objectwalker

JMN 2003-08-04 This package is sugar for objects. The idea is that it should be generic enough to use with the various TCL OO packages. After 'package require' you need to call 'ow::install' to get the object systems to recognize the traversal method. I'm not sure if all OO systems will allow this approach - but if people would like to fill in the ow::install_<pkgname> methods where possible, that'd be great.

I suspect this

 "parentObject . referredObject . anotherReferredObject . method arg1 args"

syntax may make some TCLers skin crawl, and at first I was keen to keep it simpler as demonstrated by SLB on The ghosts of VB haunt this TCLer i.e

 "Traverse parentObject referredObject anotherReferredObject {method arg1 args}"

However when playing around on the commandline I found that a separator to delimit object references and methods was more pleasing to me and a little less prone to silly mistakes. If someone prefers the plain traversal syntax, perhaps the ow::install method could be hacked with an option to allow this.

Usage Example:

 %package require XOTcl
 %namespace import XOTcl::*
 %package require objectwalker
 1.0
 %ow::install
 %Class thing
 ::thing
 %thing instproc next args {
 %    if {[string length $args]} {
 %        eval "[self] set next $args"
 %    } else {
 %        [self] set next
 %    }
 %}
 %thing a
 %thing b
 %a set next b
 %b set next a
 #now we can follow our object references in a manner that may almost be pleasing to us VB-weaned wimps ;)
 %a . next . next . next
 b
 %a . set test blah ;# The separator is of course unnecessary here
 blah
 %a . next . next . next . next . info vars
 test next

I'm not sure if the use of the . operator will cause problems in some contexts, but if so, you can always do something like this:

 %ow::install -separator ->
 %a -> next -> next
 a

 #JMN 2003-08-03
 # with thanks to the contributors at http://wiki.tcl.tk/9507
 # This code is hereby placed in the public domain.
 #
 package provide objectwalker 1.0
 package require cmdline
 
 namespace eval ow {
         variable version 1.0
         variable OOPackages [list XOTcl]
         variable sep .
 }
 proc ow::=> {object args} {
       foreach arg $args {
          set object [eval {$object} $arg]
       }
       return $object
 }
 proc ow::traverse {object args} {
         set items [list]
         
         set argl [list]
         foreach arg $args {
                 if {$::ow::sep eq $arg} {
                         lappend items $argl
                         set argl [list]
                 } else {
                         lappend argl $arg
                 }
         }
         lappend items $argl
         
         #puts "items-> $items"
         
         foreach itm $items {
                     set object [eval {$object} $itm]
         }
         return $object
 } 
 
 proc ow::install args {
         set options {
                 {separator.arg . "Object method separator string"}
                 {suppress.arg {} "List of OO packages with which we do NOT want to use objectwalker"}
                 {only.arg {} "Only install objectwalker for these OO packages"}
         }
         array set params [::cmdline::getoptions args $options]
         
         if {[llength $params(only)]} {
                 set packageList $params(only)
         } else {
                 set packageList [list]
                 foreach known $::ow::OOPackages {
                         if {[lsearch $params(suppress) $known] == -1} {
                                 lappend packageList $known
                         }
                 }
         }
 
         set ::ow::sep $params(separator)
         
         foreach pkg $packageList {
                 eval "::ow::install_$pkg"
         }
         
 }
 proc ow::install_XOTcl args {
         if {![catch {package require XOTcl} msg]} {
                 uplevel #0 [string map "@sep@ $::ow::sep" {
                         xotcl::Class objectwalker
                         objectwalker instproc @sep@ args {
                                 eval "::ow::traverse [self] @sep@ $args"
                         }        
                         xotcl::Object instmixin objectwalker
                 }]
         }
 }
 proc ow::install_snit args {
     #???        
 }

Artur Trzewik Some comments to example above. next have special meaning in XOTcl for calling base (overwritten) method. I suggest to change the name of this variable. I suppose the thing class is something like chaining list with pointer to next object. Without using objectwalker the syntax of this call will be

   [[[[a next] next] next] next] info vars

For better understand next is in this case getter and setter method in one depending of arguments count. It can be also defined as XOTcl parameter

    Class thing -parameter next

I think it is quite interesting idea to have syntax more like another OO-Languages. It would also work for more parameters (If I understand it good)

    a.getSomeInstance(2,3).childAt(3).doIt();  # C++
    a . getSomeInstance 2 3 . childAt 3 . doIt  # objectwalker
    [[a getSomeInstance 2 3] childAt 3] doIt # Tcl without objectwalkter

By the way ow::traverse procedure can be also programmed with recursion in it (parse only till separator) The shortest XOTcl implementation of it (do not delegate to objectwalker package). You also can define -> directly as proc method in Object.

    Class Traversal
    Traversal instproc -> args {
        set index [lsearch $args ->]
        if {$index==-1} {
           eval [self] $args
        } else {
           set obj [eval [self] [lrange $args 0 [expr {$index-1}]]]
           eval $obj [lrange $args $index end]
        }
    }
    Object instmixin Traversal

or in primary implementation

    proc ow::traverse {object args} {
        set index [lsearch $args $::ow::sep]
        if {$index==-1} {
           eval $object $args
        } else {
           set obj [eval $object [lrange $args 0 [expr {$index-1}]]]
           eval $obj [lrange $args $index end]
        }
    }

Category Object Orientation