Provided by: tcl9.0-doc_9.0.1+dfsg-2_all 
NAME
return - Return from a procedure, or set return code of a script
SYNOPSIS
return ?result?
return ?-code code? ?result?
return ?option value ...? ?result?
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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 (or 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 (or 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 (or 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 (or 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.
Applications and packages should use values in the range 5 to 1073741823 (0x3fffffff) for
their own purposes. Values outside this range are reserved for use by Tcl.
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).
-errorstack list
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).
-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, 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
Tcl 8.5 return(3tcl)