TIP #276: Specify and Unify Variable Linking Commands
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TIP:	276
Title:	Specify and Unify Variable Linking Commands
Version:	$Revision: 1.20 $
Authors:	Miguel Sofer <msofer@users.sourceforge.net>
State:	Draft
Type:	Project
Tcl-Version:	8.7
Vote:	Pending
Created:	Sun Oct 01 00:00:00 UTC 2006
Keywords:	Tcl, global, variable, upvar, namespace upvar
Abstract
The purpose of this TIP is to simplify and clarify the semantics of the commands in Tcl that couple variables in different scopes together.
Rationale
This TIP proposes to specify and document the behaviour of the different variable linking commands in Tcl: global, variable, upvar and namespace upvar.
In particular, as many of these commands were initially designed to be mainly useful from within procedure bodies, the documentation does not specify their behaviour with respect to qualified variable names.
This TIP proposes to specify and document this behaviour, insuring that it is essentially the same in all these commands.
Current Situation
There have been a few bug reports concerning the behaviour of variable linking commands with respect to qualified variable names: 604226, 1274916, 1274918. Some are real bugs, some are just surprising but correct behaviour, some are surprising unspecified behaviour.
Within proc bodies all of these commands create local variables that are linked to original variables elsewhere (the following assumes that local is a non-qualified name):
"global var" creates a local variable "[[namespace tail var]]" that is linked to "var" as resolved from the global namespace.
"variable var" creates a local variable "[[namespace tail var]]" that is linked to "var" as resolved from [[namespace current]].
"upvar level var local" creates a local variable "local" that is linked to "var" as resolved in the context of the frame identified by level. It is an error to try to link to a proc-local variable from a namespace context. It is an error to link a variable to itself.
"namespace upvar ns var local" creates a local variable "local" that is linked to "var" as resolved from namespace ns (itself resolved in the current context). It is an error to link a variable to itself.
One undocumented issue is what should happen when local is a qualified name.
Another issue is the behaviour of these commands when invoked outside of procedure bodies:
"global var" is documented to be a no-op.
"variable var" is documented to create a namespace variable in the current namespace, without any mention of the behaviour when "var" is qualified. Currently that is a a no-op in terms of creating a variable, but it can change the value of the target, see [[Bug #604226]]. If var is a simple name and the corresponding variable already existed, variable may set its value (and an internal flag), but does no linking.
"upvar level var local" creates a variable "local" (as resolved from [[namespace current]]) that is linked to "var" as resolved in the context of the frame identified by level. It is an error to try to link to a proc-local variable from a namespace context. It is an error to link a variable to itself.
"namespace upvar ns var local creates a variable "local" (as resolved from [[namespace current]]) that is linked to "var" as resolved from namespace ns (itself resolved in the current context). It is an error to link a variable to itself.
Proposal
This TIP proposes to unify the criteria, making all of these commands essentially implementable in Tcl from the most general, upvar. The behaviour should not depend on the commands being invoked within or outside of a procedure body.
In all of the following, it is an error for local to be a qualified variable name.
"global var" always creates a variable "[[namespace tail var]]" in the current context, linked to a global variable "var".
"variable var" always creates a variable "[[namespace tail var]]" in the current context, linked to a variable "var" as resolved from [[namespace current]].
"upvar level var local" always creates a variable "local" in the current context, linked to "var" as resolved in the context of the frame identified by level. It is an error to try to link to a proc-local variable from a namespace context.
"namespace upvar ns var local" always creates a variable "local" in the current context, linked to "var" as resolved from namespace ns (itself resolved in the current context).
In all cases, attempting to link a variable to itself will be a no-op.
Compatibility
This TIP may cause breakage in some scripts relying on undocumented behaviour.
The specification changes for global and variable outside of proc bodies is almost certainly implementing what the programmer meant them to do, hence likely to fix more bugs than it causes. 
Reference Implementation and Documentation
Forthcoming at SF.
Copyright
This document has been placed in the public domain.
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