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ActiveTcl User Guide
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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.
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% 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:
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% 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