'\"
'\" Copyright (c) 1993-1998  Lucent Technologies, Inc.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\" 
.TH ensemble 3itcl 3.0 itcl "[incr\ Tcl]"
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
ensemble \- create or modify a composite command
.SH SYNOPSIS
\fBitcl::ensemble \fIensName\fR ?\fIcommand arg arg...\fR?
.br
or
.br
\fBensemble \fIensName\fR {
.br
    \fBpart \fIpartName args body\fR
.br
    \fI...\fR
.br
    \fBensemble \fIpartName\fR {
.br
        \fBpart \fIsubPartName args body\fR
.br
        \fBpart \fIsubPartName args body\fR
.br
    \fI...\fR
    }
.br
}

.SH DESCRIPTION
.PP
The \fBensemble\fR command is used to create or modify a composite
command.  See the section \fBWHAT IS AN ENSEMBLE?\fR below for a
brief overview of ensembles.
.PP
If the \fBensemble\fR command finds an existing ensemble called
\fIensName\fR, it updates that ensemble.  Otherwise, it creates an
ensemble called \fIensName\fR.  If the \fIensName\fR is a simple name
like "foo", then an ensemble command named "foo" is added to the
current namespace context.  If a command named "foo" already exists
in that context, then it is deleted.  If the \fIensName\fR contains
namespace qualifiers like "a::b::foo", then the namespace path is
resolved, and the ensemble command is added that namespace context.
Parent namespaces like "a" and "b" are created automatically, as needed.
.PP
If the \fIensName\fR contains spaces like "a::b::foo bar baz", then
additional words like "bar" and "baz" are treated as sub-ensembles.
Sub-ensembles are merely parts within an ensemble; they do not have
a Tcl command associated with them.  An ensemble like "foo" can
have a sub-ensemble called "foo bar", which in turn can have a
sub-ensemble called "foo bar baz".  In this case, the sub-ensemble
"foo bar" must be created before the sub-ensemble "foo bar baz"
that resides within it.
.PP
If there are any arguments following \fIensName\fR, then they are
treated as commands, and they are executed to update the ensemble.
The following commands are recognized in this context:  \fBpart\fR
and \fBensemble\fR.
.PP
The \fBpart\fR command defines a new part for the ensemble.
Its syntax is identical to the usual \fBproc\fR command, but
it defines a part within an ensemble, instead of a Tcl command.
If a part called \fIpartName\fR already exists within the ensemble,
then the \fBpart\fR command returns an error.
.PP
The \fBensemble\fR command can be nested inside another \fBensemble\fR
command to define a sub-ensemble.

.SH "WHAT IS AN ENSEMBLE?"
The usual "info" command is a composite command--the command name
\fBinfo\fR must be followed by a sub-command like \fBbody\fR or \fBglobals\fR.
We will refer to a command like \fBinfo\fR as an \fIensemble\fR, and to
sub-commands like \fBbody\fR or \fBglobals\fR as its \fIparts\fR.
.PP
Ensembles can be nested.  For example, the \fBinfo\fR command has
an ensemble \fBinfo namespace\fR within it.  This ensemble has parts
like \fBinfo namespace all\fR and \fBinfo namespace children\fR.
.PP
With ensembles, composite commands can be created and extended
in an automatic way.  Any package can find an existing ensemble
and add new parts to it.  So extension writers can add their
own parts, for example, to the \fBinfo\fR command.
.PP
The ensemble facility manages all of the part names and keeps
track of unique abbreviations.  Normally, you can abbreviate
\fBinfo complete\fR to \fBinfo comp\fR.  But if an extension adds the
part \fBinfo complexity\fR, the minimum abbreviation for \fBinfo complete\fR
becomes \fBinfo complet\fR.
.PP
The ensemble facility not only automates the construction of
composite commands, but it automates the error handling as well.
If you invoke an ensemble command without specifying a part name,
you get an automatically generated error message that summarizes
the usage information.  For example, when the \fBinfo\fR command
is invoked without any arguments, it produces the following error
message:
wrong # args: should be one of...
  info args procname
  info body procname
  info cmdcount
  info commands ?pattern?
  info complete command
  info context
  info default procname arg varname
  info exists varName
  info globals ?pattern?
  info level ?number?
  info library
  info locals ?pattern?
  info namespace option ?arg arg ...?
  info patchlevel
  info procs ?pattern?
  info protection ?-command? ?-variable? name
  info script
  info tclversion
  info vars ?pattern?
  info which ?-command? ?-variable? ?-namespace? name\fR
You can also customize the way an ensemble responds to errors.
When an ensemble encounters an unspecified or ambiguous part
name, it looks for a part called \fB@error\fR.  If it exists,
then it is used to handle the error.  This part will receive all
of the arguments on the command line starting with the offending
part name.  It can find another way of resolving the command,
or generate its own error message.

.SH EXAMPLE
We could use an ensemble to clean up the syntax of the various
"wait" commands in Tcl/Tk.  Instead of using a series of
strange commands like this:
vwait x
tkwait visibility .top
tkwait window .
we could use commands with a uniform syntax, like this:
wait variable x
wait visibility .top
wait window .
The Tcl package could define the following ensemble:
itcl::ensemble wait part variable {name} {
    uplevel vwait $name
}
The Tk package could add some options to this ensemble, with a
command like this:
itcl::ensemble wait {
    part visibility {name} {
        tkwait visibility $name
    }
    part window {name} {
        tkwait window $name
    }
}
Other extensions could add their own parts to the \fBwait\fR command
too.

.SH KEYWORDS
ensemble, part, info