| Perl 5, Perl 6, Python, and Ruby are all OO languages in some form or other, so Parrot has to have core support for objects and classes. Unfortunately, all these languages have somewhat different object systems, which made the design of Parrot's object system somewhat tricky. [8] It turns out that if you draw the abstraction lines in the right places, support for the different systems is easily possible. This is especially true if you provide core support for things like method dispatch, which the different object systems can use and override. [8] As we write this, it's still in progress, though it should be done by the time this book is in print. 8.5.1 Generic Object Interfacing Parrot's object system is very simple ”in fact, a PMC only has to handle method calls to be considered an object. Just handling methods covers well over 90% of the object functionality that most programs use, since the vast majority of object access is via method calls. This means user code that does the following: object = some_constructor(1, 2, "foo"); object.bar(12);
will work just fine, no matter what language the class that backs DEFANGED_object is written in, if DEFANGED_object even has a class backing it. It could be Perl 5, Perl 6, Python, Ruby, or even Java, C#, or Common Lisp; it doesn't matter. Objects may override other functionality as well. For example, Python objects use the basic PMC property mechanism to implement object attributes. Both Python and Perl 6 mandate that methods and properties share the same namespace, with methods overriding properties of the same name . 8.5.2 Parrot Objects When we refer to Parrot objects we're really talking about Parrot's default base object system. Any PMC can have methods called on it and act as an object, and Parrot is sufficiently flexible to allow for alternate object systems, such as the one Perl 5 uses. However, in this section, we're talking about what we provide in our standard object system. Parrot's standard object system is pretty traditional ”it's a class-based system with multiple inheritance, interface declarations, and slot-based objects. Each object is a member of a class, which defines how the object behaves. Each class in an object's hierarchy can have one or more attributes ”that is, named slots that are guaranteed to be in each object of that class. The names are all class-private so there's no chance of collision. Objects are essentially little fixed- sized arrays that know what class they belong to. Most of the "smarts" for an object lives in that object's class. Parrot allows you to add attributes at runtime to a class. If you do, then all objects with that class in their inheritance hierarchy will get the new attribute added into it. Though this is potentially expensive, it's a very useful feature for languages that may extend a class at runtime. Parrot uses a multiple inheritance scheme for classes. Each class can have two or more parent classes, and each of those classes can have multiple parents. A class has control over how methods are searched for, but the default search is a left-most, depth-first search, the same way that Perl 5 does it. Individual class implementers may change this if they wish, but only the class an object is instantiated into controls the search order. Parrot also fully supports correct method redispatch, so a method may properly call the next method in the hierarchy even in the face of multiple parents. One limitation we place on inheritance is that a class is instantiated in the hierarchy only once, no matter how many times it appears in class and parent class inheritance lists. Each class has its own vtable, which all objects of that class share. This means that with the right vtable methods every object can behave like a basic PMC type in addition to an object. For unary operations such as load or store, the default class vtable first looks for the appropriately named method in the class hierarchy. For binary operators such as addition and subtraction, it first looks in the multimethod dispatch table. This is only the default, and individual languages may make different choices. Objects that implement the proper methods can also act as arrays or hashes. Finally, Parrot implements an interface declaration scheme. You may declare that a class does one or more named interfaces, and later query objects at runtime to see if they implement an interface. This doesn't put any methods in a class. For that you need to either inherit from a class that does or implement them by hand. All it does is make a declaration of what your class does. Interface declarations are inheritable as well, so if one of your parent classes declares that it implements an interface then your class will as well. This is used in part to implement Perl 6's roles. 8.5.3 Mixed Class-Type Support The final piece of Parrot's object system is the support for inheriting from classes of different types. This could be a Perl 6 class inheriting from a Perl 5 class, or a Ruby class inheriting from a .NET class. It could even involve inheriting from a fully compiled language such as C++ or Objective C, if proper wrapping is established. [9] As we talked about earlier, as long as a class either descends from the base Parrot class or has a small number of required properties, Parrot can subclass it. This potentially goes both ways, as any class system that knows how to subclass from Parrot's base class can inherit from it. [9] DEFANGED_Objective C is particularly simple, as it has a fully introspective class system that allows for run-time class creation. Inheritance can go both ways between it and Parrot. Allowing classes to inherit from other classes of a different base type does present some interesting technical issues. The inheritance isn't 100% invisible, though you have to head off into the corner cases to find the cracks. It's an important feature to design into Parrot, so we can subclass Perl 5 style classes, and they can subclass Parrot classes. Being able to subclass C++ and Objective C classes is a potential bonus. Python, Ruby, and Perl 6 all share a common (but hidden) base class in Parrot's base object type, so they can inherit from each other without difficulty. |
