Re: [Caml-list] Narrowing class's public interface

[Jacques Garrigue <garrigue@kurims.kyoto-u.ac.jp>]

From: Tony Edgin <edgin@slingshot.co.nz>

> What is the reasoning behind Ocaml not allowing the public methods
> of a class to be hidden?  

This is just a type soundness problem: as an ocaml object type only
describes the interface of an object, you have no way to make sure
that an object had originally a specific method (with the right type)
once you have hidden it.
Of course we could add some kind of declaration like
   method hidden m : type
and allow a subclass to convert a public method into such an "hidden"
one, but I'm quite sure this is not what you are asking for?
(Note that such an addition would be rather heavy, as "hidden" method
must appear in object types too, i.e. < hidden m : type; ... >)

> Some times its nice for a class to have "global" public methods
> (Java's public methods) as well as public methods local to its
> module (Java's unprefixed methods).  This allows for progressive
> encapsulation, where more distance modules no very little about a
> class, while the more local modules know more about the class.
> Inheritance could be used to approximate this, but sometimes
> subtyping makes more sense.

This problem has been thought of :-)
Look at the user's manual, section 3.17, for how to define "friend"
methods, i.e. methods accessible only from code in the same module.
Note that since ocaml modules are hierarchical, this actually allows
you more precision than Java's packages.

http://wwwfun.kurims.kyoto-u.ac.jp/soft/ocaml/htmlman/manual005.html#ss:friends

Jacques Garrigue

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