Difference between revisions of "Language Reference/Basic Concepts/Object System"

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==== External View ====
==== External View ====


The description in this section is not supposed to be an introduction to classes; it is intended as a clarification of the class notions in Visual Prolog. The reader is expected to be familiar with common class notions. The description is purely conceptual in the sense that it does not refer to any syntax, implementation, etc. Moreover, this description does not consider any operational or programmatic aspects. While the reasons for introducing classes, objects, etc are mostly of programmatic nature, we find it valuable to describe the underlying concepts without reference to these programmatic reasons.
This section clarifies the class-related notions in Visual Prolog at a conceptual level. It does not discuss syntax or implementation details.


The class concept of Visual Prolog is based on the following three semantic entities:
The class concept in Visual Prolog is based on three semantic entities:
*objects
* objects
*interfaces
* interfaces
*classes
* classes


===== Object =====
===== Object =====


An ''object'' is set of named ''object member predicates'' and a set of ''supported'' interfaces. Objects actually also have a state, but this state can only be changed and observed through the member predicates. We say that the state is ''encapsulated'' in the object.
An ''object'' is a set of named ''object member predicates'' together with a set of ''supported'' interfaces. Objects also have state, but this state can only be observed and changed through member predicates; the state is therefore ''encapsulated'' in the object.
 
Here '''encapsulation''' means the ability of an object to hide its internal data and methods, making only the intended parts of the object programmatically accessible. The importance of encapsulation and modularity is well known. Encapsulation helps building more structured and readable programs, because objects are treated like black boxes. Look at a complex problem, find a part, which you can declare and describe. Encapsulate it into an object, construct an interface and continue so, until you have declared all the sub-problems. When you have encapsulated the objects of the problem, and ensured, that they work correctly, you can abstract from them.


===== Interface =====
===== Interface =====


An ''interface'' is an object type. It has a name and defines a set of named object predicates.
An ''interface'' is an object type. It has a name and defines a set of named object predicates.


Interfaces are structured in a ''supports'' hierarchy (the structure is a semi-lattice rooted in the interface object). If an object has a type denoted by an interface, then it also has the type of any supported interfaces. Therefore, the supports hierarchy is also a type hierarchy. An interface is a subtype of all its supported interfaces.  We also say that the object ''supports'' the interface. If the interface is named ''X'', then we say that the object is an ''X'', or an ''X'' object.
Interfaces form a ''supports'' hierarchy (a [[wikipedia:semilattice|semi-lattice]]) rooted at the <vp>object</vp> interface. If an object has a type denoted by some interface, it also has the types of all supported interfaces. Thus the supports hierarchy is a type hierarchy: an interface is a subtype of each interface it supports.


===== Class =====
===== Class =====


A ''class'' is a named object factory; it can create objects corresponding to a certain interface. Any object is created by a class, if an object was created by the class, which uses the interface <vp>C</vp> to construct objects then we call it a &quot;C object&quot;.
A ''class'' is a named object factory; it constructs objects that correspond to a given interface. If a class constructs objects of interface <vp>iii</vp>, then its objects are "<vp>iii</vp> objects". All objects constructed by the same class share the same definitions of the object member predicates, but each object has its own state; the predicate definitions belong to the class, while the state belongs to each object.
 
All objects that are constructed by a certain class share the same definition of the object member predicates, but each object has its own state. Thus, the object member predicates is actually part of the class, whereas the state of the object is part of the object itself.
 
A class also contains another set of named predicates and an encapsulated state, known as the ''class'' members and ''class'' state, respectively. The class members and the class state exist on a per class basis, whereas the object members and object state exist on a per object basis. The class state can be accessed both by class members and by object members.


