combined in an arbitrary compound term. E.g. `A1-A2 == v1-v2`

      test(copy, [ true(Copy =@= hello(X,X))
                 ]) :-
              copy_term(hello(Y,Y), Copy).

AnswerTerm Cmp Value(E)
  quivalent to `true(AnswerTerm Cmp Value)` if Cmp is one of the comparison operators given above.
fail
  Body must fail.
throws(Error)
  Body must throw Error. The thrown error term is matched against term Error using `subsumes_term(Error, ThrownError)`. I.e., the thrown error must be more specific than the specified Error. See subsumes_term/2.
error(Error)
  Body must throw `error(Error, _Context)`. See keyword `throws` (as well as predicate throw/1 and library(error)) for details.
all(AnswerTerm Cmp Instances)
  Similar to `true(AnswerTerm Cmp Values)`, but used for non-deterministic predicates. Each element is compared using Cmp. Order matters. For example:

      test(or, all(X == [1,2])) :-
              ( X = 1 ; X = 2 ).

set(AnswerTerm Cmp Instances)
  Similar to `all(AnswerTerm Cmp Instances)`, but before testing both the bindings of AnswerTerm and Instances are sorted using sort/2. This removes duplicates and places both sets in the same order.2The result is only well-defined of Cmp is ``==``.
nondet
  If this keyword appears in the option list, non-deterministic success of the body is not considered an error.
sto(Terms)
  Declares that executing body is subject to occurs-check (STO). The test is executed with Terms. Terms is either `rational_trees` or `finite_trees`. STO programs are not portable between different kinds of terms. Only programs not subject to occurs-check (NSTO) are portable3See 7.3.3 of ISO/IEC 13211-1 PROLOG: Part 1 - General Core, for a detailed discussion of STO and NSTO. Fortunately, most practical programs are NSTO. Writing tests that are STO is still useful to ensure the robustness of a predicate. In case sto4 and sto5 below, an infinite list (a rational tree) is created prior to calling the actual predicate. Ideally, such cases produce a type error or fail silently.

      test(sto1, [sto(rational_trees)]) :-
              X=s(X).
      test(sto2, [sto(finite_trees),fail]) :-
              X=s(X).
      test(sto3, [sto(rational_trees), fail]) :-
              X=s(X), fail.
      test(sto4, [sto(rational_trees),error(type_error(list,L))]) :-
              L = [_|L], length(L,_).
      test(sto5, [sto(rational_trees),fail]) :-
              L = [_|L], length(L,3).

  Programs that depend on STO cases tend to be inefficient, even incorrect, are hard to understand and debug, and terminate poorly. It is therefore advisable to avoid STO programs whenever possible.

  SWI's Prolog flag occurs_check must not be modified within plunit tests.

### 2.1 Test Unit options

begin_tests(+Name)
  Start named test-unit. Same as `begin_tests(Name, [])`.
begin_tests(+Name, +Options)
  Start named test-unit with options. Options provide conditional processing, setup and cleanup similar to individual tests (second argument of test/2 rules).

  Defined options are:

  blocked(+Reason)
    Test-unit has been blocked for the given Reason.
  condition(:Goal)
    Executed before executing any of the tests. If Goal fails, the test of this unit is skipped.
  setup(:Goal)
    Executed before executing any of the tests.
  cleanup(:Goal)
    Executed after completion of all tests in the unit.
  sto(+Terms)
    Specify default for subject-to-occurs-check mode. See section 2 for details on the sto option.

### 2.2 Writing the test body

The test-body is ordinary Prolog code. Without any options, the body must be designed to succeed deterministically. Any other result is considered a failure. One of the options `fail`, `true`, `throws`, `all` or `set` can be used to specify a different expected result. See section 2 for details. In this section we illustrate typical test-scenarios by testing SWI-Prolog built-in and library predicates.

#### 2.2.1 Testing deterministic predicates

Deterministic predicates are predicates that must succeed exactly once and, for well behaved predicates, leave no choicepoints. Typically they have zero or more input- and zero or more output arguments. The test goal supplies proper values for the input arguments and verifies the output arguments. Verification can use test-options or be explicit in the body. The tests in the example below are equivalent.

    test(add) :-
            A is 1 + 2,
            A =:= 3.

    test(add, [true(A =:= 3)]) :-
            A is 1 + 2.

The test engine verifies that the test-body does not leave a choicepoint. We illustrate that using the test below:

    test(member) :-
            member(b, [a,b,c]).

