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uplevel(n) Tcl Built-In Commands uplevel(n)
_________________________________________________________________________________________________
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 con-
text, and d will execute at level 3, as if called from b. If it in turn executes the com-
mand
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 exam-
ple, 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 names-
pace 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 com-
mand. 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)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache/2.4.6 (CentOS)
Under GNU General Public License
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