module type Convert = sig
    type t
    val to_string : t -> string
    val of_string : string -> t
end

type (_,_) compose =
    | Cons : (module Convert with type t = 'a) * ('b, 'c) compose -> ('a -> 'b, 'c) compose
    | Nil  : ('c, 'c) compose

let rec serialize : type f. (f, unit) compose -> f -> string = function
    | Nil -> fun () -> ""
    | Cons ((module C), rest) -> fun x -> (C.to_string x) ^ (serialize rest)