NAME
ensemble - create or modify a composite command
SYNOPSIS
ensemble ensName ?command arg arg...?
or
ensemble ensName {
part partName args body
...
ensemble partName {
part subPartName args body
part subPartName args body
...
}
}
_________________________________________________________________
DESCRIPTION
The ensemble command is used to create or modify a compos
ite command. See the section WHAT IS AN ENSEMBLE? below
for a brief overview of ensembles.
If the ensemble command finds an existing ensemble called
ensName, it updates that ensemble. Otherwise, it creates
an ensemble called ensName. If the ensName 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 ensName 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 automati
cally, as needed.
If the ensName 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.
If there are any arguments following ensName, then they
are treated as commands, and they are executed to update
the ensemble. The following commands are recognized in
this context: part and ensemble.
The part command defines a new part for the ensemble. Its
syntax is identical to the usual proc command, but it
mand. If a part called partName already exists within the
ensemble, then the part command returns an error.
The ensemble command can be nested inside another ensemble
command to define a sub-ensemble.
WHAT IS AN ENSEMBLE?
The usual "info" command is a composite command--the com
mand name info must be followed by a sub-command like body
or globals. We will refer to a command like info as an
ensemble, and to sub-commands like body or globals as its
parts.
Ensembles can be nested. For example, the info command
has an ensemble info namespace within it. This ensemble
has parts like info namespace all and info namespace chil
dren.
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 info
command.
The ensemble facility manages all of the part names and
keeps track of unique abbreviations. Normally, you can
abbreviate info complete to info comp. But if an exten
sion adds the part info complexity, the minimum abbrevia
tion for info complete becomes info complet.
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 speci
fying a part name, you get an automatically generated
error message that summarizes the usage information. For
example, when the info 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 protection ?-command? ?-variable? name
info script
info tclversion
info vars ?pattern?
info which ?-command? ?-variable? ?-namespace? name
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 @error.
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.
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:
ensemble wait part variable {name} {
uplevel vwait $name
}
The Tk package could add some options to this ensemble,
with a command like this:
ensemble wait {
part visibility {name} {
tkwait visibility $name
}
part window {name} {
tkwait window $name
}
}
Other extensions could add their own parts to the wait
command too.
KEYWORDS
ensemble, part, info
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