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Date: Sun, 13 Jul 2008 10:39:07 -0700 (PDT)
From: Dario Teixeira <darioteixeira@yahoo.com>
Subject: Re: [Caml-list] Troublesome nodes
To: caml-list@yquem.inria.fr
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X-Spam: no; 0.00; nodes:01 foo:01 foo:01 node:01 node:01 intersection:01 sig:01 val:01 val:01 struct:01 seq:01 seq:01 show-stopper:01 nodes:01 bypassing:01 

Hi again,

Sorry, but in the meantime I came across two problems with the supposedly
ultimate solution I just posted.  I have a correction for one, but not
for the other.

The following statements trigger the first problem:

let foo1 =3D text "foo"
let foo2 =3D see "ref"
let foo3 =3D bold [foo1; foo2]

Error: This expression has type Node.link_node_t but is here used with type
         Node.nonlink_node_t
       These two variant types have no intersection

The solution that immediately comes to mind is to make the return types
for the constructor functions open:  (I can see no disadvantage with
this solution; please tell me if you find any)


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


The second problem, while not a show-stopper, may open a hole for misuse of
the module, so I would rather get it fixed.  Basically, while the module
provides constructor functions to build nodes, nothing prevents the user
from bypassing them and constructing nodes manually.  The obvious solution
of declaring the types "private" results in an "This fixed type has no row
variable" error.  Any way around it?

Thanks once more for your time,
Dario Teixeira
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