uplevel(3tcl)

NAME

   uplevel - Execute a script in a different stack frame

SYNOPSIS

   uplevel ?level? arg ?arg ...?
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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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