TIP #331: ALLOW [LSET] TO EXTEND LISTS 
========================================
 Version:      $Revision: 1.5 $
 Author:       Kevin B. Kenny <kennykb_at_acm.org>
 State:        Final
 Type:         Project
 Tcl-Version:  8.6
 Vote:         Done
 Created:      Monday, 22 September 2008
 URL:          https://tip.tcl-lang.org331.html
 Post-History: 

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 ABSTRACT 
==========

 This TIP proposes to modify the *lset* command to allow it to extend 
 lists. 

 PROPOSAL 
==========

 The *lset* command shall be modified to allow the index /end+1/ (or any 
 equivalent index designating the element one beyond the last element of 
 the list). For the simple usage: 

       lset list end+1 foo

 the effect is the same as if the script had evaluated the command: 

       lappend list foo

 For the usage: 

       lset list $n end+1 foo

 the effect is much the same as if the script had evaluated: 

       set temp [lindex $list $n]
       lappend temp foo
       lset list $n $temp

 (except, of course that no /temp/ variable is created) 

 The usage: 

       lset list end+1 0 foo

 or, equivalently, 

       lset list end+1 end+1 foo

 is equivalent to 

       lappend list [list foo]

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

 Clearly, there are equivalent constructs to all of these usages. 
 Nevertheless, it is convenient not to have to worry about whether a 
 given index is in bounds, particularly when iterating through a vector 
 or matrix. With the proposed change, many of the standard algorithms in 
 linear algebra will just work, without either having to preinitialize a 
 list to a given size or to have a test to determine whether to use 
 *lappend* or *lset* to store a given element. 

 ALTERNATIVES 
==============

 Explicitly out of scope is the treatment of indices greater than or 
 equal to /end+2/. Setting non-contiguous elements would raise false 
 expectations of sparse lists, *NULL* elements, and so on. 

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

 Copyright © 2008 by Kevin B. Kenny. 

 This document may be distributed subject to the terms and conditions 
 set forth in the Open Publication License, version 1.0 
 [<URL:http://www.opencontent.org/openpub/>]. 

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