Prometheus is a prototype-based message-passing object system for Scheme similar to the Self language
In a prototype-based object system, an object is just a set of slots. A slot has a name and a value, or a /handler procedure/ which reacts on messages associated with the slot. Some slots are special, so-called /parent slots/, whose use will become apparent shortly.
Objects receive messages. A message consists of a symbol called a /selector/, and zero or more arguments. When an object receives a message, the object searches for a slot whose name is equal (=eq?=, actually) to the message selector. When it finds such a slot, it invokes the slot's handler, or returns the slot's value, as appropriate. When the slot is not in the object, all objects in parent slots are queried for that slot.
An object is created by /cloning/ an existing object. The new object is empty except for a single parent slot, which points to the cloned object. This way, the new object behaves exactly like the old one.
In a prototype-based object system, objects are created and modified until they behave as required. Then, that object is cloned to create the real objects to work with — it forms the /prototype/ for the other objects.
Hermes is a very small object system based solely on messages. It's the basis upon which Prometheus is built. It should be considered internal information of Prometheus, but someone might find it useful, and it is visible in every Prometheus object.
A Hermes object contains messages. A message has a name, a procedure to handle this message, and a type flag on whether the return value of this message should be treated as a parent object. A message-handling procedure is applied to the arguments of the message in addition to two arguments, probably known from Prometheus' method slots by now: /Self/ and /resend/. Indeed, Prometheus' method slots are just a very thing wrapper around Hermes messages.
The =hermes= structure exports a single procedure.
*** API
**** Procedure: ~(make-hermes-object)~
Return a new Hermes object which knows the basic messages.
This adds a message named =name= to the object, upon receiving which, =handler= is applied to =self=, =resend=, and the arguments to the message. If =parent?= is supplied and not false, this message returns a parent object. In this case, it must be callable with no arguments, and must not use =resend=.
#+name: hermes-add-message-message
#+begin_src scheme
(messages-add! msg 'add-message!
(lambda (_ resend name handler . parentl)
(messages-add! msg name handler
(if (null? parentl)
#f
(car parentl))))
#f)
#+end_src
**** Message: *delete-message!* /name/
Remove the handler for the message named =name=. This causes such messages to be handled by parent objects in the future again.
The message upon which inheritance in Hermes is built. This returns a procedure of no arguments which handles the receiving of the message. This is delayed so that Hermes can check for duplicate handlers, which would be an error.
/Name/ is the name of the message we are looking for. /Receiver/ is the original receiver of the message, to be used as the =self= argument to the handler procedure. /Args/ is a list of arguments. /Visited/ is a list of objects we have seen so far. This is used to detect cycles in the inheritance graph.
This message returns two values. The first one is the handler and the other one the object in which this handler was found. The handler value can also be one of two symbols to signify an error condition. If it's the symbol message-not-understood, then neither this object nor any parents knew how to handle this message. If it's ambiguous-message-send, the same message could be handled by multiple parents in the inheritance graph. The user needs to add a message which resends the ambiguous message unambiguously to the correct parent. In either case, the second return value is #f. The handler procedure itself accepts no arguments, and just runs the message.
#+name: hermes-%get-handler-message
#+begin_src scheme
(messages-add! msg '%get-handler
(lambda (_ resend name receiver args visited)
(get-handler msg self name receiver args visited))
#f)
#+end_src
It is absolutely sufficient for an object to handle only the =%get-handler= message to participate in the inheritance handling of Hermes.
*** Implementation :noexport:
**** Messages record
The messages record stores an association of message names and message handlers. It also stores such an association for parents. This uses two lists for efficiency reasons: The list of parents is needed much more often.
#+name: hermes-messages-record
#+begin_src scheme :tangle hermes-impl.scm
(define (alist-set! alist name value)
(cond
((assq name alist)
=> (lambda (entry)
(set-cdr! entry value)
alist))
(else
(alist-cons name value alist))))
(define-record-type messages
(%make-messages alist parents)
messages?
(alist messages-alist set-messages-alist!)
(parents messages-parents set-messages-parents!))
(define (make-messages)
(%make-messages '() '()))
(define (messages-add! msg name handler parent?)
(set-messages-alist! msg
(alist-set! (messages-alist msg)
name handler))
(if parent?
(set-messages-parents! msg
(alist-set! (messages-parents msg)
name handler))))
(define (messages-delete! msg name)
(set-messages-alist! msg
(alist-delete! name
(messages-alist msg)
eq?))
(set-messages-parents! msg
(alist-delete! name
(messages-parents msg)
eq?)))
;;; Do a direct lookup (as opposed to asking the parents) for a
;;; message handler in the record.
