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
	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




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