[Self Programming Language%|%Self] is a prototype-based OO language that uses objects that only have slots. This page presents a Tcl package by the same name that was inspired by Self. (For the [self - TclOO%|%self] [TclOO] command, see [self - TclOO]) ** See Also ** [Selfish]: the pure Tcl extension which inspired this extension ** Description ** [MJ]: In a recent (oct-2006) [OO] frenzy on the [Tcl chatroom] resulting in code gems like [neo] and [eos], I decided to write a [Self Programming Language%|%Self]-like extension for Tcl. Initially this was a C extension to allow payload with procedure definitions. But I have rewritten it in plain Tcl using [[interp alias]] available from https://github.com/mpcjanssen/self After loading the extension, one object (Object) is known and the two commands (''self'' and ''next'') are imported. Every other object in the system will be cloned or copied from Object or its clones and will have slots defined containing values or methods. '''Object''' The generic prototype Object can be used to create new objects with ''clone'' or ''copy''. The generic object contains the following slots: * '''clone ''' ''new'' - creates a new object with name $new * '''copy''' ''new'' - creates a copy of the object with name $new * '''name:''' ''value'' - create a value slot '''name''' that will return $value * '''name:''' ''args'' ''body'' - will create a method slot '''name''' * '''parents*''' - a value or method slot return a list with all the parents of the object. This information is used for method dispatch. Object initially has no parents. * '''_state''' - Show the internal object state (use for debugging only). NB: The slot assignment syntax has be changed from ''slot name'' to ''name:'' to be less verbose and better in line with [Self]. '''Cloning''' clone will create a new object with only the parents* slot defined. The parents* slot will contain the name of the receiver of the clone message. The parents* slot will be used in slot dispatch resulting in inheritance and can be updated during runtime, allowing for mixin-like behaviour. ''cloned object'' * parents* {receiver of the clone message} '''Copying''' copy will create an exact copy of the receiver (in other words: ''_state'' is exactly the same). '''Command dispatch''' Slots are executed by sending messages to the object. A depth-first search of the object slots and the parents* list will be done to find the implementation of a slot. When a slot is found the slot is executed in the context of the object receiving the message. If no implementation of a slot is found, the dispatcher tries to call an ''unknown'' slot with the slotname and arguments as args. If that also fails, a standard error is returned. During evaluation of a slot, three additional commands are available: * self ''slotname'' ''args'': will call slot $slotname on the receiver of the initial message. Without arguments it will return the name of the receiver. * next: will call the same slot with the same args as currently executing, but the dispatcher will start looking for the slot only in the parents of the implementer of the currently executing slot, which is not the same as self for inherited slots. * next ''slotname'' ''args'': will call a different slot $slotname on the parents of the implementor of the currently executing slot. '''Problems''' Currently there are some missing features which would be nice to have when using the extension: * Variable slots are not real variables, so it is not possible to add traces to them. * Currently method dispatch in a very deep parent child chain is slow. Doing dispatch on a slot that is defined 999 objects higher in the inheritance tree takes approximately 1000 microseconds on my machine, where as in the [TIP] 257 [http://wiki.tcl.tk/14754] implementation it takes only 12. This can be resolved using slot/call chain caching. * It would be nice to be able to delegate methods based on their signature (an unknown on steroids), which is very useful for building [megawidget]s. For instance: a slot delegate* {} a delegate* {{cget .t} {* {self unknown}}} # the unknown slot is now a normal slot. # delegates will be called with the slotname and args '''Examples''' ====== package require self # create a Point object. Object clone Point # add a to_s slot to display information of the object Object to_s: {} { return "[self]" } # add x and y slots for the point, notice that these slots cannot be called for now. Point x: {args} {error "abstract slot, override in clone"} Point y: {args} {error "abstract slot, override in clone"} # extend default behavior from parent (Object) Point to_s: {} { return "id: [next] ([self x],[self y])" # Here next will search for a slot named to_s in the parents of the implementor of the current method (Point) # finding the Object slot to_s and the execute it in the context of the receiver (which will be a clone