NAME

       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