Note that you should use a catch with unset if there is a chance that a variable being unset hasn't been created yet.

---

Did you know that an unset can appear to fail? Erik Leunissen wrote about this on comp.lang.tcl during Oct, 2002:

Well, I've been bitten by this one on a *regular* basis, but not on a *frequent* basis. So infrequently, that I tend to forget about the issue, and once it happens again, it takes me by surprise and I need hours to find the cause of evil, which I'll explain:

The following happens when you bind a variable to an entry widget through the -textvariable option and subsequently issue [unset textvariable] :

1. the textvariable may become unset for a fraction of a millisecond, I'm not sure. Perhaps somebody else can elaborate on this ...
2. the textvariable binding will immediately reassign the value of the contents of the entry widget to the textvariable. (If it has been unset for a short moment, it is recreated)

So what's the recipe then?

1. release the binding to the entry widget by:

• doing [ .entry -textvariable {} ], or
• destroying the widget entirely

1. unset the variable: [unset textvariable]
2. try to remember this issue (better than I do) when it takes you by surprise the next time.

For the coming releases of Tcl (still in CVS), you can also set an unset trace on the textvariable, and let the trace handler do the above step one and/or two. For current releases this will not yet work because of a (recently fixed) bug.

---

TV 24 apr '03 I just found behaviour I didn't get:

 (Tcl) 68 % info var http::*
 ::http::urlTypes ::http::http ::http::1 ::http::alphanumeric ::http::encodings ::http::formMap ::http::defaultCharset
 (Tcl) 68 % unset ::http::1
 can't unset "::http::1": no such variable
 (Tcl) 69 % info var http::*
 ::http::urlTypes ::http::http ::http::alphanumeric ::http::encodings ::http::formMap ::http::defaultCharset

Its wish 8.4.1, and it runs bwise, a webserver (tclhttpd with some alterations), and this is clearly from the http package to fetch a webpage. Maybe the manual gives a neat answer, I just found it noteworthy that a unset in error still seems to do it's unsetting.

RS: ..or that the variable was removed by the web server between the first two commands? What happens if you just call the first command repeatedly?

TV: It would seem to be stable. Its the page content and url info etc array variable, which sticks around it seems until deleted, that's the whole reason I was looking for some garbage collection, or delayed freeing. It could be there is an event linked with some element, I don't know, I didn't write the at least handy http package...

CHL: I don't know why, but it seems to happen when a variable in a namespace is defined but is not set. For instance...

 1 % namespace eval nspace { variable novalue ; variable value 0 }
 2 % info vars ::nspace::*
 ::nspace::value ::nspace::novalue
 3 % unset ::nspace::novalue
 can't unset "::nspace::novalue": no such variable
 4 % info vars ::nspace::*
 ::nspace::value