Re: [Caml-list] C interop: Return values in parameters

[Marek Kubica <marek@xivilization.net>]

Hello Gabriel,

Thanks for your mail. I read it multiple times to grasp all information.

On Sun, 30 Dec 2012 15:36:16 +0100
Gabriel Scherer <gabriel.scherer@gmail.com> wrote:

> References are a derived concept defined as:
> 
>   type 'a ref = { mutable contents : 'a }
> 
> You can update them from the C side just as you would handle a
> polymorphic record, with Field and Store_field.

Turns out Field is also valid as L-Value so I can even use Field(x,0) to
set. Fun :) With this solution, I managed to fix my immediate problem.

> In the code you show, the OCaml value corresponding to the pointer is
> exactly the pointer, hidden as a 'value' type. This is correct as
> OCaml detects out-of-(OCaml)-heap pointer. However, if you used a
> custom block instead (
> http://caml.inria.fr/pub/docs/manual-ocaml-4.00/manual033.html#toc150
> ), OCaml would do the boxing for you: Data_custom_val(v) already
> returns a pointer than can be dereferenced or mutated.

I read this as well as
<http://www.linux-nantes.org/~fmonnier/ocaml/ocaml-wrapping-c.php#ref_custom>
and while I seem to understand how it works, I don't get how to modify
it.

Do I understand right that I'd just create such a custom block and just
wrap the pointer into it? I'll probably implement it tomorrow and see
how I like it.

> I think you have a choice between using references explicitly for
> those functions of the API that mutate input references, or uniformly
> representing this type of data as a custom block. The latter option
> may be valuable if you have uses for the other features of custom
> blocks, eg. the user-defined comparison and finalization operations,
> and probably not worth the trouble otherwise. Finally, explicitly
> using references to signal mutability in some part of your API is
> probably clearer and a better design.

Well, except for finalization I don't really have much use for the
operations that custom blocks expose, but the finalization might be
worth the work.

Thanks again, I really felt productive today, by getting things done.

regards,
Marek