(define (messages-direct-lookup msg name)
(cond
((assq name (messages-alist msg))
=> cdr)
(else
#f)))
;;; Ask the parents in the messages record for handlers. This returns
;;; two values as explained above. To enable the
;;; AMBIGUOUS-MESSAGE-SEND error, the parent list is searched
;;; completely even when a handler is found.
(define (messages-parent-lookup msg self name receiver args visited)
(let loop ((alis (messages-parents msg))
(handler #f)
(found #f))
(if (null? alis)
(if handler
(values handler found)
(values 'message-not-understood #f))
(receive (new new-found)
(((cdar alis)
receiver
(lambda args
(error "Parent slots must not use resend."
receiver name args)))
'%get-handler name receiver args (cons self visited))
(case new
((message-not-understood)
(loop (cdr alis)
handler
found))
((ambiguous-message-send)
(values 'ambiguous-message-send
#f))
(else
(if (and handler
(and (not (eq? found new-found))))
(values 'ambiguous-message-send
#f)
(loop (cdr alis)
new
new-found))))))))
#+end_src
**** Handling of Messages
#+name: hermes-message-handling
#+begin_src scheme :tangle hermes-impl.scm
;;; Handle a single message, checking for errors.
(define (messages-handle msg self name args)
(receive (handler found)
(get-handler msg self name self args '())
(run-with-error-checking handler self name args)))
;;; Return the appropriate handler procedure.
(define (get-handler msg self name receiver args visited)
(if (memq self visited)
(values 'message-not-understood #f)
(cond
((messages-direct-lookup msg name)
=> (lambda (handler)
(values (lambda ()
(apply handler
receiver
(make-resender msg self receiver visited)
args))
self)))
(else
(messages-parent-lookup msg self name receiver args visited)))))
;;; Create a resender for the message.
(define (make-resender msg self receiver visited)
(lambda (target name . args)
(receive (handler found)
(cond
((eq? target #f) ; ask parents
(messages-parent-lookup msg self name receiver args visited))
((or (eq? target self) ; ask this object
(eq? target #t)) ; hystorical syntax
(get-handler msg self name receiver args visited))
((symbol? target) ; ask named parent
((self target) '%get-handler name receiver args (cons self visited)))
(else ; ask that object
(target '%get-handler name receiver args (cons self visited))))
(run-with-error-checking handler self name args))))
;;; Signal the appropriate errors, if handler is not a procedure.
;;; Else, call that handler.
(define (run-with-error-checking handler self name args)
(case handler
((message-not-understood)
(if (eq? name 'message-not-understood)
(error "Message MESSAGE-NOT-UNDERSTOOD not understood"
self args)
(self 'message-not-understood name args)))
((ambiguous-message-send)
(if (eq? name 'ambiguous-message-send)
(error "Message AMBIGUOUS-MESSAGE-SEND is ambiguous"
Prometheus objects are implemented as closures. To send a message to that object, the closure is applied to the message selector (i.e., the slot name), followed by a number of arguments. The return value(s) of the message are returned from this application.
*** Slots
Prometheus knows about three kinds of slots.
/Value slots/ merely store a value which is returned when the corresponding message is received.
/Parent slots/ are just like value slots, but have a special flag marking them as parents.
/Method slots/ contain a procedure which is invoked for messages corresponding to this slot.
The procedure is called with at least two arguments, conventionally called =self= and =resend=. If the message received any arguments, they are also passed, after =resend=. =Self= is the object which received the messages, as opposed to the object where this message handler was found in. =Resend= is a procedure which can be used to resend the message to further parents, if the current method does not wish to handle the message. See [[Inheritance]], for more information about this.
A typical method handler could thus look like:
#+begin_example
(lambda (self resend a b)
(/ (+ a b)
2))
#+end_example
Every slot, regardless of its kind, can be associated with a setter method when it is created. Setter methods receive a single argument, and replaces the value of the corresponding slot with this argument. Setter methods can be created automatically when a given slot is created, and are removed when the corresponding getter slot is removed (but not vice-versa). Because of this, they are sometimes not considered to be slots, even if they are. See [[Setters are Methods]], for an example where this distinction is important.
((make-getter-setter 'MESSAGE VALUE TYPE PURE-GETTER)
(make-getter-setter 'MESSAGE VALUE TYPE PURE-GETTER PURE-GETTER))
((make-getter-setter 'MESSAGE VALUE TYPE PURE-GETTER SETABLE-GETTER)
(case-lambda
((self resend getter VALUE)
(self 'delete-slot! getter)
(self 'set-immediate-slot-list!
