upvar man page on Archlinux

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upvar(n)		     Tcl Built-In Commands		      upvar(n)


       upvar - Create link to variable in a different stack frame

       upvar ?level? otherVar myVar ?otherVar myVar ...?

       This  command  arranges	for one or more local variables in the current
       procedure to refer to variables in an enclosing procedure  call	or  to
       global  variables.   Level  may have any of the forms permitted for the
       uplevel command, and may be omitted (it defaults to 1).	For each  oth‐
       erVar  argument, upvar makes the variable by that name in the procedure
       frame given by level (or at global level, if level is #0) accessible in
       the  current  procedure	by  the	 name given in the corresponding myVar
       argument.  The variable named by otherVar need not exist at the time of
       the  call;  it will be created the first time myVar is referenced, just
       like an ordinary variable.  There must not exist a variable by the name
       myVar  at  the  time  upvar is invoked.	MyVar is always treated as the
       name of a variable, not an array element.  An error is returned if  the
       name  looks like an array element, such as a(b).	 OtherVar may refer to
       a scalar variable, an array, or an array	 element.   Upvar  returns  an
       empty string.

       The  upvar command simplifies the implementation of call-by-name proce‐
       dure calling and also makes it easier to build new  control  constructs
       as Tcl procedures.  For example, consider the following procedure:

	      proc add2 name {
		  upvar $name x
		  set x [expr {$x + 2}]

       If  add2	 is invoked with an argument giving the name of a variable, it
       adds two to the value of that variable.	Although add2 could have  been
       implemented  using uplevel instead of upvar, upvar makes it simpler for
       add2 to access the variable in the caller's procedure frame.

       namespace eval is another way (besides procedure calls)	that  the  Tcl
       naming context can change.  It adds a call frame to the stack to repre‐
       sent the namespace context.  This means	each  namespace	 eval  command
       counts as another call level for uplevel and upvar commands.  For exam‐
       ple, info level 1 will return a	list  describing  a  command  that  is
       either  the  outermost  procedure  call or the outermost namespace eval
       command.	 Also, uplevel #0 evaluates a script at top-level in the  out‐
       ermost namespace (the global namespace).

       If  an upvar variable is unset (e.g. x in add2 above), the unset opera‐
       tion affects the variable it is linked  to,  not	 the  upvar  variable.
       There is no way to unset an upvar variable except by exiting the proce‐
       dure in which it is defined.  However, it is possible  to  retarget  an
       upvar variable by executing another upvar command.

       Upvar  interacts	 with  traces  in a straightforward but possibly unex‐
       pected manner.  If a variable trace is defined on otherVar, that	 trace
       will  be triggered by actions involving myVar.  However, the trace pro‐
       cedure will be passed the name of myVar, rather than the name of other‐
       Var.   Thus, the output of the following code will be “localVar” rather
       than “originalVar”:

	      proc traceproc { name index op } {
		  puts $name
	      proc setByUpvar { name value } {
		  upvar $name localVar
		  set localVar $value
	      set originalVar 1
	      trace variable originalVar w traceproc
	      setByUpvar originalVar 2

       If otherVar refers to an element of an array, then variable traces  set
       for  the	 entire	 array will not be invoked when myVar is accessed (but
       traces on the particular element will still be invoked).	  In  particu‐
       lar, if the array is env, then changes made to myVar will not be passed
       to subprocesses correctly.

       A decr command that works like incr except it subtracts the value  from
       the variable instead of adding it:

	      proc decr {varName {decrement 1}} {
		  upvar 1 $varName var
		  incr var [expr {-$decrement}]

       global(n), namespace(n), uplevel(n), variable(n)

       context, frame, global, level, namespace, procedure, upvar, variable

Tcl								      upvar(n)

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