Language Reference/Predicates

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 :
   LowerCaseIdentifier

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.

Decorated

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:

predicates
    myPredicate : (string X)  language stdcall as decorated.

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

predicates
    myPredicate : (pointer X)  language stdcall as decoratedA.

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

predicates
    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
    constructors
        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:

predicates
    new : (integer).

Also consider the following implementation:

implement test2_class inherits test_class
    clauses
        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.

Example

interface aaa
    predicates
        ppp : ().
        qqq : ().
end interface aaa
 
interface bbb
    predicates from aaa
        ppp
    predicates
        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.

Example

interface aaa
    predicates
        ppp : ().
        qqq : ().
end interface aaa
 
interface bbb
    predicates from aaa
        ppp
    predicates
        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
    clauses
        qqq().
end implement aaa_class

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

Extension Predicates

 

Extension predicates is a syntactic sugaring, which makes it possible use class predicates as if they were object predicates.

Example A typical use case is for code like this:

clauses
    writeMsg(Stream, fct(Receiver, Sender)) :-
        Stream:write("Hello "),
        writeReceiver(Stream, Receiver),
        Stream:write(", how do you do?\nRegards "),
        writeSender(Stream, Sender).

This code writes a number of things to Stream, but some of the calls are object calls Stream:write(...) with the stream in front of a colon, while others are class calls writeReceiver(Stream, ...) with the stream as first argument.

Declaring writeReceiver and writeSender as extension predicates will make all the calls have the same syntactic form:

clauses
    writeMsg(Stream, fct(Receiver, Sender)) :-
        Stream:write("Hello "),
        Stream:writeReceiver(Receiver),
        Stream:write(", how do you do?\nRegards "),
        Stream:writeSender(Sender).

In this code the predicates writeReceiver and writeSender seems to be part of the Streams interface. So they appear to have extended the Streams interface.

Extension predicates can also be used to give give a more natural code flow for binary operations:

Example (Binary operations

class matrix
    add : (matrix A, matrix B) -> matrix R.
    add2 : (matrix A [this], matrix B) -> matrix R.  % extension
 
clauses
   xxx(A, B, C) = add(add(A, B), C).  % add must be used in front of the operands
clauses
   yyy(A, B, C) A:add2(B):add2(C). % add2 can be written infix

You should notice that such infix operations are left associative, if you want add(A, add(B, C)) you must write: A:add2(B:add2(C)). You should also notice that there is no precedence in this context.

A + B * C = A + (B * C) -> A:add2(B:mult2(C))
A:add2(B):mult2(C) -> (A + B) * C

Extension predicates can also support a cascading data flow style:

Example Cascading data flow style:

clauses
    getQ(A, X, F) = qqq::new(A):select(12, X, getList()):filter( { (V) :- V = F } ).

The created qqq object flows into the select predicate the result of this flows into the filter predicate and the result of this is returned. Here select and filter could be regular object predicates declared in an interface, but they can also be extension predicates. Assuming they are extension predicates then the corresponding code with regular class predicates would/could look like this:

clauses
    getQ(A, X, F) = filter(select(qqq::new(A), 12, X, getList()), { (V) :- V = F } ).

An extension predicate is a class predicate whose first argument is marked with the attribute [this].

Example If the regular version of the writeReceiver and writeSender predicates above are declared like this:

class predicates
    writeReceiver : (outputStream Stream, receiver Receiver).
    writeSender : (outputStream Stream, sender Sender).

Then the corresponding extension predicates are declared like this:

class predicates
    writeReceiver : (outputStream Stream [this], receiver Receiver).
    writeSender : (outputStream Stream [this], sender Sender).

I.e. the first argument is marked with the attribute [this].

Extension predicates can also be declared for non-object types.

Example We can for example declare a length extension predicate for lists like this:

class predicates
    length : (Type* List [this]) -> positive Length.

And then we can call it like this:

L1 = List:length(),
L2 = [1, 2]:length(),
L3 = Obj:theList:length() + 7,

Notice that :length() can be put after anything that evaluates to a list.

It is illegal to declare an extension predicate on an interface type if that extension predicate is conflicting with a predicate that exist in the interface itself.

Example Given the interface aaa with predicate ppp/0:

interface aaa
predicates
    ppp : ().
end interface aaa

the following extension predicate is illegal:

class predicates
    ppp : (aaa A [this]).  % Illegal: aaa already has a ppp/0 predicate

Extension predicates follows the visibility rules of class predicates.

Example Given this class

class listExt
predicates
    length : (A* List) -> integer Length.
end class listExt

This code is illegal

implement myClass
clauses
    ppp(L) = L:length(). % length is not visible
end implement myClass

opening the listExt class makes the predicate visible:

implement myClass
    open listExt
clauses
    ppp(L) = L:length(). % length is visible because listExt is opened
end implement myClass

Extension predicates can be qualified with namespace and class:

Example Using qualification

implement myClass
clauses
    ppp(L) = L:listExt::length(). % length is visible due to the qualification listExt::...
end implement myClass

Several suitable extension predicates may be visible in a certain context, and thus cause ambiguity. Qualification can resolve such ambiguity.

Example Given listExt::length above and a similar/identical extension predicate in listExt2:

class listExt2
predicates
    length : (A* List) -> integer Length.
end class listExt2

The following code have an ambiguous reference to length:

implement myClass
    open listExt, listExt2
clauses
    ppp(L) = L:length(). % length is ambiguous because it is visible both in listExt and listExt2
end implement myClass

Qualification can be used to resolve the ambiguity:

implement myClass
    open listExt, listExt2
clauses
    ppp(L) = L:listExt2::length(). % length is the one from listExt2
end implement myClass

Arity

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.

programPoint

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:

predicates
    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:

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

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

Example

clauses
    test() :-
        raiseAnException(17).

will actually correspond to:

clauses
    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.

Example

clauses
    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).