(alist-cons getter
(list #f TYPE)
(self 'immediate-slot-list)))
(self 'add-message! getter PURE-GETTER (eq? TYPE 'parent)))
((self resend getter setter VALUE)
(self 'delete-slot! getter)
(self 'delete-slot! setter)
(self 'set-immediate-slot-list!
(alist-cons getter
(list setter TYPE)
(self 'immediate-slot-list)))
(self 'add-message! getter SETABLE-GETTER (eq? TYPE 'parent))
(self 'add-message! setter
(lambda (self2 resend new)
(if (eq? self2 self)
(set! VALUE new)
(self2 'MESSAGE getter setter new)))))))))
#+end_src
*** Inheritance
When a slot for a message is not found in the current object, all its parent slots are queried recursively, i.e. parent objects which don't know the slot query their parents, etc.
If no parent knows the slot, the original message receiving object is sent a =message-not-understood= message. If more than one parent knows the slot, the original message receiving object is sent a =ambiguous-message-send= message. See [[Root Objects]], for a documentation of those messages. By default, they signal an error.
Method slots can decide not to handle the message, but rather search the inheritance tree for other handlers. For this purpose, they are passed a procedure commonly called =resend=. See [[Slots]], for an explanation of method slots.
It is important to understand the difference between sending a message to an object, and resending it to the object. When a message is just sent to an object, methods will get that object as the =self= argument. When the method wants information about the object it is handling messages for, this is usually not what is intended.
Consider an account object, which inherited from the account prototype. All the methods are in the account prototype. When a message to modify the account value is sent to the actual account object, the message receiver is the account object. It does not handle this message, so it resends the message to the parent object, the account prototype. The method handler to modify the account value should now know to modify the account object, not the prototype. Hence, the =self= argument should point to the account object, but if the message was just sent directly to the prototype, it =self= would be the prototype. Hence, resending exists. The =resend= procedure allows a method to manually request such a resending.
This procedure will try to find a different handler for the given =message=. The handler can be searched for further up the inheritance tree, or even in a totally different object and its parents.
/Whereto/ can be one of the following values.
| #f | Use any parent of the object where this handler was found in. |
| A symbol | Use the object in the parent slot with this name. |
| Any object | Search for handlers in that object. |
/Resend/ is roughly equivalent in concept to CLOS' ~(next-method)~.
*** Root Objects
Since objects are created by sending a =clone= message to other objects, there has to be a kind of root object. Prometheus provides a procedure to create such root objects.
**** Procedure: *make-prometheus-root-object*
This creates a new root object from which other objects can be cloned. This object is independent of any other object, and thus creates a new inheritance tree.
#+begin_src scheme :tangle prometheus-impl.scm
(define (make-prometheus-root-object)
(let ((o (make-hermes-object)))
(o 'add-message! 'clone root-clone)
(o 'add-message! 'message-not-understood root-message-not-understood)
(o 'add-message! 'ambiguous-message-send root-ambiguous-message-send)
(o 'add-message! 'immediate-slot-list root-immediate-slot-list)
(o 'add-message! 'set-immediate-slot-list! root-set-immediate-slot-list!)
(o 'add-message! 'add-value-slot! root-add-value-slot!)
(o 'add-message! 'add-method-slot! root-add-method-slot!)
(o 'add-message! 'add-parent-slot! root-add-parent-slot!)
(o 'add-message! 'delete-slot! root-delete-slot!)
o))
#+end_src
Prometheus also provides a single existing root object, created with the procedure above. Unless specifically wanted otherwise, using this object as the root object ensures that all prometheus objects share a common ancestor.
**** Message: *clone*
Return a clone of the message recipient. This creates a new object with a single slot, =parent=, which points to the cloned object.
Add a new value slot to the recipient. The value of the slot can be retrieved with the /getter/ message. If a /setter/ message is given, that message can be used to change the value of the slot.
Add a method to the recipient. Sending the object a /getter/ message now invokes /proc/ with the same arguments as the message, in addition to a /self/ argument pointing to the current object and a /resend/ procedure available to resend the message if the method does not want to handle it directly.
The /setter/ message can later be used to change the procedure.
#+begin_src scheme :tangle prometheus-impl.scm
(define root-add-method-slot!
(make-getter-setter 'add-method-slot! value 'method
Add a parent slot to the recipient. Parent slots are searched for slots not found directly in the object. The /setter/ message, if given, can be used to later change the value of the parent slot.
#+begin_src scheme :tangle prometheus-impl.scm
(define root-add-parent-slot!
