control(n) Tcl Control Flow Commands control(n) ______________________________________________________________________________
NAME
control - Procedures for control flow structures.
SYNOPSIS
package require Tcl 8.2
package require control ?0.1.3?
control::control command option ?arg arg ...?
control::assert expr ?arg arg ...?
control::do body ?option test?
control::no-op ?arg arg ...?
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DESCRIPTION
The control package provides a variety of commands that provide addi-
tional flow of control structures beyond the built-in ones provided by
Tcl. These are commands that in many programming languages might be
considered keywords, or a part of the language itself. In Tcl, control
flow structures are just commands like everything else.
COMMANDS
control::control command option ?arg arg ...?
The control command is used as a configuration command for cus-
tomizing the other public commands of the control package. The
command argument names the command to be customized. The set of
valid option and subsequent arguments are determined by the com-
mand being customized, and are documented with the command.
control::assert expr ?arg arg ...?
When disabled, the assert command behaves exactly like the no-op
command.
When enabled, the assert command evaluates expr as an expression
(in the same way that expr evaluates its argument). If evalua-
tion reveals that expr is not a valid boolean expression
(according to [string is boolean -strict]), an error is raised.
If expr evaluates to a true boolean value (as recognized by if),
then assert returns an empty string. Otherwise, the remaining
arguments to assert are used to construct a message string. If
there are no arguments, the message string is "assertion failed:
$expr". If there are arguments, they are joined by join to form
the message string. The message string is then appended as an
argument to a callback command, and the completed callback com-
mand is evaluated in the global namespace.
The assert command can be customized by the control command in
two ways:
[control::control assert enabled ?boolean?] queries or sets
whether control::assert is enabled. When called without a
boolean argument, a boolean value is returned indicating whether
the control::assert command is enabled. When called with a
valid boolean value as the boolean argument, the control::assert
command is enabled or disabled to match the argument, and an
empty string is returned.
[control::control assert callback ?command?] queries or sets
the callback command that will be called by an enabled assert on
assertion failure. When called without a command argument, the
current callback command is returned. When called with a com-
mand argument, that argument becomes the new assertion failure
callback command. Note that an assertion failure callback com-
mand is always defined, even when assert is disabled. The
default callback command is [return -code error].
Note that control::assert has been written so that in combina-
tion with [namespace import], it is possible to use enabled
assert commands in some namespaces and disabled assert commands
in other namespaces at the same time. This capability is useful
so that debugging efforts can be independently controlled module
by module.
% package require control
% control::control assert enabled 1
% namespace eval one namespace import ::control::assert
% control::control assert enabled 0
% namespace eval two namespace import ::control::assert
% one::assert {1 == 0}
assertion failed: 1 == 0
% two::assert {1 == 0}
control::do body ?option test?
The do command evaluates the script body repeatedly until the
expression test becomes true or as long as (while) test is true,
depending on the value of option being until or while. If option
and test are omitted the body is evaluated exactly once. After
normal completion, do returns an empty string. Exceptional
return codes (break, continue, error, etc.) during the evalua-
tion of body are handled in the same way the while command han-
dles them, except as noted in LIMITATIONS, below.
control::no-op ?arg arg ...?
The no-op command takes any number of arguments and does noth-
ing. It returns an empty string.
LIMITATIONS
Several of the commands provided by the control package accept argu-
ments that are scripts to be evaluated. Due to fundamental limitations
of Tcl's catch and return commands, it is not possible for these com-
mands to properly evaluate the command [return -code $code] within one
of those script arguments for any value of $code other than ok. In
this way, the commands of the control package are limited as compared
to Tcl's built-in control flow commands (such as if, while, etc.) and
those control flow commands that can be provided by packages coded in
C. An example of this difference:
% package require control
% proc a {} {while 1 {return -code error a}}
% proc b {} {control::do {return -code error b} while 1}
% catch a
1
% catch b
0
BUGS, IDEAS, FEEDBACK
This document, and the package it describes, will undoubtedly contain
bugs and other problems. Please report such in the category control of
the Tcllib SF Trackers [http://source-
forge.net/tracker/?group_id=12883]. Please also report any ideas for
enhancements you may have for either package and/or documentation.
SEE ALSO
break(n), continue(n), expr(n), if(n), join(n), namespace(n), return(n), string(n), while(n)
KEYWORDS
assert, control, do, flow, no-op, structure
CATEGORY
Programming tools
control 0.1.3 control(n)
Mac OS X 10.8 - Generated Wed Sep 5 16:42:37 CDT 2012