return(3) Return from a procedure

SYNOPSIS

return ?-code code? ?-errorinfo info? ?-errorcode code? ?string?




DESCRIPTION

Return immediately from the current procedure (or top-level command or source command), with string as the return value. If string is not specified then an empty string will be returned as result.

EXCEPTIONAL RETURN CODES

In addition to the result of a procedure, the return code of a procedure may also be set by return through use of the -code option. In the usual case where the -code option isn't specified the procedure will return normally. However, the -code option may be used to generate an exceptional return from the procedure. Code may have any of the following values:

ok (or 0)
Normal return: same as if the option is omitted. The return code of the procedure is 0 (TCL_OK).
error (1)
Error return: the return code of the procedure is 1 (TCL_ERROR). The procedure command behaves in its calling context as if it were the command error result. See below for additional options.
return (2)
The return code of the procedure is 2 (TCL_RETURN). The procedure command behaves in its calling context as if it were the command return (with no arguments).
break (3)
The return code of the procedure is 3 (TCL_BREAK). The procedure command behaves in its calling context as if it were the command break.
continue (4)
The return code of the procedure is 4 (TCL_CONTINUE). The procedure command behaves in its calling context as if it were the command continue.
value
Value must be an integer; it will be returned as the return code for the current procedure.

The -code option is rarely used. It is provided so that procedures that implement new control structures can reflect exceptional conditions back to their callers.

Two additional options, -errorinfo and -errorcode, may be used to provide additional information during error returns. These options are ignored unless code is error.

The -errorinfo option specifies an initial stack trace for the errorInfo variable; if it is not specified then the stack trace left in errorInfo will include the call to the procedure and higher levels on the stack but it will not include any information about the context of the error within the procedure. Typically the info value is supplied from the value left in errorInfo after a catch command trapped an error within the procedure.

If the -errorcode option is specified then code provides a value for the errorCode variable. If the option is not specified then errorCode will default to NONE.

EXAMPLES

First, a simple example of using return to return from a procedure, interrupting the procedure body. 
proc printOneLine {} {
   puts "line 1"    ;# This line will be printed.
   return 
   puts "line 2"    ;# This line will not be printed.
}

Next, an example of using return to set the value returned by the procedure. 

proc returnX {} {return X}
puts [returnX]    ;# prints "X"

Next, a more complete example, using return -code error to report invalid arguments. 

proc factorial {n} {
   if {![string is integer $n] || ($n < 0)} {
      return -code error \
            "expected non-negative integer,\
             but got \"$n\""
   }
   if {$n < 2} {
      return 1
   }
   set m [expr {$n - 1}]
   set code [catch {factorial $m} factor]
   if {$code != 0} {
      return -code $code $factor
   }
   set product [expr {$n * $factor}]
   if {$product < 0} {
      return -code error \
            "overflow computing factorial of $n"
   }
   return $product
}

Next, a procedure replacement for break. 

proc myBreak {} {
   return -code break
}

KEYWORDS

break, catch, continue, error, procedure, return

LAST SEARCHED