uplevel(3tcl)						       Tcl Built-In Commands						     uplevel(3tcl)

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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 indi-
       cated 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 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 out-
       ermost 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(3tcl), namespace(3tcl), upvar(3tcl)

KEYWORDS

       context, level, namespace, stack frame, variables

Tcl																     uplevel(3tcl)