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author | William Joye <wjoye@cfa.harvard.edu> | 2016-12-21 22:46:09 (GMT) |
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committer | William Joye <wjoye@cfa.harvard.edu> | 2016-12-21 22:46:09 (GMT) |
commit | 768f87f613cc9789fcf8073018fa02178c8c91df (patch) | |
tree | ec633f5608ef498bee52a5f42c12c49493ec8bf8 /tcl8.6/doc/uplevel.n | |
parent | 07e464099b99459d0a37757771791598ef3395d9 (diff) | |
parent | 05fa4c89f20e9769db0e6c0b429cef2590771ace (diff) | |
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Merge commit '05fa4c89f20e9769db0e6c0b429cef2590771ace' as 'tcl8.6'
Diffstat (limited to 'tcl8.6/doc/uplevel.n')
-rw-r--r-- | tcl8.6/doc/uplevel.n | 103 |
1 files changed, 103 insertions, 0 deletions
diff --git a/tcl8.6/doc/uplevel.n b/tcl8.6/doc/uplevel.n new file mode 100644 index 0000000..4decc6d --- /dev/null +++ b/tcl8.6/doc/uplevel.n @@ -0,0 +1,103 @@ +'\" +'\" 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. +'\" +.TH uplevel n "" Tcl "Tcl Built-In Commands" +.so man.macros +.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 +.PP +.CS +\fBuplevel\fR 1 {set x 43; d} +.CE +.PP +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 +.PP +.CS +\fBuplevel\fR {set x 42} +.CE +.PP +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: +.PP +.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, variable +.\" Local Variables: +.\" mode: nroff +.\" End: |