uplevel(n)

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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 call-
       ing 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  exe-
       cuting.   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 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).

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(n), upvar(n)

KEYWORDS

       context, level, namespace, stack frame, variables

Tcl                                                                 uplevel(n)