Although this test succeeds, member/2 leaves a choicepoint which is reported by the test subsystem. To make the test silent, use one of the alternatives below.

    test(member) :-
            member(b, [a,b,c]), !.

    test(member, [nondet]) :-
            member(b, [a,b,c]).

#### 2.2.2 Testing semi-deterministic predicates

Semi-deterministic predicates are predicates that either fail or succeed exactly once and, for well behaved predicates, leave no choicepoints. Testing such predicates is the same as testing deterministic predicates. Negative tests must be specified using the option `fail` or by negating the body using `\+/1`.

    test(is_set) :-
            \+ is_set([a,a]).

    test(is_set, [fail]) :-
            is_set([a,a]).

#### 2.2.3 Testing non-deterministic predicates

Non-deterministic predicates succeed zero or more times. Their results are tested either using findall/3 or setof/3 followed by a value-check or using the `all` or `set` options. The following are equivalent tests:

    test(member) :-
            findall(X, member(X, [a,b,c]), Xs),
            Xs == [a,b,c].

    test(member, all(X == [a,b,c])) :-
            member(X, [a,b,c]).

#### 2.2.4 Testing error conditions

Error-conditions are tested using the option `throws(Error)` or by wrapping the test in a catch/3. The following tests are equivalent:

    test(div0) :-
         catch(A is 1/0, error(E, _), true),
         E =@= evaluation_error(zero_divisor).

    test(div0, [error(evaluation_error(zero_divisor))]) :-
         A is 1/0.

#### 2.2.5 One body with multiple tests using assertions

PlUnit is designed to cooperate with the assertion/1 test provided by library(debug).4This integration was suggested by Günter Kniesel. If an assertion fails in the context of a test, the test framework reports this and considers the test failed, but does not trap the debugger. Using assertion/1 in the test-body is attractive for two scenarios:

- Confirm that multiple claims hold. Where multiple claims about variable bindings can be tested using the == option in the test header, arbitrary boolean tests, notably about the state of the database, are harder to combine. Simply adding them in the body of the test has two disadvantages: it is less obvious to distinguish the tested code from the test and if one of the tests fails there is no easy way to find out which one.
- Testing‘scenarios' or sequences of actions. If one step in such a sequence fails there is again no easy way to find out which one. By inserting assertions into the sequence this becomes obvious.

Below is a simple example, showing two failing assertions. The first line of the failure message gives the test. The second reports the location of the assertion.5If known. The location is determined by analysing the stack. The second failure shows a case where this does not work because last-call optimization has already removed the context of the test-body. If the assertion call originates from a different file this is reported appropriately. The last line gives the actually failed goal.

    :- begin_tests(test).

    test(a) :-
            A is 2^3,
            assertion(float(A)),
            assertion(A == 9).

    :- end_tests(test).

    ?- run_tests.
    % PL-Unit: test
    ERROR: /home/jan/src/pl-devel/linux/t.pl:5:
            test a: assertion at line 7 failed
            Assertion: float(8)
    ERROR: /home/jan/src/pl-devel/linux/t.pl:5:
            test a: assertion failed
            Assertion: 8==9
    . done
    % 2 assertions failed

## 3 Using separate test files

Test-units can be embedded in normal Prolog source-files. Alternatively, tests for a source-file can be placed in another file alongside the file to be tested. Test files use the extension `.plt`. The predicate load_test_files/1 can load all files that are related to source-files loaded into the current project.

## 4 Running the test-suite

### 4.1 Running the test suite from Prolog

To run tests from the Prolog prompt, first load the program and then run run_tests/0 or `run_tests(+Unit)`.

run_tests
  Run all test-units.
run_tests(+Spec)
  Run only the specified tests. Spec can be a list to run multiple tests. A single specification is either the name of a test unit or a term <Unit>:<Tests>, running only the specified test. <Tests> is either the name of a test or a list of names. Running particular tests is particularly useful for tracing a test:6Unfortunately the body of the test is called through meta-calling, so it cannot be traced. The called user-code can be traced normally though.

      ?- gtrace, run_tests(lists:member).

To identify nonterminating tests, interrupt the looping process with Control-C. The test name and location will be displayed.

### 4.2 Running the test suite from the command line

To run a file's tests from the command line, run the following command, replacing `your/file.pl` with the path to your file.

    swipl -g run_tests -t halt your/file.pl

Prolog will (1) load the file you specify, as well as any modules it depends on; (2) run all tests in those files, and (3) exit with status 0 or 1 depending on whether the test suite succeeds or fails.

