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[Caml-list] Alternate constructors for a class
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Date: 2003-09-02 (16:07)
From: Gerd Stolpmann <info@g...>
Subject: Re: [Caml-list] Alternate constructors for a class
Am Die, 2003-09-02 um 15.45 schrieb Richard Jones:
> I have a class defined currently like this:
>   class view ?width ?height ?packing ?show ?(title = "untitled") () =
>     (* ... *)
> I want to provide an alternate way of constructing this class, in this
> case by unmarshalling it from a channel (using the Marshall module).
> Callers should be able to construct a view either using the explicit
> parameters, or by doing:
>   let chan = open_in_bin "serialized_class" in
>   let view = new view ~chan in
>   (* ... *)
> Now in the case above I guess I could add an extra optional ?chan
> argument, but that seems a bit ugly because it doesn't prevent users
> from supplying incorrectly both ~chan and ~title, for example.
> Is there an easy way to provide two different constructors for a
> class?
> Other languages do allow different sets of arguments to different
> constructors for the same class quite naturally.

You may consider a union type, e.g.

type view_intf = View_explicitly of (int * int * ...)
               | View_from_channel of in_channel

class view (intf : view_intf) =
  let (width, height, ...) =
    match intf with
      View_explicitly(w,h,...) -> (w,h,...)
    | View_from_channel ch  -> ...
object ... end

class view_explicitly ?width ?height ... = 
  view (View_explicitly(width,height,...))

class view_from_channel ch =
  view (View_from_channel ch)

Ok, this is not very natural, but at the end the way of implementation
is hidden from the users.

Other options would be independent implementations of view_explicitly
and view_from_channel, which is possible if both classes have the same
type (which is very natural in O'Caml but not in other languages), or
you can also try to map view_from_channel directly to view, e.g.

class view_from_channel ch =
  let (width, height, ...) = ... unmarshal ch ... in
  view ~width ~height ...

You could also inherit from virtual classes only providing components of
view, and by different ways of inheriting you get different interfaces.
Just be creative.

Gerd Stolpmann * Viktoriastr. 45 * 64293 Darmstadt * Germany

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