ensemble(n)

NAME
itcl::ensemble - create or modify a composite command

SYNOPSIS
itcl::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 composite 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 automatically, 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 defines a part within an ensemble, instead of a Tcl command. 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 command 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 children.

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 extension adds the part info complexity, the minimum abbreviation 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 specifying 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 procs ?pattern?
  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:

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 wait command too.

KEYWORDS
ensemble, part, info