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Date: -- (:)
From: John Prevost <j.prevost@g...>
Subject: Re: [Caml-list] method name collision ?
On Sun, 26 Dec 2004 07:57:48 +0900 (JST), Jacques Garrigue
<garrigue@math.nagoya-u.ac.jp> wrote (in response to
briand@aracnet.com):
> The "as drawable" doesn't mean that the methods are distinct: you can
> still call all inherited methods from the outside, and even through
> self.
> This only allows you to access the old definition of a method, if you
> want to extend it incrementally.
> 
> method point ~x ~y =
>   ...  drawable#point ~x ~y ...

And just to make sure it's clear, the reason that the type has to
match is that *other* methods of the inherited class have to be able
to do self#point calls on the new object and work properly.  Here's an
illustration:

-----
class test_1 =
  object (self)
    method test a = a + 1
    method test_self () = self#test 5
  end

class test_2 =
  object
    inherit test_1 as t1
    method test x =
      Printf.printf "Called with %d\n" x;
      t1#test x
  end
-----

In this example code, you can see that the inherited methods from
test_1, when they make self#... calls, use the overridden methods in
test_2.  So if test_2 were allowed to change the types of the methods
in incompatible ways, there would be a problem.

If you *do* want to just use the methods from the other class, I
recommend using the following idiom:

Instead of this:

class broken =
  object
    inherit some_other_class as soc
    method replace_and_change_type = ...
    (* method pass_through_unchanged is inherited from soc *)
  end

write this:

class working =
  let soc = new some_other_class in
  object
    method replace_and_change_type = ...
    method pass_through_unchanged = soc#pass_through_unchanged
  end

The first one brings inheritance into play--which is important if you
want to override methods, but is not neccessary (or sufficient) in
OCaml to define a subtype.  If, instead, you want to make use of the
other class's features in order to implement your class, and pass some
methods through unchanged, use the second model.  If you need a
mixture of both, first extend the base class in order to override its
methods (for the base class's internal use, calling self#...), then
use the second model to provide a different interface.

Finally, I just realized that you could also do this if you want to
avoid defining more than one class:

class embrace_and_extend =
  let extended_object = object
    inherit class_you_want_to_extend
    method override_something = ...
  end in
  object
    ... provide a different interface ...
  end

It hadn't occurred to me before that the new immediate objects feature
allows you to inherit from a class when defining an immediate object. 
(Although, of course, there is no way to inherit from an immediate
object further down the line.)

John.