test2.ml

type literal
type param

type _ t =
  | QueryString : string -> literal t
  | IntParam : string -> param t
  | StringParam : string -> param t

type query =
  | Nil : query
  | Cons : 'a t * query -> query

let to_string : type a . a t -> string = function
  | QueryString s -> s
  | IntParam s -> ":" ^ s
  | StringParam s -> ":" ^ s

let rec query_to_string = function
  | Nil -> ""
  | Cons (t, rest) -> to_string t ^ query_to_string rest