If you want to test multiple files, you can pass multiple `..pl` files.

## 5 Tests and production systems

Most applications do not want the test-suite to end up in the final application. There are several ways to achieve this. One is to place all tests in separate files and not to load the tests when creating the production environment. Alternatively, use the directive below before loading the application.

    :- set_test_options([load(never)]).

## 6 Controlling the test suite

set_test_options(+Options)
  Defined options are:
  load(+Load)
    Determines whether or not tests are loaded. When `never`, everything between begin_tests/1 and end_tests/1 is simply ignored. When `always`, tests are always loaded. Finally, when using the default value `normal`, tests are loaded if the code is not compiled with optimisation turned on.
  run(+Run)
    Specifies when tests are run. Using `manual`, tests can only be run using run_tests/0 or run_tests/1. Using `make`, tests will be run for reloaded files, but not for files loaded the first time. Using `make(all)` make/0 will run all test-suites, not only those that belong to files that are reloaded.
  silent(+Bool)
    When `true` (default is `false`), send informational messages using the‘silent' level. In practice this means there is no output except for errors.
  sto(+Bool)
    When `true` (default `false`), assume tests are not subject to occurs check (non-STO) and verify this if the Prolog implementation supports testing this.
load_test_files(+Options)
  Load `.plt` test-files that belong to the currently loaded sources.
running_tests
  Print all currently running tests to the terminal. It can be used to find running thread in multi-threaded test operation or find the currently running test if a test appears to be blocking.
test_report(+What)
  Print report on the executed tests. What defines the type of report. Currently this only supports `fixme`, providing details on how the fixme-flagged tests proceeded.

## 7 Auto-generating tests

Prolog is an interactive environment. Where users of non-interactive systems tend to write tests as code, Prolog developers tend to run queries interactively during development. This interactive testing is generally faster, but the disadvantage is that the tests are lost at the end of the session. The test-wizard tries to combine the advantages. It collects toplevel queries and saves them to a specified file. Later, it extracts these queries from the file and locates the predicates that are tested by the queries. It runs the query and creates a test clause from the query.

Auto-generating test cases is experimentally supported through the library `library(test_wizard)`. We briefly introduce the functionality using examples. First step is to log the queries into a file. This is accomplished with the commands below. `Queries.pl` is the name in which to store all queries. The user can choose any filename for this purpose. Multiple Prolog instances can share the same name, as data is appended to this file and write is properly locked to avoid file corruption.

    :- use_module(library(test_wizard)).
    :- set_prolog_flag(log_query_file, 'Queries.pl').

Next, we will illustrate using the library by testing the predicates from library `library(lists)`. To generate test cases we just make calls on the terminal. Note that all queries are recorded and the system will select the appropriate ones when generating the test unit for a particular module.

    ?- member(b, [a,b]).
    Yes
    ?- reverse([a,b], [b|A]).
    A = [a] ;
    No

Now we can generate the test-cases for the module list using make_tests/3:

    ?- make_tests(lists, 'Queries.pl', current_output).
    :- begin_tests(lists).

    test(member, [nondet]) :-
            member(b, [a, b]).
    test(reverse, [true(A==[a])]) :-
            reverse([a, b], [b|A]).

    :- end_tests(lists).

## 8 library(test_cover): Clause coverage analysis

bug
  Relies heavily on SWI-Prolog internals. We have considered using a meta-interpreter for this purpose, but it is nearly impossible to do 100% complete meta-interpretation of Prolog. Example problem areas include handling cuts in control-structures and calls from non-interpreted meta-predicates.

The purpose of this module is to find which part of the program has been used by a certain goal. Usage is defined in terms of clauses for which the head unification succeeded. For each clause we count how often it succeeded and how often it failed. In addition we track all call sites, creating goal-by-goal annotated clauses.

This module relies on the SWI-Prolog tracer hooks. It modifies these hooks and collects the results, after which it restores the debugging environment. This has some limitations:

- The performance degrades significantly (about 10 times)
- It is not possible to use the debugger during coverage analysis
- The cover analysis tool is currently not thread-safe.

The result is represented as a list of clause-references. As the references to clauses of dynamic predicates cannot be guaranteed, these are omitted from the result.

