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uplevel(n) [centos man page]

uplevel(n)						       Tcl Built-In Commands							uplevel(n)

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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 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 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 out- ermost 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(n), namespace(n), upvar(n) KEYWORDS
context, level, namespace, stack frame, variables Tcl uplevel(n)

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upvar(n)						       Tcl Built-In Commands							  upvar(n)

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NAME
upvar - Create link to variable in a different stack frame SYNOPSIS
upvar ?level? otherVar myVar ?otherVar myVar ...? _________________________________________________________________ DESCRIPTION
This command arranges for one or more local variables in the current procedure to refer to variables in an enclosing procedure call or to global variables. Level may have any of the forms permitted for the uplevel command, and may be omitted if the first letter of the first otherVar isn't # or a digit (it defaults to 1). For each otherVar argument, upvar makes the variable by that name in the procedure frame given by level (or at global level, if level is #0) accessible in the current procedure by the name given in the corresponding myVar argu- ment. The variable named by otherVar need not exist at the time of the call; it will be created the first time myVar is referenced, just like an ordinary variable. There must not exist a variable by the name myVar at the time upvar is invoked. MyVar is always treated as the name of a variable, not an array element. Even if the name looks like an array element, such as a(b), a regular variable is created. Oth- erVar may refer to a scalar variable, an array, or an array element. Upvar returns an empty string. The upvar command simplifies the implementation of call-by-name procedure calling and also makes it easier to build new control constructs as Tcl procedures. For example, consider the following procedure: proc add2 name { upvar $name x set x [expr $x+2] } Add2 is invoked with an argument giving the name of a variable, and it adds two to the value of that variable. Although add2 could have been implemented using uplevel instead of upvar, upvar makes it simpler for add2 to access the variable in the caller's procedure frame. 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). If an upvar variable is unset (e.g. x in add2 above), the unset operation affects the variable it is linked to, not the upvar variable. | There is no way to unset an upvar variable except by exiting the procedure in which it is defined. However, it is possible to retarget an | upvar variable by executing another upvar command. | Traces and upvar | Upvar interacts with traces in a straightforward but possibly unexpected manner. If a variable trace is defined on otherVar, that trace | will be triggered by actions involving myVar. However, the trace procedure will be passed the name of myVar, rather than the name of oth- | erVar. Thus, the output of the following code will be localVar rather than originalVar: | proc traceproc { name index op } { | puts $name | } | proc setByUpvar { name value } { | upvar $name localVar | set localVar $value | } | set originalVar 1 | trace variable originalVar w traceproc | setByUpvar originalVar 2 | } | If otherVar refers to an element of an array, then variable traces set for the entire array will not be invoked when myVar is accessed (but | traces on the particular element will still be invoked). In particular, if the array is env, then changes made to myVar will not be passed | to subprocesses correctly. SEE ALSO
global(n), namespace(n), uplevel(n), variable(n) KEYWORDS
context, frame, global, level, namespace, procedure, variable Tcl upvar(n)
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