Re: [Caml-list] Object Features

[Gerd Stolpmann <info@gerd-stolpmann.de>]

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Am Samstag, den 29.11.2014, 20:24 -0800 schrieb Jordan W:

> 2. Object extension: I believe that OCaml immediate objects are fairly
> under-appreciated,

I wouldn't say so. As I'm not a big fan of inheritance immediate objects
are the simpler and more expressive of dealing with objects.

>  and most people could find useful applications for them (at least) in
> the form of row polymorphic records. However, there are several
> features that would make them even more powerful. The feature I long
> for the most is object extension (on immediate objects). OCaml has
> support for extending objects (inheritance), but only via classes.

Basically, the availability of inheritance is the big difference between
classes and functions returning immediate objects. As of now, you need
to define classes if you want extensibility. But inheritance is of
course more than that; in particular you can also override existing
methods (and that's what I in particular don't like about it, as this
leads to hard to understand program flows).

What could in deed be useful is a more light-weight construction that
layers objects, i.e. you define a new object around an existing one:
Something like

let new_object (old_object : t) =
  object
    (* Define some new methods: *)
    method foo = ...

    (* And make all methods of old_object also available *)
    method _ = old_object # _

    (* This notation shall mean that we define all methods x as
       method x = old_object # x
       and do this for all methods from t that haven't been defined
       otherwise
    *)
  end

This is very different from inheritance, because you have an outer
object and an inner, and the interface of the inner object is respected
(i.e. if another method of old_object calls foo, it is sure it calls the
version of foo in old_object, and not the new definition in new_object).
And this layering or wrapping construction is of course compatible with
immediate objects.

Gerd

-- 
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Gerd Stolpmann, Darmstadt, Germany    gerd@gerd-stolpmann.de
My OCaml site:          http://www.camlcity.org
Contact details:        http://www.camlcity.org/contact.html
Company homepage:       http://www.gerd-stolpmann.de
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