Notice! The set of object member predicates that a class ''defines'' is the '''union''' of the predicates ''declared'' (transitively) in the interfaces of that class. More specifically this means that, if the same predicate is declared in two different interfaces then the class will only provide '''one''' ''definition'' of that predicate. So the class only sounds if that makes sense, i.e. if the intended semantics of these two inherited predicates are the same.
The set of object member predicates a class ''defines'' is the '''union''' of the predicates ''declared'' (transitively) in its interfaces. If the same predicate is declared in multiple interfaces, the class provides '''one''' common definition; this is only valid if the intended semantics coincide. Interface support must be specified explicitly.
 
Notice that interface support must be specified explicitly. The fact that some class provides the predicates corresponding to some interface does not imply that the class supports the interface.


===== Module =====
===== Module =====


In fact a class need not be able to manufacture objects at all. Such class may have only class members and the class state. And therefore, such class can be considered to be a module rather than a class.
A class need not construct objects. A class without a construction type acts as a module: it may have only class members and class state.


===== Identity =====
===== Identity =====


Every object is unique: objects have a changeable state and since the state of the objects can be observed by means of their member predicates an object is only identical to itself. I.e. even if the states of two objects are identical, the objects are not identical, because we can change the state of one object without changing the state of the other object.
Every object is unique. Even if two objects currently have identical state, they are not identical: one can be changed without affecting the other. We never access object state directly; we access it through references, and many references may denote the same object. Classes and interfaces are also unique and identified by name (with namespace); duplicate names are not allowed within a program. Structural equality does '''not''' imply identity for objects, classes, or interfaces.
 
We never have direct access to an object state, we always access an object state by means of a reference to the object and while an object is only identical to itself, we can have many references to the same object. Thus, the same object can be accessed through many different references.
 
Classes and interfaces are also unique; they are identified by their names. Two interfaces or classes cannot have the same name in the same program (a class and an interface also cannot have the same name).
 
The essence is that structural equality does '''not''' imply identity for objects, classes nor interfaces.


==== Internal View ====
==== Internal View ====


Where the previous section described objects, classes, and interfaces  in terms of their external behavior, this section will extend this description  with internal issues. These internal issues have more programmatically  nature; they are concerned with splitting of classes onto the declaration  part and the implementation part.
This section complements the external view with internal aspects: how declarations and implementations are organized. From a programmatic point of view, classes are central (they contain the code). Interfaces have mainly static importance and have no direct runtime representation; objects are dynamic and exist only when the program runs.
 
From a programmatic point of view, classes are the central item: the code is contained in the classes.
 
Interfaces mainly have static importance. In fact, interfaces only exist  in the textual representation of a program; there is no (direct) runtime representation of an interface.
 
Objects, on the other hand, have mainly dynamic importance. Objects  are not directly visible in the program; they do not exist until the program  actually runs.
 
A class consists of a declaration and an implementation. The declaration  declares the ''public'' accessible parts of the class and the objects it generates. The implementation on the other hand ''defines'' the  entities ''declared'' in the class declaration. The basic implementation  of predicates is of course clauses, but predicates can also be defined  by means of ''inheritance'', or resolved to external libraries.
 
A class declaration in Visual Prolog is purely declarative. It only  states which entities you can access: not how or where they are implemented.
 
A class implementation can declare and define further entities (i.e.  domains, predicates, etc), which are only visible inside the class itself.  I.e. they are ''private''.
 
The state of an object is stored in the object as its facts. These facts  are declared as normal facts (database) sections in the implementation  of the class. Facts are local to each object (like other object entities),  whereas ''class'' facts are shared among all objects of the class.


Facts can only be declared in the implementation of a class and, therefore, cannot be accessed (directly) from outside the class.
A class has a declaration and an implementation. The declaration states the ''public'' accessible parts of the class and of the objects it constructs; the implementation ''defines'' what the declaration introduces (predicates are implemented by clauses, by inheritance, or resolved to external libraries). A class declaration is purely declarative.


