TIP #57: MOVE TCLX'S [LASSIGN] INTO THE TCL CORE 
==================================================
 Version:      $Revision: 2.4 $
 Author:       Donal K. Fellows <fellowsd_at_cs.man.ac.uk>
               Agnar Renolen <agnar.renolen_at_emap.no>
               Don Porter <dgp_at_users.sf.net>
 State:        Final
 Type:         Project
 Tcl-Version:  8.5
 Vote:         Done
 Created:      Thursday, 30 August 2001
 URL:          https://tip.tcl-lang.org57.html
 Post-History: 

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 ABSTRACT 
==========

 This TIP proposes to move the /lassign/ command from the TclX extension 
 into the Tcl core to make multiple assignment a much easier process for 
 people. 

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

 In many cases, a command needs to return more than one return value to 
 the caller. For example, suppose that the statement: 

    set coords [LocateFeature $featureID]

 would set the variable "coords" to a list containing two elements "x" 
 and "y". Assume that you need to set the "x" and "y" components 
 directly, you can do this today using the following statement: 

    foreach {x y} [LocateFeature $featureID] {}

 Now, this is not what the /foreach/ command was designed for, and it is 
 not obvious at first glance from the source code what the statement 
 does. Although it is quite useful for the purpose described in this 
 TIP, It would be more logical if the developer could write the 
 following: 

    set {x y} [LocateFeature $featureID]

 or 

    mset {x y} [LocateFeature $featureID]

 However, there is already a command in TclX for doing this kind of 
 operation: [lassign]. Given that many people already know TclX, 
 importing the command from there makes a great deal of sense. It also 
 has the nice feature of returning those list items that were not 
 assigned, making it easy to strip a few words off the front of a list. 
 That sort of operation is useful when performing tasks like 
 command-line option parsing. 

 PROPOSAL 
==========

 Define a new command in Tcl called [lassign] with the following syntax 
 (/$val/ indicates an argument that the caller would supply): 

    lassign $listValue $varName ?$varName ...?

 The command interprets its first argument as a list value and all 
 subsequent arguments as variable names. The first item in the list 
 value (i.e. at index 0) will be assigned to the first variable named, 
 the second item in the list value will be assigned to the second 
 variable named (if present), etc. When there are more variables than 
 list items, the remaining variables will be assigned the empty string. 
 The result of the command is a sublist of the input list-value that 
 contains only items that were not assigned to a variable; if all values 
 were assigned, the result is an empty list. 

 This is exactly the specification of the behaviour of the 
 correspondingly-named command in TclX. 

 NOTES 
=======

 It should be possible to efficiently compile [lassign] in many cases, 
 which would make a tremendous difference in execution speed over not 
 only the TclX version of [lassign], but also over the [foreach] 
 "idiom", especially when assigning to variables that are not simple 
 local variables (the case which [foreach] compilation is optimized 
 for.) For this reason, I'm not committing to implementing [lassign] 
 using the TclX code. 

 This TIP was substantially different in the past. Please view the CVS 
 history for details. 

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

 This document is placed in the public domain. 

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