NAME

       uplevel - Execute a script in a different stack frame

SYNOPSIS

       uplevel ?level? arg ?arg ...?
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DESCRIPTION

       All of the arg arguments are concatenated as if they had been passed to concat; the result
       is then evaluated in the variable context indicated by level.  Uplevel returns the  result
       of that evaluation.

       If  level  is an integer then it gives a distance (up the procedure calling stack) to move
       before executing the command.  If level consists of # followed by a number then the number
       gives  an absolute level number.  If level is omitted then it defaults to 1.  Level cannot
       be defaulted if the first command argument starts with a digit or #.

       For example, suppose that procedure a was invoked from top-level, and that  it  called  b,
       and that b called c.  Suppose that c invokes the uplevel command.  If level is 1 or #2  or
       omitted, then the command will be executed in the variable context of b.  If level is 2 or
       #1  then  the  command will be executed in the variable context of a.  If level is 3 or #0
       then the command will be executed at top-level (only global variables will be visible).

       The uplevel command causes the invoking procedure to disappear from the procedure  calling
       stack  while  the  command is being executed.  In the above example, suppose c invokes the
       command
              uplevel 1 {set x 43; d}
       where d is another Tcl procedure.  The set command will  modify  the  variable  x  in  b's
       context,  and  d will execute at level 3, as if called from b.  If it in turn executes the
       command
              uplevel {set x 42}
       then the set command will modify the same variable x in b's context:  the procedure c does
       not appear to be on the call stack when d is executing.  The command ``info level'' may be
       used to obtain the level of the current procedure.

       Uplevel makes it possible to implement new  control  constructs  as  Tcl  procedures  (for
       example, uplevel could be used to implement the while construct as a Tcl procedure).

       namespace  eval  is  another way (besides procedure calls) that the Tcl naming context can
       change.  It adds a call frame to the stack to represent the namespace context.  This means
       each  namespace  eval command counts as another call level for uplevel and upvar commands.
       For example, 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 outermost namespace (the global namespace).

EXAMPLE

       As stated above, the uplevel command is useful for creating new control constructs.   This
       example  shows  how (without error handling) it can be used to create a do command that is
       the counterpart of while except for always performing the  test  after  running  the  loop
       body:
              proc do {body while condition} {
                  if {$while ne "while"} {
                      error "required word missing"
                  }
                  set conditionCmd [list expr $condition]
                  while {1} {
                      uplevel 1 $body
                      if {![uplevel 1 $conditionCmd]} {
                          break
                      }
                  }
              }

SEE ALSO

       namespace(3tcl), upvar(3tcl)

KEYWORDS

       context, level, namespace, stack frame, variables