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).
Copyright © 1993 The Regents of the University of California. Copyright © 1994-1997 Sun Microsystems, Inc. Copyright © 1995-1997 Roger E. Critchlow Jr.