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From: lbaum@bcsaic.UUCP (Larry Baum)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Multiple Inheritance -- Is It A Luxury?
Message-ID: <27553@bcsaic.UUCP>
Date: 6 Jul 90 16:38:12 GMT
References: <42616@apple.Apple.COM> <2691908E.738C@intercon.com> <42665@apple.Apple.COM>
Organization: Boeing Computer Services AI Center, Seattle
Lines: 79

In article <42665@apple.Apple.COM> lins@Apple.COM (Chuck Lins) writes:
: 
: >Used properly, multiple inheritance
: >can be very useful in decomposing problems over orthgonal axes (for example,
: >take a look at the "mixin" concept in several Lisp object platforms).  Used
: >improperly, you can end up with the "it's a floor wax AND a dessert topping"
: >effect...
: 	Symbolic Lisp Machines have a wonderful class in the UI called a
: 	Listener. It inherits from 26 different classes, sometimes through
: 	multiple different paths in the inheritance graph. It's impossible
: 	to subclass properly or even figure out how to take advantage of its
: 	functionality (in case one wanted to do the same thing).

It might be impossible for you, but it certainly isn't in general.  I
think the key point is that without multiple inheritance, Symbolics
probably wouldn't have come up with this class which you admit is
wonderful.

The flavor components of Lisp-Listener are:

W:LISP-LISTENER
  W:LISTENER-MIXIN
    W:INTERACTOR-MIXIN
    W:LISTENER-MIXIN-INTERNAL
      W:PROCESS-MIXIN
      SI:STANDARD-VALUE-ENVIRONMENT-MIXIN
  W:WINDOW
    W:BORDERS-MIXIN
    W:LABEL-MIXIN
      W:ESSENTIAL-LABEL-MIXIN
        W:MARGIN-HACKER-MIXIN
    W:MINIMUM-WINDOW
      W:ESSENTIAL-EXPOSE
      W:ESSENTIAL-ACTIVATE
      W:ESSENTIAL-SET-EDGES
      W:ESSENTIAL-MOUSE
    W:STREAM-MIXIN
      SI:DISPLAY-INPUT-EDITOR
        SI:INTERACTIVE-STREAM
          SI:LINE-OUTPUT-STREAM-MIXIN
          SI:CHARACTER-STREAM
          SI:BIDIRECTIONAL-STREAM
            SI:INPUT-STREAM
    W:SELECT-MIXIN
    W:GRAPHICS-MIXIN
      W:ESSENTIAL-WINDOW
        W:SHEET
          SI:OUTPUT-STREAM
            SI:STREAM
      GRAPHICS:RASTER-GRAPHICS-MIXIN
        GRAPHICS::DRAWING-STATE-GRAPHICS-MIXIN
          GRAPHICS:STANDARD-GRAPHICS-MIXIN
      W:GRAPHICS-COMPATIBILITY-MIXIN
  FLAVOR:VANILLA

Without going thru each one individually, each of these mixins provides
different sets of behaviors which are for the most part orthogonal; e.g.
Listener-mixin makes the lisp listener a separate process with a
READ-EVAL-PRINT loop and the ability to understand command processor
commands.

Borders mixin takes care of how and when to draw or redraw borders on
windows, label-mixin takes care of window labels, etc.

It is true that often a method (i.e. the function that runs in response
to a message) is a complex combination of individual methods contributed
from each mixin and it can be difficult (but certainly not impossible)
to figure out how they combine.  That is why Symbolics includes a
workbench of extremely powerful tools such as the Flavor Examiner,
Inspector, etc.  I would certainly agree that object-oriented languages
permitting multiple inheritance should be bundled with such powerful
tools.  Of course, it is multiple inheritance that makes the building
of those tools much more feasible.

-- 
Larry Baum
Advanced Technology Center              
Boeing Computer Services     uucp:       uw-beaver!bcsaic!lbaum
(206) 865-3365               internet:   lbaum@atc.boeing.com