The Type System

    A variable x declared as
    var x : Store;
can refer to objects whose classes have at least the method signatures of Store. That is, if Store has methods
     proc set(integer)
     proc get() : integer
then the assignment
     x = MyStore.new();
will be legal if MyStore has at least methods get and set with the signature shown above. It does not matter whether MyStore inherits from Store or not. The type of a class is the set of its method signatures. A type S is subtype of T if T is in set S. Assignments of the kind
     T = S
are legal. Note parameter passing is just a kind of assignment.

    Class objects do not have classes but they do have a type. Then if one declare

object Figure
  public:
    proc get() : integer
    ...
    proc set( pn : integer )
    ...

// here are declared some methods of class Store

end

it is legal to do
     x = Figure;
since the type of Figure is subtype of the type of x, Store. In particular, Any is a supertype of any class or class object. Then, a method
     proc doIt( a : Any )
can receive as parameter any object, be it an object with a class or a classless object representing a class (as Figure). Note we added to object Figure some methods found in class Store. These methods are automatically added by the compiler to all classes but not to the class objects. Other methods are added to all class objects.

What is new ?

    The separation of subtyping from subclassing is old. What is new, and very important, is that

• a class object  does not have a class but it does have a type.  The type of a class object is used to check the assignments and parameter passings that use the class object;
• class Any is a supertype of any class object. So if one declares a variable any of type Any, she may deal with either class objects or normal objects. All objects can be handled in an uniform way.

Return