


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 ...?

_________________________________________________________________

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                       0.1.3                             1





control(n)          Tcl Control Flow Commands          control(n)


              [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



control                       0.1.3                             2





control(n)          Tcl Control Flow Commands          control(n)


       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


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, continue, expr, if, join, namespace, return, string, while

KEYWORDS
       assert, control, do, flow, no-op, structure

CATEGORY
       Programming tools

























control                       0.1.3                             3


