Tcl upvar issue

Question

Im trying to modify a variable using upvar (in an upward stack), but the value of the variable is passed to the procedure and not the variable name.

I cannot change what is passed, since it is already implemented widely on the program.

Is there a way to modify the file name in some way ?

proc check_file_exists {name} {
   upvar $name newName
   check_exists $name     #Do all checks that file is there
   set newName $name_1
}

check_file_exists $name
puts $name

this code will print the old name of the file and not the new one.

If you don't know the name, you cannot modify the variable. – n. m. could be an AI Aug 30 '15 at 13:43 — n. m. could be an AI, Aug 30 '15 at 13:43

Peter Lewerin · Accepted Answer · 2015-08-31T04:26:17.450

What I think you should do is bite the bullet and change the calls. It's a fairly simple search-and-replace, after all. The code will be more sane than if you use any of the other solutions.

check_file_exists name

Or, you could add another parameter to the parameter list and use that to pass the name, making the first argument a dummy argument.

check_file_exists $name name

Or, if you're not using the return value, you could return the new value and assign it back:

set name [check_file_exists $name]

Or, you could assign the new value to a global variable (e.g. theValue) inside the procedure, and assign that back:

check_file_exists $name
# don't need this if you're in global scope
global theValue
set name $theValue

Or, you could assign the name to a global variable (e.g. theName) and access that inside the procedure: the procedure will be able to update name directly.

# don't need this if you're in global scope
global theName
set theName name
check_file_exists $name

(There are some variations on this f.i. using upvar.)

None of the alternatives are pretty, and all of them still require you to make a change at the call (except the last, if you only ever use one variable for this value). If you're adamant about not doing that, there's always Donal's info frame solution, which only requires the procedure itself to be changed.

Let me know if you want help with the procedure code for any of these alternatives.

score 1 · Answer 2 · answered Aug 30 '15 at 11:32

This is quite difficult; it's really not the way you're supposed to work. But you can do it using info frame -1 (a facility usually used for debugging) to find out exactly how the current procedure was called. However, you need to be careful as the caller might be using the result of a command: this is an unsafe hack.

proc check_file_exists {name} {
    set caller [dict get [info frame -1] cmd]
    if {[regexp {^check_file_exists +\$(\w+)} $caller -> varName]} {
        # OK, we were called with a simple variable name
        puts "Called with variable $varName"
    } else {
        # Complicated case! Help...
        return -code error "must be called with a simple variable's contents"
    }

    upvar 1 $varName newName
    check_exists $name
    set newName $name_1
}

If the name of the variable is already known, `upvar varname localname` can also do the job. Opens a can of worms. — Johannes Kuhn, Aug 30 '15 at 16:18
This whole area is multiple high-pressure cans of worms. “I cannot change what is passed” is usually a sign that some deeply wrong way of working is about to be attempted. — Donal Fellows, Aug 30 '15 at 19:48

Tcl upvar issue

2 Answers2