Logic and control

These predicates are simply derived from the control constructs and provide additional facilities for affecting the control flow during execution.

1. \+/1 (not provable)

'\\+'(Term) is true iff call(Term) is false.

Templates and modes for the predicate are as follows:

'\\+'(@callable_term)

Note that \+ is a predefined operator.

1.1 Example tests

Let's start with some simple tests verifying success or failure of single goals.

alice.tuprolog.SimpleGoalFixture
goalsuccess()
'\\+'(true). false
\+(!). false
'\\+'((!, false)). true
'\\+'(4 = 5). true
\+(X = f(X)).

Now we run some tests also verifying the unification for some of the variables in goals.

First of all, let's start an appropriate fixture containing an engine.

fit.ActionFixture
start alice.tuprolog.EngineFixture

Then, ask the engine to solve a query, and check variable bindings.

fit.ActionFixture
enter query (X=1; X=2), \+((!, fail)).
check hasSolution true
enter variable X
check binding 1
check hasAnotherSolution true
enter variable X
check binding 2

The remaining tests cover the cases when an error or exception is thrown by the engine while solving a query.

alice.tuprolog.PrologActionFixture
enter query \+(3).
check hasSolution false
check exception type_error(callable, 3)
enter query '\\+'(X).
check hasSolution false
check exception instantiation_error

2. once/1

once(Term) is true iff call(Term) is true.

once(Term) behaves as call(Goal) but it is not re-executable.

Templates and modes for the predicate are as follows:

once(+callable_term)

1.1 Example tests

Let's start with some simple tests verifying success or failure of single goals.

alice.tuprolog.SimpleGoalFixture
goalsuccess()
once(!). true
once(repeat). true
once(fail). false
once(X = f(X)).

Now we run some tests also verifying the unification for some of the variables in goals.

First of all, let's start an appropriate fixture containing an engine.

fit.ActionFixture
start alice.tuprolog.EngineFixture

Then, ask the engine to solve a query, and check variable bindings.

fit.ActionFixture
enter query once(!), (X=1; X=2).
check hasSolution true
enter variable X
check binding 1
check hasAnotherSolution true
enter variable X
check binding 2

Note that there are no tests covering the cases when an error or exception is thrown by the engine while solving a query using this predicate.

3. repeat/0

repeat is true.

Templates and modes for the predicate are as follows:

repeat

Note that repeat is re-executable.

3.1 Example tests

Let's start with some simple tests verifying success or failure of single goals.

alice.tuprolog.SimpleGoalFixture
goalsuccess()
repeat, !, fail. false

Run the tests!

The results of the tests for Logic and control are as follows:

fit.Summary