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To: patrick@watson.org
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] Objects and private methods
In-Reply-To: <20011221001434.V91723-100000@fledge.watson.org>
References: <20011221140658Z.garrigue@kurims.kyoto-u.ac.jp>
	<20011221001434.V91723-100000@fledge.watson.org>
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Message-Id: <20011221174035L.garrigue@kurims.kyoto-u.ac.jp>
Date: Fri, 21 Dec 2001 17:40:35 +0900
From: Jacques Garrigue <garrigue@kurims.kyoto-u.ac.jp>
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From: Patrick M Doane <patrick@watson.org>

> > You cannot convert something from private to public through a coercion
> > (this is different from forcing it with an annotation). This is
> > because the values are actually different: a private method and a
> > public method have different internal representations.
> 
> Are there any papers that I could read over that would give a better sense
> for the internal representations? That might help me understand some of
> these limitations better.

I don't think so, but books like Abadi&Cardelli's Types for Objects
would give you an insight on what is safe and what is not.

> > On the other hand, you can hide a method through coercions.
> >
> >   class a_impl = object (self)
> >     method get_b = (self :> b)
> >     method b = ()
> >   end
> >   let new_a = (new a_impl :> a)
> 
> Thanks! What then are the advantages to using an annotation instead of a
> coercion for signature matching? For example, with a hypothetical syntax
> like this:
> 
> class a_impl :> a = object (self)
>   method get_b = (self :> b)
>   method b = ()
> end

You can only coerce from type to type, not from signature to
signature. Otherwise you would be able to hide a method, and then
redefine it with the same name but another type. Such inheritance
patterns are not type safe, so you are not allowed to hide a public
method at the class level, only at the expression level, where it is
safe.

Jacques Garrigue
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