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Date: Sun, 22 Feb 2009 12:59:14 -0800 (PST)
From: Dario Teixeira <darioteixeira@yahoo.com>
Subject: Unusual type declaration and Sexplib
To: caml-list@yquem.inria.fr
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X-Spam: no; 0.00; foobar:01 sexp:01 syntax:01 sig:01 foobar:01 foo:01 sexp:01 val:01 foo:01 val:01 struct:01 recursive:01 annotations:01 coercions:01 859:98 


Hi,

I'm having some trouble serialising via Sexplib a data structure defined
recursively.  Consider module M defined below; note how type foobar_t inclu=
des
a "with sexp" declaration, telling the Sexplib syntax extension to create
(de)serialisers automatically.  However, type t cannot rely on that automat=
ism,
because type declarations with the "as" operator are not fully supported.
Therefore, I need to create the (de)serialisers for this type manually.


module M:
sig
        type 'a foobar_t =3D
                [ `Foo of int
                | `Bar of 'a list
                ] with sexp

        type t =3D private [< 'a foobar_t ] as 'a

        val foo: int -> t
        val bar: t list -> t
end =3D
struct
        type 'a foobar_t =3D
                [ `Foo of int
                | `Bar of 'a list
                ] with sexp

        type t =3D 'a foobar_t as 'a

        let foo x =3D `Foo x
        let bar x =3D `Bar x
end


So basically the problem is creating the functions with the following signa=
tures:

val sexp_of_t: t -> Sexplib.Sexp.t
val t_of_sexp: Sexplib.Sexp.t -> t

Using as base the automatically created functions whose signatures are as f=
ollows:

val sexp_of_foobar_t: ('a -> Sexplib.Sexp.t) -> 'a foobar_t -> Sexplib.Sexp=
.t
val foobar_t_of_sexp: (Sexplib.Sexp.t -> 'a) -> Sexplib.Sexp.t -> 'a foobar=
_t

But in practice, I'm having trouble seeing how these later functions can
be used.  The problem lies in their recursive definition: how would I
break it?  (And yes, I realise that the definition of type t is unusual,
but it's extremely convenient for avoiding a lot of explicit annotations
and coercions [1]).

Thanks for your help!
Best regards,
Dario Teixeira

[1] http://groups.google.com/group/fa.caml/browse_thread/thread/7552095ab85=
9fb5f/9c4a6fd19812fbcc
=0A=0A=0A