'\"
'\" RCS: @(#) $Id: control.n,v 1.12 2002/01/18 21:45:42 dgp Exp $
'\" 
'\" The definitions below are for supplemental macros used in Tcl/Tk
'\" manual entries.
'\"
'\" .AP type name in/out ?indent?
'\"	Start paragraph describing an argument to a library procedure.
'\"	type is type of argument (int, etc.), in/out is either "in", "out",
'\"	or "in/out" to describe whether procedure reads or modifies arg,
'\"	and indent is equivalent to second arg of .IP (shouldn't ever be
'\"	needed;  use .AS below instead)
'\"
'\" .AS ?type? ?name?
'\"	Give maximum sizes of arguments for setting tab stops.  Type and
'\"	name are examples of largest possible arguments that will be passed
'\"	to .AP later.  If args are omitted, default tab stops are used.
'\"
'\" .BS
'\"	Start box enclosure.  From here until next .BE, everything will be
'\"	enclosed in one large box.
'\"
'\" .BE
'\"	End of box enclosure.
'\"
'\" .CS
'\"	Begin code excerpt.
'\"
'\" .CE
'\"	End code excerpt.
'\"
'\" .VS ?version? ?br?
'\"	Begin vertical sidebar, for use in marking newly-changed parts
'\"	of man pages.  The first argument is ignored and used for recording
'\"	the version when the .VS was added, so that the sidebars can be
'\"	found and removed when they reach a certain age.  If another argument
'\"	is present, then a line break is forced before starting the sidebar.
'\"
'\" .VE
'\"	End of vertical sidebar.
'\"
'\" .DS
'\"	Begin an indented unfilled display.
'\"
'\" .DE
'\"	End of indented unfilled display.
'\"
'\" .SO
'\"	Start of list of standard options for a Tk widget.  The
'\"	options follow on successive lines, in four columns separated
'\"	by tabs.
'\"
'\" .SE
'\"	End of list of standard options for a Tk widget.
'\"
'\" .OP cmdName dbName dbClass
'\"	Start of description of a specific option.  cmdName gives the
'\"	option's name as specified in the class command, dbName gives
'\"	the option's name in the option database, and dbClass gives
'\"	the option's class in the option database.
'\"
'\" .UL arg1 arg2
'\"	Print arg1 underlined, then print arg2 normally.
'\"
'\" RCS: @(#) $Id: man.macros,v 1.1 2000/03/06 21:34:53 ericm Exp $
'\"
'\"	# Set up traps and other miscellaneous stuff for Tcl/Tk man pages.
.if t .wh -1.3i ^B
.nr ^l \n(.l
.ad b
'\"	# Start an argument description
.de AP
.ie !"\\$4"" .TP \\$4
.el \{\
.   ie !"\\$2"" .TP \\n()Cu
.   el          .TP 15
.\}
.ta \\n()Au \\n()Bu
.ie !"\\$3"" \{\
\&\\$1	\\fI\\$2\\fP	(\\$3)
.\".b
.\}
.el \{\
.br
.ie !"\\$2"" \{\
\&\\$1	\\fI\\$2\\fP
.\}
.el \{\
\&\\fI\\$1\\fP
.\}
.\}
..
'\"	# define tabbing values for .AP
.de AS
.nr )A 10n
.if !"\\$1"" .nr )A \\w'\\$1'u+3n
.nr )B \\n()Au+15n
.\"
.if !"\\$2"" .nr )B \\w'\\$2'u+\\n()Au+3n
.nr )C \\n()Bu+\\w'(in/out)'u+2n
..
.AS Tcl_Interp Tcl_CreateInterp in/out
'\"	# BS - start boxed text
'\"	# ^y = starting y location
'\"	# ^b = 1
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.br
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.if n .nf
.if n .ti 0
.if n \l'\\n(.lu\(ul'
.if n .fi
..