(make-getter-setter 'add-parent-slot! value 'parent
(lambda (self resend)
value)))
#+end_src
**** Message: *delete-slot!* /name/
Delete the slot named /name/ from the receiving object. This also removes the setter corresponding to /name/, if any. Beware that the parents might contain the same slot, so a message send can still succeed even after a slot is deleted.
#+begin_src scheme :tangle prometheus-impl.scm
(define (root-delete-slot! self resend getter)
(self 'set-immediate-slot-list!
(let loop ((alis (self 'immediate-slot-list)))
(cond
((null? alis)
'())
((eq? getter (caar alis))
(self 'delete-message! (cadar alis))
(loop (cdr alis)))
(else
(cons (car alis)
(loop (cdr alis)))))))
(self 'delete-message! getter))
#+end_src
**** Message: *immediate-slot-list*
This message returns a list of slots in this object. The elements of the list are lists with four elements:
- /getter-name/
- /setter-name/
- =#f=
- /type/, which can be one of the symbols =value=, =method= or =parent=.
For the root object, this is hardcoded:
#+begin_src scheme :tangle prometheus-impl.scm
(define (root-immediate-slot-list self resend)
'((clone #f #f)
(message-not-understood #f #f)
(ambiguous-message-send #f #f)
(immediate-slot-list set-immediate-slot-list! #f)
(add-value-slot! #f #f)
(add-method-slot! #f #f)
(add-parent-slot! #f #f)
(delete-slot! #f #f)))
#+end_src
Root also gets a setter for the =immediate-slot-list=:
This is received when the message /message/ with arguments /args/ to the object would have reached multiple parents. The root object just signals an error.
This is syntactic sugar for the often-used idiom to define a method slot, by sending a =add-method-slot!= message with a /message/ name and a lambda form with /self/, /resend/ and /args/ formals, and a /body/.
Another common operation is to clone an object, and add a number of value and method slots:
This is syntactic sugar for the typical actions of cloning an object from a /parent/ object, and adding more slots.
/other-parents/ is a list of ~(name object)~ lists, where each /object/ is added as a parent slot named /name/.
/slots/ is a list of slot specifications, either ~(getter value)~ or ~(getter setter value)~ for value slots, or ~((name self resend args ...) body ...)~ for method slots.
#+begin_src scheme :tangle prometheus-impl.scm
(define-syntax define-object
(syntax-rules ()
((_ name (creation-parent (parent-name parent-object) ...)
This is syntax used internally to make /define-object/ more readable.
#+begin_src scheme :tangle prometheus-impl.scm
(define-syntax define-object/add-slots!
(syntax-rules ()
((_ o)
(values))
((_ o ((method-name . method-args) body ...)
slots ...)
(begin
(o 'add-method-slot! 'method-name (lambda method-args
body ...))
(define-object/add-slots! o slots ...)))
((_ o (slot-getter slot-setter slot-value)
slots ...)
(begin
(o 'add-value-slot! 'slot-getter 'slot-setter slot-value)
(define-object/add-slots! o slots ...)))
((_ o (slot-getter slot-value)
slots ...)
(begin
(o 'add-value-slot! 'slot-getter slot-value)
(define-object/add-slots! o slots ...)))))
#+end_src
*** Private Messages
Message names in Prometheus don't have any required type. They are only compared using =eq?=. Because of this, any kind of Scheme object can be used as a message name. This means that it is possible to use a private Scheme value—for example, a freshly-allocated list—as a slot name. This can be used to keep slot names private, since it is not possible to create an object which is =eq?= to such an object except by receiving a reference to that object.
* Examples
** Simple Account Object
This is from the file =examples/account.scm= in the Prometheus distribution:
Since Prometheus does not allow for ambiguous message sends, and setter methods are just messages, this can lead to a confusing situation. Consider the following code:
#+begin_example
(define o1 (*the-root-object* 'clone))
(o1 'add-value-slot! 'foo 'set-foo! 1)
(define o2 (o1 'clone))
(define o3 (o2 'clone))
(o3 'add-parent-slot! 'parent2 o1)
#+end_example
This creates a diamond-shaped inheritance tree. Now it is possible to send a =set-foo!= message to =o3=, though it inherits this slot from two parents, the slot is ultimately inherited from the same object. But now witness the following:
#+begin_example
> (o3 'foo)
=> 3
> (o2 'set-foo! 2)
> (o3 'set-foo! 3)
error--> Ambiguous message send
#+end_example
What happened here? The =set-foo!= message added the foo slot to =o2=, but with it, also the associated method to mutate that slot, =set-foo!=. So, sending =set-foo!= to =o3= will find the same message both in =o1= and =o2=, and cause an ambiguous message send.
Conclusion: Be extra careful with multiple inheritance.
