uplevel(3Tcl) uplevel(3Tcl)
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
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