]> Hi again, I ditched the regular variants in favour of polymorphic variants, hoping the coercion trick described by Jacques Garrigue would help [1]. I've also simplified the formulation as much as possible. Anyway, I'm still stuck with the same problem: while function 'sprint' works alright inside the Node module, its clone 'sprint2' fails to compile when placed outside. Is this a case where no amount of explicit annotations and coercions will work or am I missing something obvious? Thanks in advance! Regards, Dario Teixeira [1] http://groups.google.com/group/fa.caml/msg/855011402f1ca1b5 module Node: sig type elem_t = [ `Text of string | `Bold of elem_t list ] type +'a t = private elem_t val text: string -> [> `Basic ] t val bold: 'a t list -> [> `Complex ] t val sprint: 'a t -> string end = struct type elem_t = [ `Text of string | `Bold of elem_t list ] type +'a t = elem_t let text str = `Text str let bold seq = `Bold seq let rec sprint = function | `Text str -> Printf.sprintf "(Text %s)" str | `Bold seq -> Printf.sprintf "(Bold %s)" (List.fold_left (^) "" (List.map sprint seq)) end module Foobar: sig val sprint2: 'a Node.t -> string end = struct let rec sprint2 node = match (node : _ Node.t :> [> ]) with | `Text str -> Printf.sprintf "(Text %s)" str | `Bold seq -> Printf.sprintf "(Bold %s)" (List.fold_left (^) "" (List.map sprint2 (seq : _ Node.t list :> [>] list))) end