uplevel (n)
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 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).
namespace eval is another way (besides procedure calls)
that the Tcl naming context can change.
It adds 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).
SEE ALSO
KEYWORDS
context, level, namespace, stack frame, variables
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