Language Reference/Predicates


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Language Reference

Predicates Sections

A predicates section declares a set of object or class predicates in the current scope.

PredicatesSection :
   class-opt predicates PredicateDeclaration-dot-term-list-opt

The keyword class can be used only inside class implementations, since:

  • predicates declared in an interface are always object predicates and
  • predicates declared in a class declaration are always class predicates.

Predicate Declarations

The predicate declaration is used to declare the predicate in scopes in which the predicate declaration can be seen. When predicates are declared in an interface definition, this means that objects of the corresponding type must support these predicates. When predicates are declared in a class declaration, this means that the class publicly provides the declared predicates. And if predicates are declared in a class implementation, this means that the predicates are available locally. In all cases a corresponding definitions of the predicates must exist.

PredicateDeclaration :
   PredicateName : PredicateDomain LinkName-opt
   PredicateName : PredicateDomainName LinkName-opt
LinkName :
   as StringLiteral
PredicateName :

Here predicateDomainName is the name of a predicate domain declared in the domains section.

A predicate declaration states the name of the predicate, its type, mode, flow (see predicate domains), and optionally a link name.

Only class predicates can have link names. If the link name is not stated then a link name is derived from the predicate name, the way this name is derived depends on the calling convention.

If the calling convention is apicall then the link name stated in the as clause is decorated anyway. If this decoration is unintended, use stdcall instead.


Sometimes a name must have the _...@N decoration, but the default from apicall is wrong. In such cases decorated, decoratedA and decoratedW can be used to control the decoration:

    myPredicate : (string X)  language stdcall as decorated.

In this case the link name will be "_MyPredicate@4", where apicall would make it "_MyPredicateW@4".

    myPredicate : (pointer X)  language stdcall as decoratedA.

In this case the link name will be "_MyPredicateA@4", where apicall would make it "_MyPredicate@4".

    myPredicate : (pointer X)  language stdcall as decoratedW.

In this case the link name will be "_MyPredicateW@4", where apicall would make it "_MyPredicate@4".

All of them change the start of the name from xxxx to _Xxxx and all of them put @N behind. The first never uses a suffix; the second always uses A and the third always uses W. This means that the programmer is responsible for deciding which suffix is needed. But he needs not to worry about calculating argument size and initial "_X".

Constructors Sections

A constructors section declares a set of constructors. The constructors belong to the scope in which the constructors section occurs (see class declaration and class implementation).

ConstructorsSection :
   constructors ConstructorDeclaration-dot-term-list-opt

Constructor sections can only occur in declarations and implementations of classes that construct objects.

Constructor Declarations

A constructor declaration declares a named constructor of a class.

A constructor actually has two associated predicates:

  • A class function, which returns a new constructed object.
  • An object predicate, which is used when initializing inherited objects.

An associated constructor object predicate is used to perform an object initialization. This predicate can only be called from the constructor in the class itself and from a constructor in a class that inherits from the class (i.e. base class initialization).

ConstructorDeclaration :
   ConstructorName : PredicateDomain

It is illegal to state a predicate mode for constructors, constructors always have procedure mode.

Example Consider the following class:
class test_class : test
        new : (integer Argument).
end class test_class

The associated class level predicate has the following signature:

class predicates
    new : (integer) -> test.

Whereas the associated object level predicate has the following signature:

    new : (integer).

Also consider the following implementation:

implement test2_class inherits test_class
        new() :-
            test_class::new(7),  % invoke the base class constructor on "This"
            p(test_class::new(8)). % create a new object of the base class and pass it to p(...)

The first call to test_class::new does not return a value, therefore it is a call to the non-function object version of the constructor. I.e. it is an invocation of the base class constructor on "This".

The second call on the other hand does return a value, therefore it is a call to the class function version of the constructor. I.e. we are creating a new object.

Predicates from Interface

An interface can support a subset of another interface by stating the predicates in a predicates from section. The predicates from section names the interface and all supported predicates. The predicates are stated by name or by name and arity.

If an interface supports a subset of another interface it is neither subtype or super-type related to the other interface.