[semidet]show_coverage(:Goal)
[semidet]show_coverage(:Goal, +Options)
[semidet]show_coverage(:Goal, +Modules:list(atom))
  Report on coverage by Goal. Goal is executed as in once/1. Options processed:
  modules(+Modules)
    Provide a detailed report on Modules. For backwards compatibility this is the same as providing a list of modules in the second argument.
  annotate(+Bool)
    Create an annotated file for the detailed results. This is implied if the `ext` or `dir` option are specified.
  ext(+Ext)
    Extension to use for the annotated file. Default is‘.cov`.
  dir(+Dir)
    Dump the annotations in the given directory. If not given, the annotated files are created in the same directory as the source file. Each clause that is related to a physical line in the file is annotated with one of:

    > ### | Clause was never executed.
    > ++N | Clause was entered N times and always succeeded
    > --N | Clause was entered N times and never succeeded
    > +N-M | Clause has succeeded N times and failed M times
    > +N*M | Clause was entered N times and succeeded M times

    All call sites are annotated using the same conventions, except that `---` is used to annotate subgoals that were never called.

  line_numbers(Boolean)
    If `true` (default), add line numbers to the annotated file.
  color(Boolean)
    Controls using ANSI escape sequences to color the output in the annotated source. Default is `true`.
[semidet,multifile]report_hook(+Succeeded, +Failed)
  This hook is called after the data collection. It is passed a list of objects that have succeeded as well as a list of objects that have failed. The objects are one of
  ClauseRef
    The specified clause
  call_site(ClauseRef, PC, PI)
    A call was make in ClauseRef at the given program counter to the predicate indicated by PI.

## 9 Portability of the test-suite

One of the reasons to have tests is to simplify migrating code between Prolog implementations. Unfortunately creating a portable test-suite implies a poor integration into the development environment. Luckily, the specification of the test-system proposed here can be ported quite easily to most Prolog systems sufficiently compatible to SWI-Prolog to consider porting your application. Most important is to have support for term_expansion/2.

In the current system, test units are compiled into sub-modules of the module in which they appear. Few Prolog systems allow for sub-modules and therefore ports may have to fall-back to inject the code in the surrounding module. This implies that support predicates used inside the test unit should not conflict with predicates of the module being tested.

### 9.1 PlUnit on SICStus

The directory of `plunit.pl` and `swi.pl` must be in the `library` search-path. With PLUNITDIR replaced accordingly, add the following into your `.sicstusrc` or `sicstus.ini`.

    :- set_prolog_flag(language, iso). % for maximal compatibility
    library_directory('PLUNITDIR').

The current version runs under SICStus 3. Open issues:

- Some messages are unformatted because SICStus 3 reports all ISO errors as instantiation errors.

- Only `plunit.pl`. Both coverage analysis and the test generation wizard currently require SWI-Prolog.

- The `load` option `normal` is the same as `always`. Use `set_test_options(load, never)` to avoid loading the test suites.

- The `run` option is not supported.

- Tests are loaded into the enclosing module instead of a separate test module. This means that predicates in the test module must not conflict with the enclosing module, nor with other test modules loaded into the same module.

## 10 Motivation of choices

### Easy to understand and flexible

There are two approaches for testing. In one extreme the tests are written using declarations dealing with setup, cleanup, running and testing the result. In the other extreme a test is simply a Prolog goal that is supposed to succeed. We have chosen to allow for any mixture of these approaches. Written down as test/1 we opt for the simple succeeding goal approach. Using options to the test the user can choose for a more declarative specification. The user can mix both approaches.

The body of the test appears at the position of a clause-body. This simplifies identification of the test body and ensures proper layout and colouring support from the editor without the need for explicit support of the unit test module. Only clauses of test/1 and test/2 may be marked as non-called in environments that perform cross-referencing.

# Index

?
assertion/1
  2.2.5 2.2.5
begin_tests/1
  2 6
begin_tests/2
call_cleanup/2
  2
catch/3
  2.2.4
copy_term/2
  2
end_tests/1
  2 6
findall/3
  2.2.3
load_test_files/1
  3
make/0
  6
make_tests/3
  7
member/2
  2.2.1
report_hook/2
run_tests/0
  4.1 6
run_tests/1
  6
running_tests/0
set_test_options/1
setof/3
  2.2.3
show_coverage/1
show_coverage/2
sort/2
  2
subsumes_term/2
  2
term_expansion/2
  9
test/1
  10 10
test/2
  2.1 10
test_report/1
throw/1
  2
variant/2
  2