control(3)



control(3tcl)              Tcl Control Flow Commands             control(3tcl)

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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  (ac-
              cording 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 bool-
              ean 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  com-
              mand  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  de-
              fault 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  as-
              sert 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  re-
              turn  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 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  Trackers  [http://core.tcl.tk/tcllib/reportlist].   Please
       also  report any ideas for enhancements you may have for either package
       and/or documentation.

       When proposing code changes, please provide unified diffs, i.e the out-
       put of diff -u.

       Note  further  that  attachments  are  strongly  preferred over inlined
       patches. Attachments can be made by going  to  the  Edit  form  of  the
       ticket  immediately  after  its  creation, and then using the left-most
       button in the secondary navigation bar.

SEE ALSO
       break, continue, expr, if, join, namespace, return, string, while

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

CATEGORY
       Programming tools

tcllib                               0.1.3                       control(3tcl)

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