Re: [Caml-list] question: what is the recommended use case of `val` in class type

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

On 2016/07/04 19:19, Goswin von Brederlow wrote:
> 
> On Sun, Jul 03, 2016 at 03:23:35PM +0900, Jacques Garrigue wrote:
>> 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
> 
> What's the point of x then? Wouldn't it make more sense to only expose
> the accessors so even derived classes will go through them instead of
> accessing x directly?

Maybe, if OCaml were an object-oriented language.
But the accessor approach doesn’t mix too well with multiple inheritance, whereas
ocaml instance variables do.

> Question: Can ppx attributes generate the accessors but also hide the
> "val mutable x" when no mli file is present?


Not so easy to do, because you need to write an explicit class interface to hide
the instance variable, and for this you need to write the type of the methods x and x_set.
Here is some code that does it, and could be translated into an extension:

class c (x : int) = object
   inherit (fun x -> object val mutable x = x method x = x method x_set y = x <- y end :  'a -> object method x : 'a method x_set : 'a -> unit end) x
end;;

One could generate the inherit line.

Jacques