uplevel



uplevel

NAME
SYNOPSIS
DESCRIPTION
EXAMPLE
SEE ALSO
KEYWORDS

___________________________

NAME

uplevel − Execute a script in a different stack frame

SYNOPSIS

uplevel ?level? arg ?arg ...? ___________________________

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 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 outermost 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(n), namespace.conf(5), upvar(n)

KEYWORDS

context, level, namespace, stack frame, variable







Opportunity


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.





Free Software


Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.


Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.





Free Books


The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.


Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.





Education


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.


Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.