Hi Hongo,

You may want to have a look at a small camlp4 syntax extension I wrote a long time ago,
which allows to do what you say.
Using it, one would write:

val mutable x with accessor

to say that it can be accessed through the x and x_set methods.

The code is here:
http://www.math.nagoya-u.ac.jp/~garrigue/code/ocaml.html

Note that I didn’t try to support functional state changes, i.e. adding a method
method x_upd x’ = {< x = x’ >}

Jacques

On 2016/07/03 02:23, Hongbo Zhang (BLOOMBERG/ 731 LEX) wrote:
>
> Hi Nicolas, thanks! It matches with my expectation.
> I am thinking of overriding such syntax for bucklescript FFI, we can use either
>
> class type t = object val mutable x : int end
> or
> class type t = object method x : int method x_set : int -> unit end
>
> Currently I think the former looks better, since the user can tell it is getter or setter without relying on naming convention
> From: nicolas.ojeda.bar@lexifi.com At: 07/02/16 13:08:28
> To: HONGBO ZHANG (BLOOMBERG/ 731 LEX)
> Cc: caml-list@inria.fr, gabriel.scherer@gmail.com
> Subject: Re: [Caml-list] question: what is the recommended use case of `val` in class type
> Hi Hongbo,
>
> As you observed, `val` can be used via inheritance to expose some private state to subclasses without exposing it to the outside.
>
> Cheers
> Nicolas
>
> On Sat, Jul 2, 2016 at 7:02 PM, Hongbo Zhang (BLOOMBERG/ 731 LEX) <hzhang295@bloomberg.net> wrote:
> Thanks for your reply. But if `val` is not accessible from outside, why it is the part of class type signature, any reason for this design?
>
> From: gabriel.scherer@gmail.com At: 07/02/16 13:00:34
> To: HONGBO ZHANG (BLOOMBERG/ 731 LEX)
> Cc: caml-list@inria.fr
> Subject: Re: [Caml-list] question: what is the recommended use case of `val` in class type
> Objects have some private state, and they expose methods that can be called from the outside. "val" fields correspond to such private state, they are not accessible from outside and are thus not part of an object's type.
>
> You can always expose a value field to the outside through a "getter" method to access it (and a "setter" method to mutate it if relevant), but that is often considered dubious object-oriented style -- it tends to go against good encapsulation.
>
> On Sat, Jul 2, 2016 at 12:45 PM, Hongbo Zhang (BLOOMBERG/ 731 LEX) <hzhang295@bloomberg.net> wrote:
> Dear all,
> I have a question about val in class type, is it only useful in inheritance?
> for example
>
> class type text = object val mutable text : string end
>
> let f (x : text ) = x#text;;
> ^
> Error: This expression has type text
> It has no method text
> Thanks -- Hongbo
>
>



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