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From: Goswin von Brederlow <goswin-v-b@web.de>
To: caml-list@inria.fr
Subject: Re: [Caml-list] Subtyping
References: <873aclf1z8.fsf@frosties.localdomain>
Date: Wed, 08 Apr 2009 02:38:49 +0200
In-Reply-To: <873aclf1z8.fsf@frosties.localdomain> (Goswin von Brederlow's
	message of "Tue, 07 Apr 2009 07:48:43 +0200")
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X-Spam: no; 0.00; subtyping:01 foo:01 foo:01 ocaml:01 ocaml:01 iter:01 syntax:01 1.2:98 mfg:98 abstract:01 caml-list:01 int:01 int:01 writes:01 newline:02 

Goswin von Brederlow <goswin-v-b@web.de> writes:

> So what other ways are there of doing this? Records. Idealy I would
> like to do this:
>
> type base = { x : int }
> let make_base x = { x = x }
> let print_x r = print_int r.x
> type foo = { base with y : int }
> let make_foo x y = { x = x; y = y }
> let _ =
>   print_x (make_base 0); print_newline ();
>   print_x (make_foo 0 1); print_newline ();;
>
> Ocaml has no way of making this work like that, right?

Small extra question concerning this. Can I get ocaml to recognise a
type like this?

type base = 'a. {
  x : 'a;
  fn : 'a -> unit;
}

List.iter
  (fun r -> r.fn r)
  [{x = 1; fn = (fun r -> print_int r.x); };
   {x = 1.2; fn = (fun r -> print_float r.x); }]

The difference to a "'a base" type would be that the 'a is only
infered and fixed inside the record but remains abstract outside of
it.

For that reason this would not be allowed:

let cross_call x y = x.fn y

The 'a would have to escape the record which wouldn't be allowed.


So is there any syntax or trick to get the ocaml typesystem to
construct such a type?

MfG
        Goswin