uplevel - Execute a script in a different stack frame

       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 integer then the level gives an  absolute  level.   If
       level  is  omitted  then it defaults to 1.  Level cannot be defaulted if the first command argument is an
       integer or starts with #.

       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 infolevel 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  namespaceeval  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  namespaceeval  command counts as another call level for uplevel and upvar commands.  For example,
       infolevel1 will return a list describing a command that is either the outermost procedure call  or  the
       outermost  namespaceeval  command.   Also,  uplevel#0 evaluates a script at top-level in the outermost
       namespace (the global namespace).

       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
                      }
                  }
              }

       context, level, namespace, stack frame, variable

Tcl                                                                                                uplevel(3tcl)

       uplevel - Execute a script in a different stack frame

apply(3tcl), namespace(3tcl), upvar(3tcl)

uplevel ?level? arg ?arg...?
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