Editing Oberon (programming language) (section)

===Message handler===
This technique consists of replacing the set of methods with a single procedure, which discriminates among the various methods:

 '''MODULE''' Figures; ''<span style="color: gray">(* Abstract module *)</span>''
 
 '''TYPE'''
    Figure*    = '''POINTER TO''' FigureDesc;
 
    Message*   = '''RECORD END''';
    DrawMsg*   = '''RECORD''' (Message) '''END''';
    ClearMsg*  = '''RECORD''' (Message) '''END''';
    MarkMsg*   = '''RECORD''' (Message) '''END''';
    MoveMsg*   = '''RECORD''' (Message) dx*, dy* : '''INTEGER''' '''END''';
 
    Handler*   = '''PROCEDURE''' (f : Figure; '''VAR''' msg : Message);
 
    FigureDesc* = '''RECORD'''
       ''<span style="color: gray">(* Abstract *)</span>''
       handle : Handler;
    '''END''';
 
 '''PROCEDURE''' Handle* (f : Figure; '''VAR''' msg : Message);
 '''BEGIN'''
    f.handle(f, msg)
 '''END''' Handle;
 
 '''PROCEDURE''' Init* (f : Figure; handle : Handler);
 '''BEGIN'''
    f.handle := handle
 '''END''' Init;
 
 '''END''' Figures.

We extend the generic type Figure to a specific shape:

 '''MODULE''' Rectangles;
 
 '''IMPORT''' Figures;
 
 '''TYPE'''
    Rectangle* = '''POINTER TO''' RectangleDesc;
 
    RectangleDesc* = '''RECORD'''
       (Figures.FigureDesc)
       x, y, w, h : INTEGER;
    '''END''';
 
 '''PROCEDURE''' Draw* (r : Rectangle);
 '''BEGIN'''
   ''<span style="color: gray">(* ... *)</span>''
 '''END''' Draw;
 
 ''<span style="color: gray">(* Other procedures here *)</span>''
 
 '''PROCEDURE''' Handle* (f: Figure; '''VAR''' msg: Figures.Message);
    '''VAR'''
       r : Rectangle;
 '''BEGIN'''
    r := f(Rectangle);
    '''IF'''    msg '''IS''' Figures.DrawMsg '''THEN''' Draw(r)
    '''ELSIF''' msg '''IS''' Figures.MarkMsg '''THEN''' Mark(r)
    '''ELSIF''' msg '''IS''' Figures.MoveMsg '''THEN''' Move(r, msg(Figures.MoveMsg).dx, msg(Figures.MoveMsg).dy)
    '''ELSE'''  <span style="color: gray">(* ignore *)</span>
    '''END'''
 '''END''' Handle;
 
 '''PROCEDURE''' New* ('''VAR''' r : Rectangle);
 '''BEGIN'''
    '''NEW'''(r);
    Figures.Init(r, Handle)
 '''END''' New;
 
 '''END''' Rectangles.

In the Oberon operating system both of these techniques are used for dynamic dispatch. The first one is used for a known set of methods; the second is used for any new methods declared in the extension module. For example, if the extension module Rectangles were to implement a new Rotate() procedure, within the Figures module it could only be called via a message handler.