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 = [ `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 :> super_node_t list)
    let see ref = `See ref
    let mref ref seq = `Mref (ref, seq)
end

open Node

let foo1 = text "foo"                   (* valid *)
let foo2 = bold [text "foo"]            (* valid *)
let foo3 = mref "ref" [text "foo"]      (* valid *)
let foo4 = 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 =
struct
    open Node

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

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

    and convert_super_node node = 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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