The important thing about the predicates from section is that the mentioned predicates retain their origin interface. Therefore:

  • there will be no support conflict with any predicates from the origin interface;
  • they can be inherited as the predicates from the origin interface.
PredicatesFromInterface :
    predicates from InterfaceName PredicateNameWithArity-comma-sep-list-opt

PredicatesFromInterface can only be used in interface definitions.

interface aaa
        ppp : ().
        qqq : ().
end interface aaa
interface bbb
    predicates from aaa
        rrr : ().
end interface bbb
interface ccc supports aaa, bbb
end interface ccc

Even though aaa and bbb both declare a predicate ppp, ccc can support them both without any conflicts, because ppp has aaa as an origin interface in all cases.

interface aaa
        ppp : ().
        qqq : ().
end interface aaa
interface bbb
    predicates from aaa
        rrr : ().
end interface bbb
class aaa_class : aaa
end class aaa_class
class bbb_class : bbb
end class bbb_class
implement aaa_class inherits bbb_class
end implement aaa_class

aaa_class can inherit ppp from bbb_class, because ppp in both classes has aaa as origin interface.


A predicate that takes N arguments are said to be N-ary, or to have arity N. Predicates with different arity are always different predicates, even if they have the same name.

In most situations the arity of a predicate is obvious from the context in which the predicate is mentioned. But in, for example, predicatesFromInterface sections and resolve qualifications the arity is not obvious.

In order to distinguish between different arities of predicates in predicates from sections and in resolve qualifications, predicate names can (optionally) be stated with arity.

The following arities are possible:

  • Name/N meaning an ordinary predicate (i.e. not a function) Name of arity N.
  • Name/N-> meaning a function Name of arity N.
  • Name/N... meaning an ordinary predicate Name with N arguments followed by an Ellipsis argument (i.e. a varying number of arguments). (Ellipsis "..." can be used in predicate and predicate domain declarations as the last formal argument. In this case it means that the declared predicate (predicate domain) can have a variable number of arguments. Ellipsis flow must match an ellipsis argument and can therefore be only the last flow in the flow pattern.)
  • Name/N...-> meaning a function Name with N arguments followed by an ellipsis argument.
PredicateNameWithArity :
   PredicateName Arity-opt
Arity : one of
   / IntegerLiteral Ellipsis-opt
   / IntegerLiteral Ellipsis-opt ->

In Name/0... and Name/0...->. the zero is optional and can thus be written as Name/... and Name/...->, respectively.


A programPoint is a value that represents a specific point in a clause. The programPoint contains the class name, the predicate name, the line number and position on the line. The programPoint domain is defined in the core class

programPoint's are used by the exception mechanism to indicate where exceptions are raised and continued, but the usage is not limited to that purpose.

The compiler suppors programPoint's in a special way by means of the attribute programPoint, which can be added to a predicate declaration like this:

    raiseAnException : (integer X) [programPoint].

Adding this attribute actually means that two predicates are declared, the one you have mentioned and an another one with name raiseAnException_explicit which in addition to the arguemnts of raiseAnException takes a programPoint as first argument:

    raiseAnException : (integer X).
    raiseAnException_explicit : (programPoint ProgramPoint, integer X).

When you call raiseAnException the compiler will actually create a program point ans call raiseAnException_explicit instead.

    test() :-

will actually correspond to:

    test() :-
        raiseAnException_explicit(programPoint(...), 17).
where the program point corresponds to the point where raiseAnException is called in the test predicate.

If you have a programPoint you can directly call the explicit predicate with it.

    raiseAnExceptio17_explicit(ProgramPoint) :-
        raiseAnException_explicit(ProgramPoint, 17).
Typically, as in this example, explicit predicates will call other explicit predicates with the programPoint they receive in order to use an "original" call point in a nested explicit predicate.

Such code is treated in the usual way. I.e. when calling raiseAnException or raiseAnException_explicit will in both cases result in calling raiseAnException_explicit, so this is the only predicate that needs an implementation. In fact, it is illegal to state clauses for the non-explicit predicate that will never be called.

There is also a built-in predicate, programPoint/0->, which returns a programPoint corresponding to the place where it is called.

To summarize:

  • A predicate declaration with programPoint attribute actually declares two predicates. A non-explicit and an explicit predicate.
  • Calling the non-explicit predicate actually results in calling the explicit predicate with the call point as additional argument.
  • Only the explicit predicate should be implemented.

The introduction of the the programPoint feature simplifies the exception mechanism as known in Visual Prolog 7.3 and before. For example classInfo predicates are no longer needed (though they are still legal in Visual Prolog 7.4 and 7.5, but has been deprecated to ease transition).