In object-oriented programming class is a set (a transitive closure) of types bound by the relation of inheritance. It is said that all types derived from some base type T and the type T itself form a class T.

The first type T from the class T sometimes is called the root type of the class.

A class of types itself, as a type, has the values and operations of its own.
The operations of are usually called methods of the root type.
Both operations and values are called  polymorphic.

A polymorphic operation (method) selects an implementation depending on the actual specific type of the polymorphic argument.

The action of choice the type-specific implementation of a polymorphic operation is called dispatch. Correspondingly, polymorphic operations are often called dispatching or virtual.
Operations with multiple arguments and/or the results of the class are called multi-methods.
A further generalization of is the operation with arguments and/or results from different classes.

 single-dispatch languages are those that allow only one argument or result to control the dispatch. Usually it is the first parameter, often hidden, so that a prefix notation x.f() is used instead of mathematical f(x).
 multiple-dispatch languages allow many arguments and/or results to control the dispatch.


A polymorphic value has a type tag indicating its specific type from the class and the corresponding specific value of that type.
This type is sometimes called the most specific type of a [polymorphic] value.
The type tag of the value is used in order to resolve the dispatch.
The set of polymorphic values of a class is a transitive closure of the sets of values of all types from that class.

In many  OO languages
the type of the class of T and T itself are considered equivalent.
In some languages they are distinct (like in Ada).
When class T and T are equivalent, there is no way to distinguish
polymorphic and specific values.



Task

Create a basic class with a method, a constructor, an instance variable and how to instantiate it.




without js -- (else cffi namespaces error, classes not supported by pwa/p2js anyway)
requires ("1.0.2") -- (free up the temp used in the v.show() call)

class five nullable
    private integer n = 3
    function get_n()
        return n
    end function
    procedure set_n(integer n)
        this.n = n
    end procedure
    procedure show()
        printf(1,"show: n is %d\n",{n})
    end procedure
    function five(integer n = 4)
        printf(1,"constructor five(%d) called\n",n)
        this.n = n
        return this
    end function
    procedure ~five()
        printf(1,"destructor ~five(%d) called\n",n)
    end procedure
end class
five v = new({5})
assert(v.n=5)
v.n = 6
v.show()
v=NULL
