TIP:            45
Title:          Empty index lists for [lindex] and [lset]
Version:        $Revision: 1.9 $
Author:         Kevin Kenny <kennykb@acm.org>
Author:         Don Porter <dgp@users.sourceforge.net>
State:          Final
Type:           Project
Vote:           Done
Created:        18-Jul-2001
Post-History:   
Discussions-To: news:comp.lang.tcl,mailto:kennykb@acm.org
Keywords:       lindex,lset,multiple arguments,sublists
Tcl-Version:    8.4b1

~ 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 [33]
([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 [22] and [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 [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.

 1. The specification of ''lset'' in [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
[22] and [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

[22], [33].

~ Copyright

This document has been placed in the public domain.