[Richard Suchenwirth] 2007-05-04 - [MJ] contributed this nice example of how to extend Tcl with a new subcommand to the [chan] ensemble (new from 8.5). First, write a self-contained function that implements what you want - in this case, return a list indicating whether the given channel is readable (r) and/or writable (w):
 /*
  *----------------------------------------------------------------------
  *
  * Tcl_ModeObjCmd --
  *
  *----------------------------------------------------------------------
 */

        /* ARGSUSED */
 int
 Tcl_ModeObjCmd(
    ClientData dummy,           /* Not used. */
    Tcl_Interp *interp,         /* Current interpreter. */
    int objc,                   /* Number of arguments. */
    Tcl_Obj *const objv[])      /* Argument objects. */
 {
    Tcl_Channel chan;           /* The channel to puts on. */
    const char *channelId; /* Name of channel for puts. */
    int mode;                   /* Mode in which channel is opened. */
            
    if (objc != 2) {
        Tcl_WrongNumArgs(interp, 1, objv, "channelId");
        return TCL_ERROR;
    }

    channelId = Tcl_GetString(objv[1]);

    chan = Tcl_GetChannel(interp, channelId, &mode);
    if (chan == (Tcl_Channel) NULL) {
        return TCL_ERROR;
    }

    if ((mode & TCL_READABLE) != 0) {
        Tcl_AppendElement(interp, "r");
    }

    if ((mode & TCL_WRITABLE) != 0) {
        Tcl_AppendElement(interp, "w");
    }

    return TCL_OK;

 }
Then, make it available as a command to Tcl:
            Tcl_CreateObjCommand(interp, "::tcl::chan::mode",
                            Tcl_ModeObjCmd, NULL, NULL);

Finally, in [init.tcl], register that new command as ensemble member of [chan]:
 # Set up the 'chan' ensemble (TIP #208).
    namespace eval chan {
        # TIP #219. Added methods: create, postevent.
        # TIP 287.  Added method: pending.
        namespace ensemble create -command ::chan -map {
            blocked     ::tcl::chan::blocked
            close       ::tcl::chan::close
            configure   ::tcl::chan::configure
            copy        ::tcl::chan::copy
            create      ::tcl::chan::rCreate
            eof         ::tcl::chan::eof
            event       ::tcl::chan::event
            flush       ::tcl::chan::flush
            gets        ::tcl::chan::gets
            mode        {::tcl::chan::mode ;############### here}
            names       {::file channels}
            pending     ::tcl::chan::Pending
            postevent   ::tcl::chan::rPostevent
            puts        ::tcl::chan::puts
            read        ::tcl::chan::read
            seek        ::tcl::chan::seek
            tell        ::tcl::chan::tell
            truncate    ::tcl::chan::Truncate
        }
    }
----
[NEM] You shouldn't have to put the command in the tcl namespace or mess with init.tcl.
The [namespace ensemble] mechanism is flexible enough to be extended from elsewhere. In
this case, you could do:
            Tcl_CreateObjCommand(interp, "::mychan::mode",
                            Tcl_ModeObjCmd, NULL, NULL); 
and then in some script do:
 set map [namespace ensemble configure ::chan -map]
 dict set map mode ::mychan::mode
 namespace ensemble configure ::chan -map $map

[DKF]: I'd go further and say that what [NEM] suggests is the ''correct'' way to do it, since it does not rely on the current configuration of how [chan] has its internal subcommands. It also allows people to have other extension subcommands.

[MJ]: I agree that in the case of extending chan with new subcommands this is the way to go, there is nothing in this piece of code that will prevent the [[chan mode]] command to be loaded in an extension (this extensibility is one of the advantages of ensembles). However, I wanted to add a command in the core itself and was delighted by how easy this was. See [Extending chan] for the right way to do it.
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[[[Category Example]|[Category Channel]]]