The implementation of a class can also declare that it supports more interfaces than mentioned in the declaration. This information is, however,  only visible in the implementation itself and is, therefore, private. 
An implementation can declare additional private entities (domains, predicates, etc.). Object state is stored as facts in the implementation; such facts are per-object, while ''class'' facts are shared across all objects of the class. Facts are declared only in implementations and are not directly accessible from outside. Implementations may also (privately) support more interfaces than stated in the declaration.


===== Code Inheritance =====
===== Code Inheritance =====


In Visual Prolog code inheritance only takes place in the implementation  of a class. Visual Prolog has multiple inheritance. You inherit from a  class by mentioning the class in a special ''inherits'' section of  the implementation. The classes you inherit from are called ''parent'' classes or ''super''-classes. ''Child'' class and ''sub''-class  is the dual to parent class, we also say that the child classes ''inherit''  from the parent classes. A child class can only access its parent classes through its public interface, i.e. it does not receive any extra  privileges than anybody else that use the parent class.
Code inheritance occurs only in class implementations. Visual Prolog supports multiple inheritance via an ''inherits'' section in the implementation. The classes you inherit from are ''parent'' (''super'') classes; the inheriting class is the ''child'' (''sub'') class. A child class can only access its parent classes through their public interfaces; it receives no special privileges.


[[Category:Language Reference]]
<noinclude>{{LanguageReferenceSubarticle|Basic Concepts/Object System}}</noinclude>

Latest revision as of 14:26, 20 August 2025

External View

This section clarifies the class-related notions in Visual Prolog at a conceptual level. It does not discuss syntax or implementation details.

The class concept in Visual Prolog is based on three semantic entities:

  • objects
  • interfaces
  • classes
Object

An object is a set of named object member predicates together with a set of supported interfaces. Objects also have state, but this state can only be observed and changed through member predicates; the state is therefore encapsulated in the object.

Interface

An interface is an object type. It has a name and defines a set of named object predicates.

Interfaces form a supports hierarchy (a semi-lattice) rooted at the object interface. If an object has a type denoted by some interface, it also has the types of all supported interfaces. Thus the supports hierarchy is a type hierarchy: an interface is a subtype of each interface it supports.

Class

A class is a named object factory; it constructs objects that correspond to a given interface. If a class constructs objects of interface iii, then its objects are "iii objects". All objects constructed by the same class share the same definitions of the object member predicates, but each object has its own state; the predicate definitions belong to the class, while the state belongs to each object.

The set of object member predicates a class defines is the union of the predicates declared (transitively) in its interfaces. If the same predicate is declared in multiple interfaces, the class provides one common definition; this is only valid if the intended semantics coincide. Interface support must be specified explicitly.

Module

A class need not construct objects. A class without a construction type acts as a module: it may have only class members and class state.

Identity

Every object is unique. Even if two objects currently have identical state, they are not identical: one can be changed without affecting the other. We never access object state directly; we access it through references, and many references may denote the same object. Classes and interfaces are also unique and identified by name (with namespace); duplicate names are not allowed within a program. Structural equality does not imply identity for objects, classes, or interfaces.

Internal View

This section complements the external view with internal aspects: how declarations and implementations are organized. From a programmatic point of view, classes are central (they contain the code). Interfaces have mainly static importance and have no direct runtime representation; objects are dynamic and exist only when the program runs.

A class has a declaration and an implementation. The declaration states the public accessible parts of the class and of the objects it constructs; the implementation defines what the declaration introduces (predicates are implemented by clauses, by inheritance, or resolved to external libraries). A class declaration is purely declarative.

An implementation can declare additional private entities (domains, predicates, etc.). Object state is stored as facts in the implementation; such facts are per-object, while class facts are shared across all objects of the class. Facts are declared only in implementations and are not directly accessible from outside. Implementations may also (privately) support more interfaces than stated in the declaration.

Code Inheritance

Code inheritance occurs only in class implementations. Visual Prolog supports multiple inheritance via an inherits section in the implementation. The classes you inherit from are parent (super) classes; the inheriting class is the child (sub) class. A child class can only access its parent classes through their public interfaces; it receives no special privileges.

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