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OpenSolaris 2009.06 - man page for uplevel (opensolaris section 1t)

uplevel(1T)						       Tcl Built-In Commands						       uplevel(1T)


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 indi- cated 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 rep- resent 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).
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 } } }
namespace(1T), upvar(1T)
context, level, namespace, stack frame, variables
See attributes(5) for descriptions of the following attributes: +--------------------+-----------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +--------------------+-----------------+ |Availability | SUNWTcl | +--------------------+-----------------+ |Interface Stability | Uncommitted | +--------------------+-----------------+
Source for Tcl is available on Tcl uplevel(1T)