[Xotcl] Problem with automatic variable unsetting upon return from an instproc?

[Jim Russell]

Uwe:

   Thank you so much for the very lucid explanation.  I was hoping that 
I could use the trace mechanism to implement a garbage collection 
scheme.  I was storing the XOTcl object name in an ordinary Tcl variable 
upon which I placed the unset trace.  When the variable goes out of 
scope, the unset callback is called, and I had hoped to dereference the 
variable to call the XOTcl object's destroy method.  However, this 
doesn't appear to be a good approach.  Do you have any suggestions?

Jim

Uwe Zdun wrote:
> Jim,
> 
> at first glance, the behavior seems ok to me. You're setting 2 traces to the 
> local scope variable "f" which get unset in reverse order -> so
> the two "f" outputs should be ok.
> 
> The rhs traces "2" and "3" are for the two inner invocations. For the outer
> invocations, the callframe is deleted before the Trace is executed. The 
> problem here is that Tcl does not print the error that occurs.
> 
> Consider the following plain Tcl example:
> 
> proc tracecb { name1 name2 op } {
>    if {[catch {
>      upvar $name1 var
>      puts "tracecb \"$name1\" \"$name2\" \"$op\" \"$var\""
>    } err]} { 
>      puts $err
>    }
> }
> proc x {a} {
>   ::trace add variable a [list unset] ::tracecb
>   if {$a == 0} {return}
>   incr a -1
>   x $a
> }
> 
> x 4
> 
> This prints:
> tracecb "a" "" "unset" "0"
> tracecb "a" "" "unset" "1"
> tracecb "a" "" "unset" "2"
> tracecb "a" "" "unset" "3"
> can't read "var": no such variable
> 
> so you cannot rely on the "upvar" in the trace to a local scope variable.
> 
> You can try to refer to an XOTcl variable (an instance variable)
> with "[self] trace ..." instead.
> 
> Uwe
> 
>