universal implemented with existential.ml

#!/usr/bin/env ocaml

module type HdArg = sig
    type t
    val x: t list
    val continuation : t -> unit
end

let hd (module M : HdArg): unit =
    let fn : M.t list -> M.t = function
            | h :: _ -> h
            | _ -> failwith "got 0-length list"
    in
    M.continuation (fn M.x)

let () =
    let module M : HdArg = struct
        type t = int
        let x = [1; 2; 3]
        let continuation = Printf.printf "result: %d\n"
    end
        in
    hd (module M)