[TCLCORE] TIP #295: Enhance Arguments to lrange

[Andreas L. <av...@lo...>]

 TIP #295: ENHANCE ARGUMENTS TO LRANGE 
=======================================
 Version:      $Revision: 1.1 $
 Author:       Andreas Leitgeb <avl_at_logic.at>
 State:        Draft
 Type:         Project
 Tcl-Version:  8.5
 Vote:         Pending
 Created:      Monday, 06 November 2006
 URL:          http://purl.org/tcl/tip/295.html
 WebEdit:      http://purl.org/tcl/tip/edit/295
 Post-History: 

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 ABSTRACT 
==========

 This TIP proposes an enhancement to *lrange* that lets it take more 
 than one start-end index pair. 

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

 Sometimes you need to extract a non-continuous selection of elements 
 from some list and build a new list from them. This requires the use of 
 multiple calls to *lrange* and a surrounding *concat* to build the 
 final list. However, since the following fails with a syntax error: 

       lrange {a b c d e f}  1 3  5 5

 It would seem a reasonable extension for such a usage of *lrange* to 
 return "b c d f". This would also make following pattern of usage 
 feasible: 

    lassign [lrange $someList 0 2  10 12]  var0 var1 var2 var10 var11 var12

 The index-pairs should /not/ be required to be in ascending order (this 
 might be difficult to determine when one index is end-relative), or 
 even disjoint. Even: 

    lrange {a b c d}  3 3  0 3  0 0

 should be made legal and return "d a b c d a". 

 PROPOSED CHANGE 
=================

 Change the implementation of *lrange* to accept any odd number of 
 arguments, with semantics (upon supply of a correct number of 
 arguments) equivalent to: 

    proc lrange-new {list args} {
       set result [list]
       foreach {start end} $args {
          lappend result {*}[lrange $list $start $end]
       }
       return $result
    }

 DRAFT IMPLEMENTATION 
======================

 Just the above mentioned procedure, at the moment. 

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

 This document has been placed in the public domain. 

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