Tests are written in pure Prolog and enclosed within the directives
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
test(Name, Options), where Name is a ground term
Options is a list describing additional properties of the
test. Here is a very simple example:
The optional second argument of the test-head defines additional
processing options. Defined options are:
- 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
- Similar to
blocked(Reason), but the test it executed
anyway. If it fails, a
is printed instead of
character. If it passes a
and if it passes with a choicepoint,
summary is printed at the end of the test run and the goal
test_report(fixme) can be used to get details.
- Pre-condition for running the test. If the condition fails the test is
skipped. The condition can be used as an alternative to the
option. The only difference is that failure of a condition skips the
test and is considered an error when using the
- 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
open(Tmp, write, Out),
test(read, [ setup(create_file(Tmp)),
read_file_to_terms(Tmp, Terms, ),
Term = hello(_).
- Goal is run before the test-body. Typically used together
cleanup option to create and destroy the required
- 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>
<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
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))
- AnswerTerm Cmp Value(E)
- quivalent to
true(AnswerTerm Cmp Value) if Cmp
is one of the comparison operators given above.
- Body must fail.
- Body must throw Error. The thrown error term is matched
against term Error using
ThrownError). I.e., the thrown error must be more specific than
the specified Error. See
- Body must throw
error(Error, _Context). See keyword
(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
removes duplicates and places both sets in the same order.2The
result is only well-defined of Cmp is
- If this keyword appears in the option list, non-deterministic success of
the body is not considered an error.
- Run the test in a particular occurs check mode. Mode is one of
error. See the Prolog flag occurs_check
- Start named test-unit. Same as
- Start named test-unit with options. Options provide conditional
processing, setup and cleanup similar to individual tests (second
argument of test/2
Defined options are:
- Test-unit has been blocked for the given Reason.
- Executed before executing any of the tests. If Goal fails,
the test of this unit is skipped.
- Executed before executing any of the tests.
- Executed after completion of all tests in the unit.
- Specify default for subject-to-occurs-check mode. See section
2 for details on the
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
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.
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.
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:
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.
member(b, [a,b,c]), !.
test(member, [nondet]) :-
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
fail or by negating the body using
test(is_set, [fail]) :-
Non-deterministic predicates succeed zero or more times. Their
results are tested either using findall/3
followed by a value-check or using the
options. The following are equivalent tests:
findall(X, member(X, [a,b,c]), Xs),
Xs == [a,b,c].
test(member, all(X == [a,b,c])) :-
Error-conditions are tested using the option
or by wrapping the test in a catch/3.
The following tests are equivalent:
catch(A is 1/0, error(E, _), true),
E =@= evaluation_error(zero_divisor).
test(div0, [error(evaluation_error(zero_divisor))]) :-
A is 1/0.
PlUnit is designed to cooperate with the assertion/1
test provided by library(debug).3This
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.
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
- 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.4If
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.
A is 2^3,
assertion(A == 9).
% PL-Unit: test
test a: assertion at line 7 failed
test a: assertion failed
% 2 assertions failed