Re: Events, method iterator

Re: Events, method iterator

[Markus Mottl]

> >> There's no way to define an
> >>     explicit function to send messages to objects ?
> >
> >Yes, there is: just use unbound (=lambda-) functions. E.g.:
> >
> >  List.iter (fun obj -> obj#paint) _objs
> 
> Yeah, this is a step in the direction I intended, thanks, but I think i
> can't write a sort of
> 
> let send msg obj = obj#msg	(* I know this won't work *)

Yes, this won't work, because OCAML relies on static type checking only
(I think this is good). It is impossible to decide at compile time
whether all objects "obj" passed to "send" really implement message "msg".

> to be used like this :
> ...
> List.iter (send paint) _objs
> 
> ...
> List.iter (send (moveRel 20 20)) _objs

Two possibilities:

You can define an algebraic data type of messages. E.g.

  type action = Paint
              | MoveRel of int * int

  let send action obj = match action with
    | Paint -> obj#paint
    | MoveRel (x,y) -> obj#move_rel x y

  class graph_obj name = object
    method paint = Printf.printf "%s#paint\n" name
    method move_rel x y = Printf.printf "%s#move_rel %d %d\n" name x y
  end

  let _ =
    List.iter (send (MoveRel(20,20)))
      [new graph_obj "first"; new graph_obj "second"]

But I prefer the version, where you have to define the functions (shorter
and clearer):

  let paint obj = obj#paint
  let move_rel x y obj = obj#move_rel x y

  class graph_obj name = object
    method paint = Printf.printf "%s#paint\n" name
    method move_rel x y = Printf.printf "%s#move_rel %d %d\n" name x y
  end

  let _ =
    List.iter (move_rel 20 20) [new graph_obj "first"; new graph_obj "second"]

This is hardly any effort and works just fine.

Don't forget that functions are first class values in OCAML!! You can
pass them around easily (including functions that encapsulate method
calls)! E.g.:

  let paint obj = obj#paint
  let move_rel x y obj = obj#move_rel x y

  class graph_obj name = object
    val mutable x = 0
    val mutable y = 0
    method paint = Printf.printf "%s#paint\n" name
    method move_rel dx dy =
      x <- x + dx; y <- y + dy;
      Printf.printf "%s#move_rel %d %d -> x:%d y:%d\n" name dx dy x y
  end

  let _ =
    let obj_lst = [new graph_obj "first"; new graph_obj "second"]
    and msg_lst = [move_rel 42 10; move_rel (-5) 7; paint] in
    List.iter (fun msg -> List.iter msg obj_lst) msg_lst

Syntactically convenient enough? :-)

Well, I could imagine a slightly more convenient version if there were
some kind of operator which automatically creates a function that applies
a method to an object. Maybe something like:

  List.iter (#move_rel 10 20) obj_lst

Another use:

  let msg_lst = [#move_rel 42 10; #move_rel (-5) 7; #paint]

Thus, the encapsulating function does not have to be necessarily bound
(keeps the namespace tidy) and the programmer can see from the syntax
that it is methods that get invoked.

I don't know whether the implementors want to add such a kind of syntactic
sugar... any suggestions?

> I know the result is the same, but I prefer a solution like this,
> I like sintactic-clean code ...

Me, too. But I like semantically correct code even more - and thus I
don't want to have my programs crash, just because a message is passed
to an object, which can't handle it. But as you can see, the present
concept is just fine...

Best regards,
Markus

-- 
Markus Mottl, mottl@miss.wu-wien.ac.at, http://miss.wu-wien.ac.at/~mottl

Re: Random.int on non unix platform

[Martin Quinson]

> >From: Martin Quinson <mquinson@zeppelin-cb.de>
> 
> >How can I write this without the unix library ?
> >
> >Random.init (int_of_float (Unix.time ()));;
> 
> You can do:
> 
>     Random.init (int_of_float (Sys.time ()));;
> 
> or you can input a random seed from the user and give it as argument
> to Random.init.


                        (francais en bas)

hmmm. from htmlman/module sys :

> val time: unit -> float
>
>          Return the processor time, in seconds, used by the program since the
> beginning of execution.

So, if this call is at the begining of the programm, it will allways generate 
the same number (or almost). from the random.ml source code :
(* Simple initialisation.  The seed is an integer.
   Two seeds that are close enough will not produce uncorrelated
   pseudo-random sequences.
*)

And I really dislike the second solution...

Maybe would it be possible to make the module Random initialize itself alone 
before the first call, in the next release...


                             [en francais]

J'aimerais initialiser le generateur aléatoire de nombre sans utiliser la 
librairie Unix. Dans d'autres langage, j'ai l'habitude d'utiliser l'heure pour 
ca, mais, apparament, c'est impossible sans la librairie Unix (ce qui n'est 
pas portable, oblige a recompiler le top-level et complique la ligne de 
compilation des compilateurs)

Quant à l'idée de demander une graine à l'utilisateur, je trouve ca un peu 
préhistorique, comme approche. ;)

A mon avis, le plus simple serait encore que le module Random se charge tout 
seul comme un grand de s'initialiser sans demander a l'utilisateur de le faire.
(dans une prochaine version ?)

Merci & thanks,
Martin.

Events, method iterator

[Ubik]

Hi everybody. I premise:
1) I'm a neophyte about caml
2) I can't speak French, so my mails will be mono-lingual.

Now the questions :
a) How can i get the value associated with an event (if an event actually
"carries" a value ...) ?
    i tried to send out a function through an int->int->int channel, i
receive it from the other side with
    an int->int->int event, but i can't apply it ...
b) Is there a way to iterate a method upon a list of objects of some type ?
Suppose i have a hierarchy
    with many overridden methods, and that i use containers of base-type,
like this :
    class shape = object
	method virtual paint : unit
	...
    end
    class circle = object
	inherit shape
	method paint = ...
	...
    end
    ...
    let paintObject x = x#paint

    class container = object
	inherit shape
	val mutable _objs:shape list = []
	method paint = List.iter paintObject _objs
	...
    end
    I have to define a function for every method to iterate, in this way ?
There's no way to define an
    explicit function to send messages to objects ?

Thank you

Emiliano

Re: Events, method iterator

[Jerome Vouillon]

On Sun, May 30, 1999 at 09:44:25PM +0200, Ubik wrote:

> a) How can i get the value associated with an event (if an event actually
> "carries" a value ...) ?
> i tried to send out a function through an int->int->int channel, i
> receive it from the other side with
>     an int->int->int event, but i can't apply it ...

An event is something that may happen on a channel. You have to wait
for it to get a value from the channel. You can either wait for one
event using the function "Event.sync", wait for any of a list of
events with "select" or check if an event has already happen with
"poll".

[...]
>     let paintObject x = x#paint
> 
>     class container = object
> 	inherit shape
> 	val mutable _objs:shape list = []
> 	method paint = List.iter paintObject _objs
> 	...
>     end
>     I have to define a function for every method to iterate, in this way ?
> There's no way to define an
>     explicit function to send messages to objects ?

Yes, you need to define a function each time. But you can do it inline :
   method paint = List.iter (fun x -> x#paint) _objs

-- Jérôme