source: downloads/tcl8.5.2/doc/uplevel.n @ 44

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'\" Copyright (c) 1993 The Regents of the University of California.
'\" Copyright (c) 1994-1997 Sun Microsystems, Inc.
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'\" RCS: @(#) $Id: uplevel.n,v 1.10 2007/12/13 15:22:33 dgp Exp $
'\" 
.so man.macros
.TH uplevel n "" Tcl "Tcl Built-In Commands"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
uplevel \- Execute a script in a different stack frame
.SH SYNOPSIS
\fBuplevel \fR?\fIlevel\fR?\fI arg \fR?\fIarg ...\fR?
.BE
.SH DESCRIPTION
.PP
All of the \fIarg\fR arguments are concatenated as if they had
been passed to \fBconcat\fR; the result is then evaluated in the
variable context indicated by \fIlevel\fR.  \fBUplevel\fR returns
the result of that evaluation.
.PP
If \fIlevel\fR is an integer then
it gives a distance (up the procedure calling stack) to move before
executing the command.  If \fIlevel\fR consists of \fB#\fR followed by
a number then the number gives an absolute level number.  If \fIlevel\fR
is omitted then it defaults to \fB1\fR.  \fILevel\fR cannot be
defaulted if the first \fIcommand\fR argument starts with a digit or \fB#\fR.
.PP
For example, suppose that procedure \fBa\fR was invoked
from top-level, and that it called \fBb\fR, and that \fBb\fR called \fBc\fR.
Suppose that \fBc\fR invokes the \fBuplevel\fR command.  If \fIlevel\fR
is \fB1\fR or \fB#2\fR  or omitted, then the command will be executed
in the variable context of \fBb\fR.  If \fIlevel\fR is \fB2\fR or \fB#1\fR
then the command will be executed in the variable context of \fBa\fR.
If \fIlevel\fR is \fB3\fR or \fB#0\fR then the command will be executed
at top-level (only global variables will be visible).
.PP
The \fBuplevel\fR command causes the invoking procedure to disappear
from the procedure calling stack while the command is being executed.
In the above example, suppose \fBc\fR invokes the command
.CS
\fBuplevel\fR 1 {set x 43; d}
.CE
where \fBd\fR is another Tcl procedure.  The \fBset\fR command will
modify the variable \fBx\fR in \fBb\fR's context, and \fBd\fR will execute
at level 3, as if called from \fBb\fR.  If it in turn executes
the command
.CS
\fBuplevel\fR {set x 42}
.CE
then the \fBset\fR command will modify the same variable \fBx\fR in \fBb\fR's
context:  the procedure \fBc\fR does not appear to be on the call stack
when \fBd\fR is executing.  The \fBinfo level\fR command may
be used to obtain the level of the current procedure.
.PP
\fBUplevel\fR makes it possible to implement new control
constructs as Tcl procedures (for example, \fBuplevel\fR could
be used to implement the \fBwhile\fR construct as a Tcl procedure).
.PP
The \fBnamespace eval\fR and \fBapply\fR 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 \fBnamespace eval\fR command
counts as another call level for \fBuplevel\fR and \fBupvar\fR commands.
For example, \fBinfo level 1\fR will return a list
describing a command that is either
the outermost procedure call or the outermost \fBnamespace eval\fR command.
Also, \fBuplevel #0\fR evaluates a script
at top-level in the outermost namespace (the global namespace).
.SH EXAMPLE
As stated above, the \fBuplevel\fR command is useful for creating new
control constructs.  This example shows how (without error handling)
it can be used to create a \fBdo\fR command that is the counterpart of
\fBwhile\fR except for always performing the test after running the
loop body:
.CS
proc do {body while condition} {
    if {$while ne "while"} {
        error "required word missing"
    }
    set conditionCmd [list expr $condition]
    while {1} {
        \fBuplevel\fR 1 $body
        if {![\fBuplevel\fR 1 $conditionCmd]} {
            break
        }
    }
}
.CE
.SH "SEE ALSO"
apply(n), namespace(n), upvar(n)
.SH KEYWORDS
context, level, namespace, stack frame, variables