Re: [Oorexx-devel] object, class, metaclass... Big picture ?
Hi Jean-Louis, it seems that Rick is currently not online, so I try to give some shots, hoping they may help a little bit. [One problem in OO is the terminology, which sometimes comes into ones way (instead of clarifying concepts, sometimes they obfuscate them, unfortunately). There are quite a few synonyms, like object == instance == value etc. or class == type etc.] On 08.08.2011 10:23, Jean-Louis Faucher wrote: I try to understand the underlying model for object, class, metaclass (source excerpt at the end of the mail). Not easy... ooRexx has been quite influenced by Smalltalk, so some of the concepts are directly from there. In general there are no class methods! (This means among other things that all methods are instance methods!) :) --- * There is simply a class that allows one to define methods and attributes the methods of a class can have direct access to. o A 'method is a function/procedure that is only available for instances of the class in which the methods are defined. * An object is just an instance of a class. The object possesses the functionality/behaviour of the methods of its class. * There is a class hierarchy which is used when method resolution takes place with the concept that all methods (not attributes!) of all the superclasses (shortest path to root class) get inherited. (This is also the area where scope becomes important w.r.t. method resolution, but also access to attributes, i.e. concurrency issues and the like.) o The class hierarchy allows an object to have more methods (functionality/behaviour) than defined in its own class, by means of inheritance. * The root of a class hierarchy is usually calld Object, which is a class, as it defines methods. As all other classes in ooRexx are derived directly (by default) or indirectly from this root class, its methods (functionality/behaviour) is inherited by all other classes (and its instances)! --- In dynamic languages the class definitions (the name, the methods) are represented in objects as well. The class used for creating objects that keep the definition of a class in ooRexx (and many other languages) is called Class. As instances of this class describe the structure and behaviour of individual classes the term metaclass is used for it. So the ooRexx class Class is the metaclass of ooRexx. Of course, if one subclasses Class it inherits all abilities of Class and as such keeps on being able to maintain the definition of classes. An instance of Class or one of its subclasses (hence of any metaclass) is called class object. The instance methods of metaclasses (i.e. class objects) are called class methods, indicating that these methods belong to the class object. --- When ooRexx starts up the class hierarchy is created in native code and from the image. I.e. all ooRexx supplied classes build (and are represented in) the ooRexx class hierarchy in the form of class objects. Each of these class objects maintains e.g. the name of the class and the methods, instances of this class should get assigned to at creation time (which is done in the 'new' method of Class). At runtime one can access these class objects with their environment symbol, which is the name of the class preceeded by a dot, e.g.: .object, .class, .array and the like. --- One speciality (and very powerful feature) of ooRexx is its ability to add/remove methods from instances (one-off-objects)! Cf. e.g. the Object's methods setMethod and unsetMethod, but also the Class method enhanced to create instances of a class which get additional methods (hence one-off). It is also possible to change the set of methods instances of a class get assigned to, when they get created. Cf. the Class methods method, methods, define and delete, which allow to maintain the methods instances of a class should get assigned to at the time they got created. Note: the method instanceMethod and instanceMethods in the root class Object allow one to fetch the actual instance method(s) of an object (ie. also from one-off objects). --- Finally ad methods of a class object: all methods of a class object are instance methods, sometimes dubbed class methods, because they are the instance methods of a metclass/class object. A class object is just a normal ooRexx object and as such it is possible to add/remove instance methods to/from it! When an ooRexx class defines methods with the subkeyword class then these methods will be added to the class object (as instance methods of the class object). --- Hope that helps clarify a few of your questions. Ad ooRexx native code and implementation, this is something that probably Rick is the only one who can answer at the moment. ---rony I can't find a suitable definition for the ??? below. Extracted from rexxref : 5.1.1 The Object class is the root of the class hierarchy. The instance methods
Re: [Oorexx-devel] object, class, metaclass... Big picture ?
