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control(n) 0.1.2 "Tcl Control Flow Commands"
control - Procedures for control flow structures.
package require Tcl 8.2
package require control ?0.1.2?
The control package provides a variety of commands that provide
additional 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.
- control::control command option ?arg arg ...?
-
The control command is used as a configuration command for
customizing 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
command 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 evaluation 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 command 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 command argument, that argument becomes the
new assertion failure callback command. Note that an assertion
failure callback command 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
combination 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}
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- 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 evaluation of body are handled in the same way
the while command handles them, except as noted in
LIMITATIONS, below.
- control::no-op ?arg arg ...?
-
The no-op command takes any number of arguments and does
nothing. It returns an empty string.
Several of the commands provided by the control package accept
arguments that are scripts to be evaluated. Due to fundamental
limitations of Tcl's catch and return commands, it is not
possible for these commands 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
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break, continue, expr, if, join , namespace, return, string , while
assert , control , do , flow , no-op , structure