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return(3tcl) Tcl Built-In Commands return(3tcl) 





NAME


return - Return from a procedure, or set return code of a
  script






SYNOPSIS


return ?result?


return ?-code code? ?result?


return ?option value ...? ?result?








DESCRIPTION


In its simplest usage, the return command is used without
    options in the body of a procedure to immediately return control to the
    caller of the procedure. If a result argument is provided, its value
    becomes the result of the procedure passed back to the caller. If
    result is not specified then an empty string will be returned to the
    caller as the result of the procedure.


The return command serves a similar function within script
    files that are evaluated by the source command. When source
    evaluates the contents of a file as a script, an invocation of the
    return command will cause script evaluation to immediately cease, and
    the value result (or an empty string) will be returned as the result
    of the source command.






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 is not 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:



  

  
Normal return: same as if the option is omitted. The return code of the
      procedure is 0 (TCL_OK).

  

  
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.

  

  
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).

  

  
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.

  

  
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 must be an integer; it will be returned as the return code
      for the current procedure.




When a procedure wants to signal that it has received invalid
    arguments from its caller, it may use return -code error with
    result set to a suitable error message. Otherwise usage of the
    return -code option is mostly limited to procedures that implement a
    new control structure.


The return -code command acts similarly within script files
    that are evaluated by the source command. During the evaluation of
    the contents of a file as a script by source, an invocation of the
    return -code code command will cause the return code of
    source to be code.






RETURN OPTIONS


In addition to a result and a return code, evaluation of a command
    in Tcl also produces a dictionary of return options. In general usage, all
    option value pairs given as arguments to return become entries
    in the return options dictionary, and any values at all are acceptable
    except as noted below. The catch command may be used to capture all
    of this information — the return code, the result, and the return
    options dictionary — that arise from evaluation of a script.


As documented above, the -code entry in the return options
    dictionary receives special treatment by Tcl. There are other return options
    also recognized and treated specially by Tcl. They are:



  

  
The -errorcode option receives special treatment only when the
      value of the -code option is TCL_ERROR. Then the list
      value is meant to be additional information about the error, presented as
      a Tcl list for further processing by programs. If no -errorcode
      option is provided to return when the -code error option is
      provided, Tcl will set the value of the -errorcode entry in the
      return options dictionary to the default value of NONE. The
      -errorcode return option will also be stored in the global variable
      errorCode.

  

  
The -errorinfo option receives special treatment only when the
      value of the -code option is TCL_ERROR. Then info is
      the initial stack trace, meant to provide to a human reader additional
      information about the context in which the error occurred. The stack trace
      will also be stored in the global variable errorInfo. If no
      -errorinfo option is provided to return when the -code
      error option is provided, Tcl will provide its own initial stack trace
      value in the entry for -errorinfo. Tcl's initial stack trace will
      include only the call to the procedure, and stack unwinding will append
      information about higher stack levels, but there will be no information
      about the context of the error within the procedure. Typically the
      info value is supplied from the value of -errorinfo in a
      return options dictionary captured by the catch command (or from
      the copy of that information stored in the global variable
      errorInfo).

  

  
The -errorstack option receives special treatment only when the
      value of the -code option is TCL_ERROR. Then list is
      the initial error stack, recording actual argument values passed to each
      proc level. The error stack will also be reachable through info
      errorstack. If no -errorstack option is provided to
      return when the -code error option is provided, Tcl will
      provide its own initial error stack in the entry for -errorstack.
      Tcl's initial error stack will include only the call to the procedure, and
      stack unwinding will append information about higher stack levels, but
      there will be no information about the context of the error within the
      procedure. Typically the list value is supplied from the value of
      -errorstack in a return options dictionary captured by the
      catch command (or from the copy of that information from info
      errorstack).

  

  
The -level and -code options work together to set the return
      code to be returned by one of the commands currently being evaluated. The
      level value must be a non-negative integer representing a number of
      levels on the call stack. It defines the number of levels up the stack at
      which the return code of a command currently being evaluated should be
      code. If no -level option is provided, the default value of
      level is 1, so that return sets the return code that the
      current procedure returns to its caller, 1 level up the call stack. The
      mechanism by which these options work is described in more detail
    below.

  

  
The value options must be a valid dictionary. The entries of that
      dictionary are treated as additional option value pairs for the
      return command.








RETURN CODE HANDLING MECHANISMS


Return codes are used in Tcl to control program flow. A Tcl script
    is a sequence of Tcl commands. So long as each command evaluation returns a
    return code of TCL_OK, evaluation will continue to the next command
    in the script. Any exceptional return code (non-TCL_OK) returned by a
    command evaluation causes the flow on to the next command to be interrupted.
    Script evaluation ceases, and the exceptional return code from the command
    becomes the return code of the full script evaluation. This is the mechanism
    by which errors during script evaluation cause an interruption and unwinding
    of the call stack. It is also the mechanism by which commands like
    break, continue, and return cause script evaluation to
    terminate without evaluating all commands in sequence.


Some of Tcl's built-in commands evaluate scripts as part of their
    functioning. These commands can make use of exceptional return codes to
    enable special features. For example, the built-in Tcl commands that provide
    loops — such as while, for, and foreach —
    evaluate a script that is the body of the loop. If evaluation of the loop
    body returns the return code of TCL_BREAK or TCL_CONTINUE, the
    loop command can react in such a way as to give the break and
    continue commands their documented interpretation in loops.


Procedure invocation also involves evaluation of a script, the
    body of the procedure. Procedure invocation provides special treatment when
    evaluation of the procedure body returns the return code TCL_RETURN.
    In that circumstance, the -level entry in the return options
    dictionary is decremented. If after decrementing, the value of the
    -level entry is 0, then the value of the -code entry becomes
    the return code of the procedure. If after decrementing, the value of the
    -level entry is greater than zero, then the return code of the
    procedure is TCL_RETURN. If the procedure invocation occurred during
    the evaluation of the body of another procedure, the process will repeat
    itself up the call stack, decrementing the value of the -level entry
    at each level, so that the code will be the return code of the
    current command level levels up the call stack. The source
    command performs the same handling of the TCL_RETURN return code,
    which explains the similarity of return invocation during a
    source to return invocation within a procedure.


The return code of the return command itself triggers this
    special handling by procedure invocation. If return is provided the
    option -level 0, then the return code of the return command
    itself will be the value code of the -code option (or
    TCL_OK by default). Any other value for the -level option
    (including the default value of 1) will cause the return code of the
    return command itself to be TCL_RETURN, triggering a return
    from the enclosing procedure.






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
}




With the -level 0 option, return itself can serve as
    a replacement for break, with the help of interp alias.






interp alias {} Break {} return -level 0 -code break




An example of using catch and return -options to
    re-raise a caught error:






proc doSomething {} {


    set resource [allocate]


    catch {


        # Long script of operations


        # that might raise an error


    } result options


    deallocate $resource


    return -options $options $result
}




Finally an example of advanced use of the return options to
    create a procedure replacement for return itself:






proc myReturn {args} {


    set result ""


    if {[llength $args] % 2} {


        set result [lindex $args end]


        set args [lrange $args 0 end-1]


    }


    set options [dict merge {-level 1} $args]


    dict incr options -level


    return -options $options $result
}








SEE ALSO


break(3tcl), catch(3tcl), continue(3tcl), dict(3tcl), error(3tcl),
    errorCode(3tcl), errorInfo(3tcl), proc(3tcl), source(3tcl), throw(3tcl),
    try(3tcl)






KEYWORDS


break, catch, continue, error, exception, procedure, result,
    return







  

    8.5
    Tcl