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Date: Sat, 12 Jul 2008 11:21:09 -0700 (PDT)
From: Dario Teixeira <darioteixeira@yahoo.com>
Subject: Re: [Caml-list] Troublesome nodes
To: Wolfgang Lux <wlux@uni-muenster.de>
Cc: Jeremy Yallop <jeremy.yallop@ed.ac.uk>, caml-list@yquem.inria.fr
In-Reply-To: <F562803B-40A8-4914-8B20-03F8E9099308@uni-muenster.de>
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X-Spam: no; 0.00; nodes:01 node:01 compiler:01 nodes:01 node:01 sig:01 val:01 val:01 struct:01 seq:01 seq:01 foo:01 foo:01 struct:01 intersection:01 

Hi,

Thank you all for your help!  I think we're almost there: with Wolfgang's
suggestion, the Node module complies with the original constraints, as the
code below shows:  (the compiler complains -- as it should! -- when on the
last line I try to nest two link nodes directly)

module rec Node:
sig
    type nonlink_node_t =3D [ `Text of string | `Bold of Node.super_node_t =
list ]
    type link_node_t =3D [ `See of string | `Mref of string * nonlink_node_=
t list ]
    type super_node_t =3D [ nonlink_node_t | link_node_t ]

    val text: string -> nonlink_node_t
    val bold: [< super_node_t] list -> nonlink_node_t
    val see: string -> link_node_t
    val mref: string -> nonlink_node_t list -> link_node_t
end =3D
struct
    type nonlink_node_t =3D [ `Text of string | `Bold of Node.super_node_t =
list ]
    type link_node_t =3D [ `See of string | `Mref of string * nonlink_node_=
t list ]
    type super_node_t =3D [ nonlink_node_t | link_node_t ]

    let text txt =3D `Text txt
    let bold seq =3D `Bold (seq :> super_node_t list)
    let see ref =3D `See ref
    let mref ref seq =3D `Mref (ref, seq)
end

open Node

let foo1 =3D text "foo"                   (* valid *)
let foo2 =3D bold [text "foo"]            (* valid *)
let foo3 =3D mref "ref" [text "foo"]      (* valid *)
let foo4 =3D mref "ref" [see "ref"]       (* invalid *)


Now, the icing on the cake would be the possibility to build a Node_to_Node
set of functions without code duplication.  Ideally, I would like to do
something as follows:

module Node_to_Node =3D
struct
    open Node

    let rec convert_nonlink_node =3D function
        | `Text txt         -> Node.text txt
        | `Bold seq         -> Node.bold (List.map convert_super_node seq)

    and convert_link_node =3D function
        | `See ref          -> Node.see ref
        | `Mref (ref, seq)  -> Node.mref ref (List.map convert_nonlink_node=
 seq)

    and convert_super_node node =3D match node with
        | #nonlink_node_t   -> (convert_nonlink_node node :> super_node_t)
        | #link_node_t      -> (convert_link_node node :> super_node_t)
end


But this fails on the last line:

Error: This expression has type
         [< `Bold of 'a list & Node.super_node_t list | `Text of string ]
         as 'a
       but is here used with type
         [< `Mref of string * 'a list | `See of string ]
       These two variant types have no intersection


Do you see any way around it?

Thanks again for all your time and attention!
Cheers,
Dario
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