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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 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.

COMMANDS
       control::control command option ?arg arg ...?
              The control command is used as a configuration command for customizing the other  public  com-mands commands
              mands  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 argu-ments, arguments,
              ments, 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  names-pace. namespace.
              pace.

              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}


       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.


LIMITATIONS
       Several of the commands provided by the control package accept arguments that are scripts to be eval-uated. evaluated.
       uated.  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 argu-ments arguments
       ments 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 con-trol control
       trol 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                                         control(n)