'\"	# BE - end boxed text (draw box now)
.de BE
.nf
.ti 0
.mk ^t
.ie n \l'\\n(^lu\(ul'
.el \{\
.\"	Draw four-sided box normally, but don't draw top of
.\"	box if the box started on an earlier page.
.ie !\\n(^b-1 \{\
\h'-1.5n'\L'|\\n(^yu-1v'\l'\\n(^lu+3n\(ul'\L'\\n(^tu+1v-\\n(^yu'\l'|0u-1.5n\(ul'
.\}
.el \}\
\h'-1.5n'\L'|\\n(^yu-1v'\h'\\n(^lu+3n'\L'\\n(^tu+1v-\\n(^yu'\l'|0u-1.5n\(ul'
.\}
.\}
.fi
.br
.nr ^b 0
..
'\"	# VS - start vertical sidebar
'\"	# ^Y = starting y location
'\"	# ^v = 1 (for troff;  for nroff this doesn't matter)
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.if !"\\$2"" .br
.mk ^Y
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.el .nr ^v 1u
..
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.ev
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.nr ^v 0
..
'\"	# Special macro to handle page bottom:  finish off current
'\"	# box/sidebar if in box/sidebar mode, then invoked standard
'\"	# page bottom macro.
.de ^B
.ev 2
'ti 0
'nf
.mk ^t
.if \\n(^b \{\
.\"	Draw three-sided box if this is the box's first page,
.\"	draw two sides but no top otherwise.
.ie !\\n(^b-1 \h'-1.5n'\L'|\\n(^yu-1v'\l'\\n(^lu+3n\(ul'\L'\\n(^tu+1v-\\n(^yu'\h'|0u'\c
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.\}
.if \\n(^v \{\
.nr ^x \\n(^tu+1v-\\n(^Yu
\kx\h'-\\nxu'\h'|\\n(^lu+3n'\ky\L'-\\n(^xu'\v'\\n(^xu'\h'|0u'\c
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..
'\"	# DS - begin display
.de DS
.RS
.nf
.sp
..
'\"	# DE - end display
.de DE
.fi
.RE
.sp
..
'\"	# SO - start of list of standard options
.de SO
.SH "STANDARD OPTIONS"
.LP
.nf
.ta 4c 8c 12c
.ft B
..
'\"	# SE - end of list of standard options
.de SE
.fi
.ft R
.LP
See the \\fBoptions\\fR manual entry for details on the standard options.
..
'\"	# OP - start of full description for a single option
.de OP
.LP
.nf
.ta 4c
Command-Line Name:	\\fB\\$1\\fR
Database Name:	\\fB\\$2\\fR
Database Class:	\\fB\\$3\\fR
.fi
.IP
..
'\"	# CS - begin code excerpt
.de CS
.RS
.nf
.ta .25i .5i .75i 1i
..
'\"	# CE - end code excerpt
.de CE
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..
.de UL
\\$1\l'|0\(ul'\\$2
..
.TH control n 0.0 control "Tcl Control Flow Commands"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
control \- Procedures for control flow structures.
.SH SYNOPSIS
\fBpackage require Tcl 8.2\fR
.sp
\fBpackage require control ?0.1?\fR
.sp
\fBcontrol::control \fIcommand option \fR?\fIarg arg ...\fR?
.sp
\fBcontrol::assert \fIexpr \fR?\fIarg arg ...\fR?
.sp
\fBcontrol::do \fIbody \fR?\fIoption test\fR?
.sp
\fBcontrol::no-op \fR?\fIarg arg ...\fR?
.BE
.SH DESCRIPTION
.PP
The \fBcontrol\fR 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 \fIkeywords\fR,
or a part of the language itself.  In Tcl, control flow structures
are just commands like everything else.
.SH COMMANDS
.TP
\fBcontrol::control \fIcommand option \fR?\fIarg arg ...\fR?
The \fBcontrol\fR command is used as a configuration command
for customizing the other public commands of the control package.
The \fIcommand\fR argument names the command to be customized.
