trusty (3) uplevel.3tcl.gz

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