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 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
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, variables
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