of Point) } # define a point factory Point create: {name x y} { self clone $name $name x: $x $name y: $y } # clone a Point Point clone p1 # to_s will fail because the x and y slots in Point are called catch {p1 to_s} err puts $err # use the Point factory which will define x and y slots Point create p1 0 0 # to_s will now work puts [p1 to_s] ====== '''Intercepting slot calls for debugging purposes''' ====== Object clone A A test: args {return} A clone a a test A clone debug debug test: {args} {puts "called test with $args"; next} a parents* {debug} a test 1 2 3 ====== '''Demonstrating unknown to create a read-only text widget''' ====== # example demostrating how to override a widget package require self package require Tk proc debugtext name { text $name rename $name _$name Object clone $name $name unknown: args { puts "[self] $args" _[self] {*}$args } $name destroy: {} { destroy _[self] rename _[self] {} next } return $name } debugtext .t pack .t -expand 1 -fill both button .b -text "Make readonly" -command make_ro pack .b proc make_ro {} { # allows on the fly redefining of behaviour .t insert: args {puts stderr "readonly"} .t delete: args {puts stderr "readonly"} } ====== ---- With all of these [Self Programmin Language%|%Self]-like extensions, is it possible to make singleton objects by removing the clone function? Does that even make sense for prototype-based object systems? -- [escargo] 20 Oct 2006 [NEM]: A singleton would just be an object. Perhaps an example of what you would be using the singleton for would be useful? I tend to avoid singletons. About the only place I use them is when defining the base case of some structure (e.g., if you define a binary search tree as two cases: Branch(left,right,val) and Empty, then the Empty case can be a singleton). [MJ]: As [NEM] already mentions above, a singleton only makes sense in a class based OO system where you want to instantiate a class only once. In a prototype based OO system everything is a singleton (there are no classes). However if you just want to disallow cloning of a specific object you can use the fact that clone is just a slot and redefine it e.g.: % Object clone a % a clone: {args} {error "cannot clone"} % a clone b cannot clone ---- [NEM]: I've not tried the implementation yet, but I very much like the specification of this extension. If I make a slot contain an object, what is the syntax for sending messages to that object? From your description, it sounds like it would be something like: MyObj pos: [Point create $x $y] puts "pos = [[MyObj pos] to_s]" Is that correct? Would it be possible to make it like the following? puts "pos = [MyObj pos to_s]" [MJ]: In the point implementation from above that create call should actually be ''Point create pos $x $y'' (note that automatic clone naming is trivial to add in the clone slot). Apart from that, you are correct. I guess it would be possible to implement this, but I cannot see a clear way to add this in the current implementation and not break anything else. It will certainly make slot dispatch more complicated; it has to do number of arguments checking for instance. Even figuring out if a slot contains another object is not straightforward in the current implementation. However, one could implement it with the existing functionality something like this: ====== Object addChild: {object} { self $object: {slot args} "return \[$object \$slot \{*\}\$args \]" } Object clone a Object clone pos pos to_s: {} {return "I am [self]"} a addChild pos puts [a pos to_s] # even nested Object clone b b to_s: {} {return "I am [self]"} pos addChild b puts [a pos b to_s] Or more elaborate: package require self Object children*: {} Object addChild: {name object} { self $name: {slot args} "return \[$object \$slot \{*\}\$args \]" self children* [lappend [self children*] $object] } Object delete: {} { foreach child [self children*] { $child delete } self destroy } Object clone Point Object to_s: {} { return "[self]" } namespace eval self { namespace eval objs { variable counter } } Point x: {args} {error "abstract slot, override in clone"} Point y: {args} {error "abstract slot, override in clone"} Point to_s: {} { return "id: [next] ([self x],[self y])" } Point create: {x y} { set obj [self clone ::self::objs::obj[incr ::self::objs::counter]] $obj x: $x $obj y: $y return $obj } Object clone MousePointer MousePointer addChild pos [Point create 130 140] MousePointer pos x MousePointer pos to_s MousePointer delete # child is gone info commands ::self::objs::* ====== On a side note, implementing something like this will take away some of the simplicity of the design IMO and I have tried to make the extension as simple as possible while still offering enough flexibility. [Zarutian] 2006-10-26 15:35 UTC: I find this extension interesting but I haven't tried it out yet but plan to do just that. <> Package | Object Orientation