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Cc: Jeremy Yallop <jeremy.yallop@ed.ac.uk>, caml-list@yquem.inria.fr
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From: Wolfgang Lux <wlux@uni-muenster.de>
Subject: Re: [Caml-list] Troublesome nodes
Date: Sat, 12 Jul 2008 18:44:49 +0200
To: Dario Teixeira <darioteixeira@yahoo.com>
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X-Spam: no; 0.00; wlux:01 uni-muenster:01 nodes:01 node:01 sig:01 node:01 val:01 val:01 struct:01 seq:01 seq:01 foo:01 foo:01 module's:01 wrote:01 

Dario Teixeira wrote:

> module rec Node:
> sig
> 	type nonlink_node_t = [ `Text of string | `Bold of  
> Node.super_node_t list ]
> 	type link_node_t = [ `See of string | `Mref of string *  
> nonlink_node_t list ]
> 	type super_node_t = [ 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 =
> struct
> 	type nonlink_node_t = [ `Text of string | `Bold of  
> Node.super_node_t list ]
> 	type link_node_t = [ `See of string | `Mref of string *  
> nonlink_node_t list ]
> 	type super_node_t = [ nonlink_node_t | link_node_t ]
>
> 	let text txt = `Text txt
> 	let bold seq = `Bold seq
> 	let see ref = `See ref
> 	let mref ref seq = `Mref (ref, seq)
> end
>
> open Node
> let foo = text "foo"
> let bar = 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

This problem is apparently easy to solve. Just don't expect an exact  
super_node_t list argument for the bold function. Instead use
   val bold : [< super_node_t] list -> nonlink_node_t
in the signature and
   let bold seq = `Bold (seq :> super_node_t list)
in the body of the Node module's definition.

Wolfgang