TIP:		184
Title:		Avoid Creating Unusable Variables
Version:	$Revision: 1.4 $
Author:		Miguel Sofer <msofer@users.sf.net>
State:		Final
Type:		Project
Vote:		Done
Created:	27-Mar-2004
Post-History:	
Keywords:	Tcl, upvar, global
Tcl-Version:	8.5

~ Abstract

Both '''upvar''' and '''global''' can create unreachable variables:
scalar variables whose name looks like an array element, e.g.,
a(b). This behaviour is documented in the '''upvar''' manpage.  This
TIP proposes that both '''upvar''' and '''global''' raise errors
instead of creating such variables.

~ Rationale

As detailed in [[Bug #600812]]
[http://sf.net/tracker/?func=detail&aid=600812&group_id=10894&atid=110894],
both '''upvar''' and '''global''' can create unreachable variables:
scalar variables whose names looks like an array element.

One example is:

|   upvar 0 x(1) y(1)

which creates a ''scalar'' variable named y(1) that is linked to the
element 1 of the array x. However, there is no way for a tcl script to
read or write such scalar variables, the parser will interpret that
name as referring to an element of the array named y.

Another example is:

|   proc a {} {
|       global x(1)
|       ...
|   }

which will create a scalar local variable named 'x(1)', linked to the
element 1 of theglobal array x. Again, this variable is unreachable.

This TIP proposes '''upvar''' and '''global''' raise an error rather
than creating such a variable, mimicking in this respect the behaviour
of '''variable'''.

Note that a TIP is required because the behaviour of '''upvar''' is
documented in the manual page, so that it cannot really be described
as a bug:

 > MyVar is always treated as the name of a variable, not an array
   element. Even if the name looks like an array element, such as
   a(b), a regular variable is created.


~ Reference Implementation

There is a patch attached to the bug ticket
[http://sf.net/tracker/?func=detail&aid=600812&group_id=10894&atid=110894]
that implements this TIP.

~ Copyright

This TIP is in the public domain.