Hi Jean-Louis, overlooked the send question: in ooRexx the defined methods and the messages (represented by the tilde/twiddle/~) are represented in their own ooRexx classes. So if in a Rexx program the ~ is encountered a message object (an instance of the class Message) is created. However, the creation of a message does not yet cause that message to be dispatched to the object. In order to dispatch a message object (which actually carries out the message) one has to send it either the mssage send or start. * Using the send method causes the execution to halt until the message has been carried out in full and allows one to fetch directly a result value, if any. * Using the start method cuases the execution to go on, the message is carried out concurrently and returns an object, which can be interrogated for the status of the asynchroneous execution of the started message. Eventually, when the asynchroneous message finished, one can learn that and even fetch the result. Cf. the reference documentation of the class Message. HTH, ---rony On 08.08.2011 11:39, Rony G. Flatscher wrote: Hi Jean-Louis, it seems that Rick is currently not online, so I try to give some shots, hoping they may help a little bit. [One problem in OO is the terminology, which sometimes comes into ones way (instead of clarifying concepts, sometimes they obfuscate them, unfortunately). There are quite a few synonyms, like object == instance == value etc. or class == type etc.] On 08.08.2011 10:23, Jean-Louis Faucher wrote: I try to understand the underlying model for object, class, metaclass (source excerpt at the end of the mail). Not easy... ooRexx has been quite influenced by Smalltalk, so some of the concepts are directly from there. In general there are no class methods! (This means among other things that all methods are instance methods!) :) --- * There is simply a class that allows one to define methods and attributes the methods of a class can have direct access to. o A 'method is a function/procedure that is only available for instances of the class in which the methods are defined. * An object is just an instance of a class. The object possesses the functionality/behaviour of the methods of its class. * There is a class hierarchy which is used when method resolution takes place with the concept that all methods (not attributes!) of all the superclasses (shortest path to root class) get inherited. (This is also the area where scope becomes important w.r.t. method resolution, but also access to attributes, i.e. concurrency issues and the like.) o The class hierarchy allows an object to have more methods (functionality/behaviour) than defined in its own class, by means of inheritance. * The root of a class hierarchy is usually calld Object, which is a class, as it defines methods. As all other classes in ooRexx are derived directly (by default) or indirectly from this root class, its methods (functionality/behaviour) is inherited by all other classes (and its instances)! --- In dynamic languages the class definitions (the name, the methods) are represented in objects as well. The class used for creating objects that keep the definition of a class in ooRexx (and many other languages) is called Class. As instances of this class describe the structure and behaviour of individual classes the term metaclass is used for it. So the ooRexx class Class is the metaclass of ooRexx. Of course, if one subclasses Class it inherits all abilities of Class and as such keeps on being able to maintain the definition of classes. An instance of Class or one of its subclasses (hence of any metaclass) is called class object. The instance methods of metaclasses (i.e. class objects) are called class methods, indicating that these methods belong to the class object. --- When ooRexx starts up the class hierarchy is created in native code and from the image. I.e. all ooRexx supplied classes build (and are represented in) the ooRexx class hierarchy in the form of class objects. Each of these class objects maintains e.g. the name of the class and the methods, instances of this class should get assigned to at creation time (which is done in the 'new' method of Class). At runtime one can access these class objects with their environment symbol, which is the name of the class preceeded by a dot, e.g.: .object, .class, .array and the like. --- One speciality (and very powerful feature) of ooRexx is its ability to add/remove methods from instances (one-off-objects)! Cf. e.g. the Object's methods setMethod and unsetMethod, but also the Class method enhanced to create instances of a class which get additional methods (hence one-off). It is also possible to change the set of methods instances of a
Re: [Oorexx-devel] object, class, metaclass... Big picture ?
Thanks a lot Rony ! About ~send, I was focusing on the search order for method selection, and I found the answer in rexxref 4.2.5. There is no reference to metaclass there, so I assume that the methods of a metaclass are not visible from an object. Honestly, I can't say it's still very clear for the distinction class/metaclass. I thought that a metaclass was the class of a class, but this is not the case in the internal model. (All the details in the attached file...) RexxClass is a RexxObject. As such, it inherits the attribute 'behaviour' which is used for : ~class, ~instanceMethod, ~instanceMethods, ~setMethod, ~unsetMethod Though this behaviour, I can get the class of a RexxClass. But RexxClass has also an attribute 'metaClass' which is an array of RexxClass. The other attributes for which it's not clear are : scopes classMethodDictionary (~!DEFINE_CLASS_METHOD) metaClassMethodDictionary metaClassScopes Jean-Louis 2011/8/8 Rony G. Flatscher rony.flatsc...@wu-wien.ac.at ** Hi Jean-Louis, overlooked the send question: in ooRexx the defined methods and the messages (represented by the tilde/twiddle/~) are represented in their own ooRexx classes. So if in a Rexx program the ~ is encountered a message object (an instance of the class Message) is created. However, the creation of a message does not yet cause that message to be dispatched to the object. In order to dispatch a message object (which actually carries out the message) one has to send it either the mssage send or start. - Using the send method causes the execution to halt until the message has been carried out in full and allows one to fetch directly a result value, if any. - Using the start method cuases the execution to go on, the message is carried out concurrently and returns an object, which can be interrogated for the status of the asynchroneous execution of the started message. Eventually, when the asynchroneous message finished, one can learn that and even fetch the result. Cf. the reference documentation of the class Message. HTH, ---rony On 08.08.2011 11:39, Rony G. Flatscher wrote: Hi Jean-Louis, it seems that Rick is currently not online, so I try to give some shots, hoping they may help a little bit. [One problem in OO is the terminology, which sometimes comes into ones way (instead of clarifying concepts, sometimes they obfuscate them, unfortunately). There are quite a few synonyms, like object == instance == value etc. or class == type etc.] On 08.08.2011 10:23, Jean-Louis Faucher wrote: I try to understand the underlying model for object, class, metaclass (source excerpt at the end of the mail). Not easy... ooRexx has been quite influenced by Smalltalk, so some of the concepts are directly from there. In general there are no class methods! (This means among other things that all methods are instance methods!) :) --- - There is simply a class that allows one to define methods and attributes the methods of a class can have direct access to. - A 'method is a function/procedure that is only available for instances of the class in which the methods are defined. - An object is just an instance of a class. The object possesses the functionality/behaviour of the methods of its class. - There is a class hierarchy which is used when method resolution takes place with the concept that all methods (not attributes!) of all the superclasses (shortest path to root class) get inherited. (This is also the area where scope becomes important w.r.t. method resolution, but also access to attributes, i.e. concurrency issues and the like.) - The class hierarchy allows an object to have more methods (functionality/behaviour) than defined in its own class, by means of inheritance. - The root of a class hierarchy is usually calld Object, which is a class, as it defines methods. As all other classes in ooRexx are derived directly (by default) or indirectly from this root class, its methods (functionality/behaviour) is inherited by all other classes (and its instances)! --- In dynamic languages the class definitions (the name, the methods) are represented in objects as well. The class used for creating objects that keep the definition of a class in ooRexx (and many other languages) is called Class. As instances of this class describe the structure and behaviour of individual classes the term metaclass is used for it. So the ooRexx class Class is the metaclass of ooRexx. Of course, if one subclasses Class it inherits all abilities of Class and as such keeps on being able to maintain the definition of classes. An instance of Class or one of its subclasses (hence of any metaclass) is called class object. The instance methods of metaclasses (i.e. class objects) are called class methods, indicating that these methods belong to the class object. --- When ooRexx
Re: [Oorexx-devel] object, class, metaclass... Big picture ?
