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uplevel.3tcl
Langue: en
Version: 349676 (ubuntu - 24/10/10)
Section: 3 (Bibliothèques de fonctions)
NAME
uplevel - Execute a script in a different stack frameSYNOPSIS
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}
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uplevel {set x 42}
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(3tcl), namespace(3tcl), upvar(3tcl)KEYWORDS
context, level, namespace, stack frame, variablesContenus ©2006-2024 Benjamin Poulain
Design ©2006-2024 Maxime Vantorre