The set of valid \fIoption\fR and subsequent arguments are
determined by the command being customized, and are documented
with the command.
.TP
\fBcontrol::assert \fIexpr \fR?\fIarg arg ...\fR?
When disabled, the \fBassert\fR command behaves exactly like
the \fBno-op\fR command.
.sp
When enabled, the \fBassert\fR command evaluates \fIexpr\fR as
an expression (in the same way that \fBexpr\fR evaluates its
argument).  If evaluation reveals that \fIexpr\fR is not a valid
boolean expression (according to [\fBstring is boolean -strict\fR]),
an error is raised.  If \fIexpr\fR evaluates to a true boolean value
(as recognized by \fBif\fR), then \fBassert\fR returns an empty string.
Otherwise, the remaining arguments to \fBassert\fR 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 \fBjoin\fR 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.
.sp
The \fBassert\fR command can be customized by the \fBcontrol\fR
command in two ways:
.sp
[\fBcontrol::control assert enabled \fR?\fIboolean\fR?] queries or
sets whether \fBcontrol::assert\fR is enabled.  When called without
a \fIboolean\fR argument, a boolean value is returned indicating
whether the \fBcontrol::assert\fR command is enabled.  When called
with a valid boolean value as the \fIboolean\fR argument, the
\fBcontrol::assert\fR command is enabled or disabled to match the
argument, and an empty string is returned.
.sp
[\fBcontrol::control assert callback \fR?\fIcommand\fR?] queries or sets
the callback command that will be called by an enabled \fBassert\fR on
assertion failure.  When called without a \fIcommand\fR argument, the
current callback command is returned.  When called with a \fIcommand\fR
argument, that argument becomes the new assertion failure callback
command.  Note that an assertion failure callback command is always
defined, even when \fBassert\fR is disabled.  The default callback
command is [\fBreturn -code error\fR].
.sp
Note that \fBcontrol::assert\fR has been written so that in
combination with [\fBnamespace import\fR], it is possible to
use enabled \fBassert\fR commands in some namespaces and disabled
\fBassert\fR commands in other namespaces at the same time.  
This capability is useful so that debugging efforts can be independently
controlled module by module.
.sp
.CS
\fB% 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}\fR
.CE
.TP
\fBcontrol::do \fIbody \fR?\fIoption test\fR?  
The \fBdo\fR command evaluates the script \fIbody\fR repeatedly
\fBuntil\fR the expression \fBtest\fR becomes true or as long as
(\fBwhile\fR) \fBtest\fR is true, depending on the value of
\fIoption\fR being \fBuntil\fR or \fBwhile\fR. If \fIoption\fR and
\fItest\fR are omitted the body is evaluated exactly once. After
normal completion, \fBdo\fR returns an empty string.  Exceptional
return codes (\fBbreak\fR, \fBcontinue\fR, \fBerror\fR, etc.) during
the evaluation of \fIbody\fR are handled in the same way the
\fBwhile\fR command handles them, except as noted in
\fBLIMITATIONS\fR, below.
.TP
\fBcontrol::no-op \fR?\fIarg arg ...\fR?
The \fBno-op\fR command takes any number of arguments and does nothing.
It returns an empty string.

.SH LIMITATIONS

Several of the commands provided by the \fBcontrol\fR package
accept arguments that are scripts to be evaluated.  Due to
fundamental limitations of Tcl's \fBcatch\fR and \fBreturn\fR
commands, it is not possible for these commands to properly
evaluate the command [\fBreturn -code $code\fR] within one
of those script arguments for any value of \fI$code\fR other
than \fIok\fR.  In this way, the commands of the \fBcontrol\fR
package are limited as compared to Tcl's built-in control flow
commands (such as \fBif\fR, \fBwhile\fR, etc.) and those
control flow commands that can be provided by packages coded
in C.  An example of this difference:
.sp
.CS
\fB% 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
.CE

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

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