1uplevel(n)                   Tcl Built-In Commands                  uplevel(n)
2
3
4
5______________________________________________________________________________
6

NAME

8       uplevel - Execute a script in a different stack frame
9

SYNOPSIS

11       uplevel ?level? arg ?arg ...?
12______________________________________________________________________________
13

DESCRIPTION

15       All of the arg arguments are concatenated as if they had been passed to
16       concat; the result is then evaluated in the variable context  indicated
17       by level.  Uplevel returns the result of that evaluation.
18
19       If level is an integer then it gives a distance (up the procedure call‐
20       ing stack) to move before executing the command.  If level consists  of
21       #  followed by a number then the number gives an absolute level number.
22       If level is omitted then it defaults to 1.  Level cannot  be  defaulted
23       if the first command argument starts with a digit or #.
24
25       For  example,  suppose that procedure a was invoked from top-level, and
26       that it called b, and that b called c.   Suppose  that  c  invokes  the
27       uplevel  command.   If  level  is 1 or #2  or omitted, then the command
28       will be executed in the variable context of b.  If level  is  2  or  #1
29       then  the  command  will  be executed in the variable context of a.  If
30       level is 3 or #0 then the command will be executed at  top-level  (only
31       global variables will be visible).
32
33       The uplevel command causes the invoking procedure to disappear from the
34       procedure calling stack while the command is being  executed.   In  the
35       above example, suppose c invokes the command
36
37              uplevel 1 {set x 43; d}
38
39       where  d  is  another  Tcl  procedure.  The set command will modify the
40       variable x in b's context, and d will execute at level 3, as if  called
41       from b.  If it in turn executes the command
42
43              uplevel {set x 42}
44
45       then  the  set  command will modify the same variable x in b's context:
46       the procedure c does not appear to be on the call stack when d is  exe‐
47       cuting.   The info level command may be used to obtain the level of the
48       current procedure.
49
50       Uplevel makes it possible to implement new control  constructs  as  Tcl
51       procedures  (for  example, uplevel could be used to implement the while
52       construct as a Tcl procedure).
53
54       The namespace eval and apply commands offer other ways (besides  proce‐
55       dure  calls)  that  the Tcl naming context can change.  They add a call
56       frame to the stack to represent the namespace context.  This means each
57       namespace  eval  command  counts  as another call level for uplevel and
58       upvar commands.  For example, info level 1 will return a list  describ‐
59       ing a command that is either the outermost procedure call or the outer‐
60       most namespace eval command.  Also, uplevel #0 evaluates  a  script  at
61       top-level in the outermost namespace (the global namespace).
62

EXAMPLE

64       As stated above, the uplevel command is useful for creating new control
65       constructs.  This example shows how (without error handling) it can  be
66       used to create a do command that is the counterpart of while except for
67       always performing the test after running the loop body:
68
69              proc do {body while condition} {
70                  if {$while ne "while"} {
71                      error "required word missing"
72                  }
73                  set conditionCmd [list expr $condition]
74                  while {1} {
75                      uplevel 1 $body
76                      if {![uplevel 1 $conditionCmd]} {
77                          break
78                      }
79                  }
80              }
81

SEE ALSO

83       apply(n), namespace(n), upvar(n)
84

KEYWORDS

86       context, level, namespace, stack frame, variable
87
88
89
90Tcl                                                                 uplevel(n)
Impressum