Re: AW: Re: [Xotcl] Using instforward to wrap other packages

From: Gustaf Neumann <neumann_at_wu-wien.ac.at>
Date: Fri, 24 Feb 2006 11:35:30 +0100

mail_at_xdobry.de schrieb:
> Learnable lazy wrapper per unknown mechanism.
> Should one do such magic thinks?
>
one can use "unknown" by itself for implementing the task, or to
register a forwarder (i have used the same approach in
http://wiki.tcl.tk/11033).

I am not a particular friend of unknown, except for error handling.
i see three reasons against it.

1) "unknown" does not mix well with mixins. The program semantics
    can change, when mixins are added (see below)
2) only unknown commands are forwarded. What if you want to
    forward a method, that happens to be defined by some classes.
    Try to forward e.g. append by this approach (somewhat related to (1)
3) all unknown commands are forwarded (can easily be fixed)

generally i prefer to use the explicit "create" over the unknown handler
to create objects (using "Object create o1" instead of "Object o1"),
except that the shortcut is much nicer to read. If one defines user
methods on classes, the unknown is dangerous as well, one might
define a method o1, typos can easily turn up as created objects.

This as a side node "unknown considered harmful".

So, the apporach via "unknown" is interesting, but has its dangers.
It only buys us a slight optimization by delaying the generation of the
forwarders. Note, that the forward command only registers a command
and is fairly efficient (comparable with a single eval).

To be complete, yet another approach is to use filter or instfilters,
here a filter solution:

   Class Queue -array set forwarder {clear 1 get 1 put 1 peek 1 unget 1
size 1}
   Queue instproc qfilter args {
     if {[[self class] exists forwarder([self calledproc])]} {
       return [eval [self]::queue [self calledproc] $args]
     }
     next
   }
   Queue instproc init {} {
     ::struct::queue [self]::queue
     my filter qfilter
   }
   Queue instproc destroy {} {
     [self]::queue destroy
     next
   }

-gustaf

ad 1) If we take the example based on unknown and add a mixin,
 as with this tricky oneliner

   Class M -instproc get args next
   Class Queue -parameter {{queue [self]::queue}} -instmixin M

  then the command "get" is defined, therefore "unknown" is not called
  when "myqueue get" is invoked. Therefore, there will be no forwarding
  to the queue...