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uplevel(n)                                  Tcl Built-In Commands                                 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 calling stack) to move before exe-cuting executing
       cuting 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 toplevel
       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 info level command 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).

       The  namespace eval and apply commands offer other ways (besides procedure calls) that the Tcl naming
       context can change.  They add 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
       apply(n), namespace(n), upvar(n)

KEYWORDS
       context, level, namespace, stack frame, variables



Tcl                                                                                               uplevel(n)