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:
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.
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:
-errorcode list
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.
-errorinfo info
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).
-level level
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.
-options options
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.
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), proc(3tcl), source(3tcl), tclvars(3tcl)
KEYWORDS
break, catch, continue, error, procedure, return
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