TIP #45: EMPTY INDEX LISTS FOR [LINDEX] AND [LSET] 
====================================================
 Version:        $Revision: 1.9 $
 Author:         Kevin Kenny <kennykb_at_acm.org>
                 Don Porter <dgp_at_users.sourceforge.net>
 State:          Final
 Type:           Project
 Tcl-Version:    8.4b1
 Vote:           Done
 Created:        Wednesday, 18 July 2001
 URL:            https://tip.tcl-lang.org45.html
 Discussions-To: news:comp.lang.tcl
                 mailto:kennykb@acm.org
 Post-History:   

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 ABSTRACT 
==========

 TIP's #22 and #33 contain an oversight in specifying the behavior of 
 the multi-argument forms of /lset/ and /lindex/ when an empty index 
 list is specified. The intended behavior is that an empty list of 
 indices designates the entire list. 

 RATIONALE 
===========

 In the discussion of [TIP #33] 
 ([<URL:http://www.geocrawler.com/archives/3/7375/2001/5/0/5784409/>]), 
 Jim Ingham pointed out that the list of indices presented to the 
 multi-argument forms of /lindex/ and /lset/ is analogous to a database 
 cursor. This cursor is conceptually navigating a tree structure; the 
 command: 

     lindex $list {1 2 3}

 means, "extract the sublist at index 1 in $list, the sublist at index 2 
 in that list, and the element at index 3 in that list". 

 When implementing this functionality, the author of this TIP realized 
 that [TIP #22] and [TIP #33] provide no way to address the root of the 
 tree -- the entire list being manipulated. An empty list of indices is 
 a convenient means of specifying the root. 

 SPECIFICATION 
===============

    1. The specification of /lindex/ in [TIP #22] shall be amended so 
       that the forms: 

     lindex list

       and 

     lindex list {}

       will return the value of the entire list. The /list/ parameter is 
       not required to be a well-formed Tcl list when this form is used. 

    2. The specification of /lset/ in [TIP #33] shall be amended so that 
       the forms: 

     lset var value

       and 

     lset var {} value

       will simply store the supplied value into the variable named 
       /var/. Neither the old nor the new value of /var/ is required to 
       be a well-formed Tcl list when this form is used. The return 
       value of the operation, as with all other uses of /lset/, is the 
       new value of /var/. 

 REFERENCE IMPLEMENTATION 
==========================

 Work progresses on implementing this functionality; the currently 
 committed version is on SourceForge in the branch labeled, 
 /kennykb-tcl-22-33/. 

 DISCUSSION 
============

 Since this proposed change introduces syntax that is expressly 
 forbidden in [TIP #22] and [TIP #33], it does not have any impact on 
 backward compatibility. For the same reason, the author thought it 
 unwise to proceed with its implementation without a vote of the TCT. 

 SEE ALSO 
==========

 [TIP #22], [TIP #33]. 

 COPYRIGHT 
===========

 This document has been placed in the public domain. 

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