Hi Jean-Louis, On 08.08.2011 14:40, Jean-Louis Faucher wrote: About ~send, I was focusing on the search order for method selection, and I found the answer in rexxref 4.2.5. There is no reference to metaclass there, so I assume that the methods of a metaclass are not visible from an object. No, they are not! (Unlike in C++ or Java or the like.) If one needs to get at the class methods in ooRexx one first needs to get at the class object (which created the object in question) by sending the message class to the object in question. Then you can send the names of the class methods to the class object that the message class returned. (Again, the class object is just a plain object, so all rules about objects apply.) Honestly, I can't say it's still very clear for the distinction class/metaclass. I thought that a metaclass was the class of a class, but this is not the case in the internal model. This depends on your definition of class of a class, class and metaclass, for which you seem to expect a distinction to be present. There is none in ooRexx. A metaclass is a plain class, i.e. it allows one to define methods that are regarded to belong together to serve some purpose. The term meta just points at the usage of a particular class, usually named Class: this class allows one to define, maintain and interrogate the methods of classes at runtime. But the mechanics (allowing to define methods and sending messages) are the same as for any other class: one can create instances, send the names of existing methods, etc. The metaclass also has a method defined to create an instance/object/value of the class (possessing all methods that were defined at object creation time and which can be retrieved from the metaclass with the message methods) and after initialization returns it: the method new. Maybe a different application/point-of-view: every time an ooRexx program is processed the ::class-directive will cause the runtime system to create an instance of Class assigning that instance (a class object) the name found with the class directive. Then, for each ::method-directive an instance of the class Method is created by the found name, which also stores the Rexx code; that method object is then added to the class object collection of methods that should be assigned to objects it creates, using the class object method define. In the case that a ::method-directive has the subkeyword class that method is added directly to the class object instead using the Object's method setMethod. (The major distraction, I think, stems from the metaphysically sounding name metaclass, as if something special, secret is allotted to it.) The speciality of the class named Class is simply: it allows for maintaining the name of an ooRexx class, define/delete methods that instances of an ooRexx class should be assigned to an object, when it gets created. That's it. (Maybe, if you state what you think a class of a class is/should be about, then we could discuss those concepts/expectations in more detail.) --- Ad implementation details/rationales/structures/mechanics, again, only Rick can explain them. (All the details in the attached file...) RexxClass is a RexxObject. As such, it inherits the attribute 'behaviour' which is used for : ~class, ~instanceMethod, ~instanceMethods, ~setMethod, ~unsetMethod Though this behaviour, I can get the class of a RexxClass. Yes, these are available by inheritance, they are not locally defined in the class object for RexxClass! But RexxClass has also an attribute 'metaClass' which is an array of RexxClass. metaClass is an instance method of the class object, defined in RexxClass (along with e.g. define, delete. enhanced, new, superclasses, subclasses, etc.). Not sure why it is backed by an array in the implementation (Rick would know, as he implemented it that way). The other attributes for which it's not clear are : scopes A scope generally defines the visibility of variables, labels. In the context of ooRexx it also defines the visibility of routines and classes, e.g. routines and classes are always seen/accessible in the program that defined them. Public routines and classes become visible once the program was called/required that defined them. In the context of methods there is one important item in addition: access to attributes! Usually all methods that are defined for a class share the same scope, i.e. if they define attributes (using the EXPOSE-statement as the first instruction) then they are shared/visible among all methods. Accessing attributes in parallel running/executing methods would incur the classic problems, hence access to attributes are guarded in ooRexx by default. Now, it is possible in ooRexx to define methods in a program that are not assignable to a specific class, because the method directives preceed class-directives. Such floating methods get collected by the interpreter and from the defining program one can access them via the .methods environment
