Re: [Xotcl] Dynamic dispatch and subclasses

From: Gustaf Neumann <neumann_at_wu-wien.ac.at>
Date: Fri, 21 May 2004 21:12:24 +0200

On Wednesday 19 May 2004 19:30, Neil Madden wrote:
> I was kind of expecting the last to say "Foo in Base", as the method
> call of "foo" was dispatched from the Base class level. This is an
> interesting problem, as I can see reasons why you might want to do the
> current behaviour. However, I'm worried about some of the implications
> of implementation details and name clashes. Suppose I implement some
> class Base, which as part of it's implementation has a (private) method
> DoStuff.

 XOTcl has no private methods, every method is public.
 the evaluation context does not change the semantics of
 an invocation.

 OO breaks sometimes locality, and refining existing classes
 is not the best concept for components.

 without thinking long, i can offer two approaches.
 1) The first one is rather for fun, it changes the class
     of the object on the fly:

    Object instproc local args {
      set oldclass [my info class]
      my class [uplevel self class]
      set r [eval my $args]
      my class $oldclass
      return r
   }
   ...
   Base instproc bar {} {
     my local foo
   }
 
 since it might have some unwanted effects,
 when the "local" called method (here "foo")
 calls other methods, which should be dispatched
 based on the true objects class. The help in
 mixins is also rather limited.

 2) following your prefix approach:

   Object instproc own {} {
       namespace tail [uplevel self class]
   }
   Object instproc private {method name arguments body} {
      set prefix [namespace tail [self]]
      [self] $method $prefix-$name $arguments $body
   }

  one can define now:

   Class Base
   Base private instproc foo {} {
        puts "Foo in Base"
   }
   ....
 
 and call this via:

   Base instproc bar {} { my [my own]-foo }

 to avoid stuttering while reading, one can define a global funciton

   proc own {} { namespace tail [self class] }
   Base instproc bar {} { my [own]-foo }

 in genereal, instead of [own], [self class] could be used, but currently,
 the leading colons are not allowed in xotcl. i guess, we can change this.

-gustaf neumann

-- 
Univ.Prof. Dr.Gustaf Neumann
Abteilung für Wirtschaftsinformatik
WU-Wien, Augasse 2-6, 1090 Wien