'\" '\" Copyright (c) 1993 The Regents of the University of California. '\" Copyright (c) 1994-1997 Sun Microsystems, Inc. '\" '\" See the file "license.terms" for information on usage and redistribution '\" of this file, and for a DISCLAIMER OF ALL WARRANTIES. '\" '\" RCS: @(#) $Id: uplevel.n,v 1.1.1.1 2007/07/10 15:04:23 duncan 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 command ``\fBinfo level\fR'' 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 \fBnamespace eval\fR 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 \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" namespace(n), upvar(n) .SH KEYWORDS context, level, namespace, stack frame, variables