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uplevel(3tcl) Tcl Built-In Commands uplevel(3tcl) 





NAME


uplevel - 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 integer then the level gives
    an absolute level. If level is omitted then it defaults to 1.
    Level cannot be defaulted if the first command argument is an
    integer or starts with #.


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 info level command 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).


The namespace eval and apply commands offer other
    ways (besides procedure calls) that the Tcl naming context can change. They
    add 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).






EXAMPLE


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


        }


    }
}








SEE ALSO


apply(3tcl), namespace(3tcl), upvar(3tcl)






KEYWORDS


context, level, namespace, stack frame, variable







  

    
    Tcl