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[Caml-list] Objects and private methods
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Date: -- (:)
From: Jacques Garrigue <garrigue@k...>
Subject: Re: [Caml-list] Objects and private methods
> I'm having some trouble getting the following type of structure to
> type-check:
> 
>   class type a = object method get_b : a end
> 
>          and b = object method b : unit end
> 
>   class a_impl =
>   (object (self)
>     method get_b = (self :> b)
>     method private b = ()
>   end : a)
>   ;;
> 
> This produces the error message:
> 
>   The class type object method b : unit method get_b : b end
>   is not matched by the class type a
>   The public method b cannot be hidden
> 
> I would have expected the failure to occur when coercing to type 'b' as
> the method is declared private (which does not match the signature).

Due to the way privacy attributes are implemented in the type checker,
a type annotation can force a private method to become public (but
never the opposite). You can see it on the following example.

  # class c = object (self : < b : 'a; .. >) method private b = 1 end;;
  class c : object method b : int end

This explains why the type checker doesn't complain about b not being
public: it makes it public as needed. And since b is public, you
cannot hide it.
(This may seem strange from the point of view of privacy, but you
should just understand private methods as "may be hidden in a
subclass"; if unified with a public method it may no longe be hidden.)

> The basic idea is that the class 'a_impl' implements 'b' privately. This
> seems like something that should be possible to do.  I suspect that the
> coercion of self to 'b' is causing the problem here.

I don't understand very well what you mean by "implement privately":
a and b do not seem to be related in your code.
I will suppose that you meant
  class type a = object method get_b : b end

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.
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)

---------------------------------------------------------------------------
Jacques Garrigue      Kyoto University     garrigue at kurims.kyoto-u.ac.jp
		<A HREF=http://wwwfun.kurims.kyoto-u.ac.jp/~garrigue/>JG</A>
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