NEM 6 Oct 2006: While contemplating all things OO and TIP 257 [L1 ], I came across an interesting interaction between features available in Tcl 8.5. It is possible to use namespace ensemble without needing any namespace to back it up! In particular, the -map option can be used to model simple slot-based objects. Together with the apply command, methods can then be added. To demonstrate, let's consider writing a simple stateless object to represent a rectangle on a canvas:
proc rect {canv x0 y0 x1 y1} {
set id [$canv create rect $x0 $y0 $x1 $y1]
namespace ensemble create -command $canv.rect$id \
-map [dict create id [list const $id] \
coords [list const [list $x0 $y0 $x1 $y1]]]
}
proc const val { return $val }We now have a simple constructor that creates a rectangle on a canvas and creates an object command for that rectangle, with two slots to get the canvas id and coordinates associated with the rectangle:
pack [canvas .c] set r [rect .c 20 20 100 100] puts "id = [$r id], coords = [$r coords]"
These slots are just entries in the -map of the ensemble that alias to the "const" command to simply return their value. Note that no namespace is involved at all! (DKF: Actually, the namespace involved is the global namespace, but getting rid of that is equivalent to deleting the interpreter.)
We can write a general method for creating slot aliases on these objects:
proc alias {object slot command args} {
set map [namespace ensemble configure $object -map]
dict set map $slot [linsert $args 0 $command]
namespace ensemble configure $object -map $map
}We can now add extra slots to the rectangle object:
alias $r type const "rectangle" puts "$r is a [$r type]"
Using anonymous functions via apply in 8.5, we can even add new methods to the object. These are simply aliases to an anonymous function. We add a "self" parameter and arrange for it to be filled with the object command:
proc method {object name params body} {
set params [linsert $params 0 self]
alias $object $name ::apply [list $params $body ::] $object
}
method $r width {} {
lassign [$self coords] x0 y0 x1 y1
expr {abs($x1-$x0)}
}
method $r height {} {
lassign [$self coords] x0 y0 x1 y1
expr {abs($y1-$y0)}
}We can even add method and alias as methods on the object itself:
alias $r method ::method $r
alias $r alias ::alias $r
$r method area {} { expr {[$self width] * [$self height]} }
puts "area = [$r area]"We could add mutable slots by creating a slot alias that can rewrite itself. This seems to be a fascinating new way of creating relatively lightweight objects, benefiting from the fast namespace ensemble mechanism, without having the overhead of an actual namespace.