Here's a simple bit of code to extend any [ensemble]-like command by means of tcl8.5's [namespace ensemble] command.  [CMcC] 6Mar2006

[Larry Smith]: [stacking] does a similar job.

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 package provide extend 1.0
 package require Tcl 8.5
 
 # extend a command with a new subcommand
 proc extend {cmd body} {
    if {![namespace exists ${cmd}]} {
        set wrapper [string map [list %C $cmd %B $body] {
            namespace eval %C {}
            rename %C %C::%C
            namespace eval %C {
                proc _unknown {junk subc args} {
                    return [list %C::%C $subc]
                }
                namespace ensemble create -unknown %C::_unknown
            }
        }]
    }

    append wrapper [string map [list %C $cmd %B $body] {
        namespace eval %C {
            %B
            namespace export -clear *
        }
    }]
    uplevel 1 $wrapper
 }
======

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Here's the [file] command extended with '''newer''' and '''newerthan''' subcommands:

======
 extend file {
    proc newer {a b} {
       return [expr {[file mtime $a] > [file mtime $b]}]
    }

    proc newerthan {mtime path} {
       return [expr {[file exists $path] && ([file mtime $path] > $mtime)}]
    }
 }
======

Here's the [dict] command extended with the '''modify''' subcommand:

======
 # extra useful dict commands
 extend dict {
    proc modify {var args} {
       upvar 1 $var dvar
       foreach {name val} $args {
          dict set dvar $name $val
       }
    }
 }
======

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[LV] So, this seems like a nice bit of functionality. Would it be useful enough to include either in Tcl itself or at least [Tcllib]?

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''quick hacks'' ?

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[AMG]: See also my [[dict getnull]] example in [[[dict get]]].

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[samoc]: oclib.tcl[https://github.com/samoconnor/oclib.tcl] has a similar "extend_proc" [https://github.com/samoconnor/oclib.tcl/blob/master/oclib/oc_ensemble-1.0.tm#L17] command.
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In a [comp.lang.tcl] posting dated Fri, 04 Apr 2014 09:25:30 [DKF] posted an example of using the ensemble's `-unknown` parameter to lazily apply extensions.  A version of [extend] using this technique:

======
proc extend {ens script} {
    namespace eval $ens [concat {
        proc _unknown {ens cmd args} {
            if {$cmd in [namespace eval ::${ens} {::info commands}]} {
                set map [namespace ensemble configure $ens -map]
                dict set map $cmd ::${ens}::$cmd
                namespace ensemble configure $ens -map $map
            }
            return "" ;# back to namespace ensemble dispatch
                      ;# which will error appropriately if the cmd doesn't exist
        }
    }   \; $script]
    namespace ensemble configure $ens -unknown ${ens}::_unknown
}
======
Note that new extensions defined in this way will not appear in the ensemble's map until they are used, so the default error message is misleading.

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