TIP #57 Version 1.2: Multiple Assignment Command

This is not necessarily the current version of this TIP.

Abstract

This TIP proposes a new function mset (or a modification to set) to perform multiple assignment.

Introduction

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]

Design Alternatives

The following Tcl code is an implementation I have made myself for mset. Because it returns the remaining elements in the list that were not assigned to a value, the proposed specification is not suitable as an extension to set, but rather as a new command mset.

 ##
 #   mset - set multiple variables with values from a list
 #
 # SYNOPSIS
 #   [mset <varlist> <valuelist>]
 #
 # DESCRIPTION
 #   Sets multiple variables with values from a list.  The <varlist>
 #   contains variable names and each variable will be assigned the
 #   corresponding value in <valuelist>.  For example
 #
 #   [mset \{name age sex\} \{Bill 42 male\}]
 #
 #   It is an error if <valuelist> contains fewer elements than the
 #   <varlist>.  If the valuelist contains more elements than the
 #   <varlist> the exceeding values will not be assigned to any
 #   variable.
 #
 #   The procedure returns the a list containing the remaining 
 #   elements in <valuelist> that were no assigned to a variable.
 ##
 
 proc misc::mset {varList valueList} {
     if {[llength $varList] > [llength $valueList]} {
 	error "mset error: more variables than values"
     }
     set i 0
     foreach varName $varList {
 	upvar $varName var
 	set var [lindex $valueList $i]
 	incr i
     }
     return [lrange $valueList 0 [expr $i - 1]]
 }

If set is extended rather than introducing a new mset command, it should return a list comprising the elements that were set, so

 set {name age} {Bill 42 male developer}

would return the list,

 {Bill 42}

while

 mset {name age} {Bill 42 male developer}

as specified above, would return:

 {male developer}

Evaluation of Alternatives

Since the syntax proposed for an extended set command would produce an error in its current form, it will create no problems of backward compatibility. Returning the list of the elements that were assigned to variables would also be consistent with the current set command.

The advantage of the mset command is that it provides a convenient way to take care of exceeding values when you have assigned the first values to variables. However, because it is inconsistent with the set command in terms of the returned vale, it should be implemented as a different command.

A third theoretical alternative is to implement a new mset command having the specification outlined for the extended set command. Personally, I think it is unnecessary to introduce a new command when you can extend an existing function consistently and without problems with backward compatibility.

Copyright

This document is placed in the public domain.

This is not necessarily the current version of this TIP.