TIP:            58
Title:          Extend [set] to Assign Multiple Values to Multiple Variables
Version:        $Revision: 1.6 $
Author:         Anselm Lingnau <anselm@strathspey.org>
State:          Rejected
Type:           Project
Vote:           Done
Created:        02-Sep-2001
Post-History:   
Tcl-Version:    8.5

~ Abstract

This TIP proposes a multiple assignment command as a
backwards-compatible extension to the Tcl ''set'' command.

~ Introduction

Often one needs to assign values to several variables in close
proximity.  Right now several ''set'' commands are necessary:

|  set a 123
|  set b 456

or

|  set a 123; set b 456

Or one abuses the ''foreach'' command:

|  foreach {a b} {123 456} break

However, by analogy to the ''variable'' and ''array set'' commands,
the following would be useful:

|  set a 123 b 456

This would assign 123 to the variable ''a'' and 456 to the variable
''b''.

Note that this extension is backwards-compatible to existing uses of
the ''set'' command since until now only one or two arguments to
''set'' were allowed.

~ Specification

The ''set'' command is extended to allow either one or an even number
of arguments.  The behaviour in the case of one argument remains the
one documented in the ''set'' manual page; when an even number of
arguments is specified, the behaviour of ''set v0 e0 ... vn en'' is
identical to that of the sequence of commands ''set v0 e0; ...; set vn
en'' according to the traditional semantics, except that the way
Tcl processes commands means that ''e0'' ... ''en'' are all
evaluated before any assignments are performed. I.e., the commands

|  set a 1
|  set a 2 b $a
|  puts $b

print ''1'', not ''2''. If this is an issue you must use separate
''set'' statements.

The command ''set v0 e0 ... vn en'' returns the value of ''en''.

~ Rationale

This extension is an obvious analogy to the ''variable'' and ''array
set'' commands of Tcl, both of which allow an alternating list of
names and expressions to be given as arguments.  It is completely
backwards-compatible (''set'' invocations with more than two arguments
used to be syntax errors) and very easily implemented.

This extension in no way prejudices against the adoption and use of
other multiple-assignment commands, such as ''lassign'' (see [57]).
In particular, the ''set'' extension is unsuitable for assigning a
list result to a number of variables element by element.  However, its
simplicity and consistency to other similar Tcl commands is appealing.

~ Reference Implementation

A patch to Tcl 8.4a3 which implements the ''set'' extension may be
found at http://anselm.our-isp.org/set-patch.diff - a patched Tcl
8.4a3 passes the Tcl 8.4a3 regression test suite with no test
failures.  No test cases nor documentation for the ''set'' extensions
have been devised yet but this is easy to do once there is a consensus
that this feature is actually desirable.

~ Copyright

This document is placed in the public domain.