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Date: Sat, 12 Jul 2008 05:37:14 -0700 (PDT)
From: Dario Teixeira <darioteixeira@yahoo.com>
Subject: Re: [Caml-list] Troublesome nodes
To: Jeremy Yallop <jeremy.yallop@ed.ac.uk>
Cc: caml-list@yquem.inria.fr
In-Reply-To: <4877CE91.6010107@ed.ac.uk>
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X-Spam: no; 0.00; nodes:01 compiler:01 node:01 subset:01 node:01 compiler:01 sig:01 val:01 val:01 struct:01 seq:01 seq:01 foo:01 foo:01 ingenious:98 

Hi,

Thanks Jeremy, that's quite ingenious.  However, I've hit a problem in the
definition of the constructor functions (the full code + compiler error
is below).  Now, I know that nonlink_node_t is a subset of super_node_t,
and therefore any variant valid for nonlink_node_t is also acceptable for
super_node_t.  But how do I tell this to the compiler?  (Note that having
users of the module manually casting types with :> is something I would
rather avoid).

Thanks again,
Dario


module rec Node:
sig
=09type nonlink_node_t =3D [ `Text of string | `Bold of Node.super_node_t l=
ist ]
=09type link_node_t =3D [ `See of string | `Mref of string * nonlink_node_t=
 list ]
=09type super_node_t =3D [ nonlink_node_t | link_node_t ]

=09val text: string -> nonlink_node_t
=09val bold: super_node_t list -> nonlink_node_t
=09val see: string -> link_node_t
=09val mref: string -> nonlink_node_t list -> link_node_t
end =3D
struct
=09type nonlink_node_t =3D [ `Text of string | `Bold of Node.super_node_t l=
ist ]
=09type link_node_t =3D [ `See of string | `Mref of string * nonlink_node_t=
 list ]
=09type super_node_t =3D [ nonlink_node_t | link_node_t ]

=09let text txt =3D `Text txt
=09let bold seq =3D `Bold seq
=09let see ref =3D `See ref
=09let mref ref seq =3D `Mref (ref, seq)
end

open Node
let foo =3D text "foo"
let bar =3D bold [text "bar"]


Error: This expression has type Node.nonlink_node_t
       but is here used with type Node.super_node_t
       The first variant type does not allow tag(s) `Mref, `See

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