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Date: Fri, 27 Jun 2008 13:35:57 -0700 (PDT)
From: Dario Teixeira <darioteixeira@yahoo.com>
Subject: Re: [Caml-list] Phantom types: transparency vs rogue unification
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X-Spam: no; 0.00; unification:01 functor:01 non-trivial:01 node:01 node:01 opaqueness:01 sig:01 val:01 val:01 struct:01 parser:01 parser:01 sig:01 subset:01 struct:01 

> I may have spoken too soon about the functor solving the
> problem.  In fact,
> in a non-trivial example the problem is merely shifted to a
> different place.

Hi,

Just to let you know that I have found an embarrassingly simple workaround:
the Document module can just provide node_of_complex and node_of_basic
functions to overcome the opaqueness of Document.t.  I haven't found (yet)
any major disadvantages to this solution, so I reckon the problem can be
solved this way (code below).

Thanks again for your attention,
Dario Teixeira

___________________________________________________________________________

(************************************************************************)
(* Document.=09=09=09=09=09=09=09=09*)
(************************************************************************)

module type DOCUMENT =3D
sig
=09type inline_t =3D node_t list
=09 and node_t =3D
=09=09private
=09=09| Text of string
=09=09| Bold of inline_t
=09=09| Italic of inline_t

=09type 'a t

=09val node_of_complex: [< `Basic | `Complex] t -> node_t
=09val node_of_basic: [< `Basic] t -> node_t

=09val text: string -> [> `Basic] t
=09val bold: 'a t list -> 'a t
=09val italic: [< `Basic | `Complex] t list -> [> `Complex] t
end


module Document : DOCUMENT =3D
struct
=09type inline_t =3D node_t list
=09 and node_t =3D
=09=09| Text of string
=09=09| Bold of inline_t
=09=09| Italic of inline_t

=09type 'a t =3D node_t

=09let node_of_complex node =3D node
=09let node_of_basic node =3D node

=09let text str =3D Text str
=09let bold inl =3D Bold inl
=09let italic inl =3D Italic inl
end


(************************************************************************)
(* Document_parser.=09=09=09=09=09=09=09*)
(************************************************************************)

module Document_parser:
sig
=09exception Invalid_subset

=09val parse_complex: string -> [> `Basic | `Complex] Document.t
=09val parse_basic: string -> [> `Basic] Document.t
end =3D
struct
=09open Document

=09exception Invalid_subset

=09let parse_complex =3D function
=09=09| "complex"     -> italic [bold [text "ola"]]
=09=09| _             -> bold [text "ola"]

=09let complex_to_basic node =3D
=09=09let rec complex_to_basic_aux =3D function
=09=09=09| Text str      -> text str
=09=09=09| Bold inl      -> bold (List.map complex_to_basic_aux inl)
=09=09=09| Italic inl    -> raise Invalid_subset
=09=09in complex_to_basic_aux (node_of_complex node)

=09let parse_basic str =3D complex_to_basic (parse_complex str)
end


(************************************************************************)
(* top-level.=09=09=09=09=09=09=09=09*)
(************************************************************************)

open Document
open Document_parser

let foo =3D bold [text "foo"];;

let bar =3D italic [text "bar"];;

let a =3D parse_complex "complex";;

let b =3D parse_complex "basic";;

let c =3D parse_basic "basic";;

let d =3D parse_basic "complex";;  (* Raises exception *)
___________________________________________________________________________

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