Tcl Built-In Commands


uplevel - Execute a script in a different stack frame


uplevel ?level? arg ?arg ...?


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 command ``info level'' 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).


context, stack frame, variables

Last change:

[ tcl8.0a1 | tk8.0a1 | X-ref ]

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