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 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
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
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).
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
}
}
}
apply(3tcl),
namespace(3tcl),
upvar(3tcl)
context, level, namespace, stack frame, variable