type (_, _) term =
  | Int: (int -> 'a, 'a) term
  | String: (string -> 'a, 'a) term
  | Const: string -> ('a, 'a) term

type (_, _) term_list =
  | []   : ('a, 'a) term_list
  | (::) : ('a, 'b) term * ('b, 'c) term_list -> ('a, 'c) term_list

(* Construct a function that takes same arguments and produces a string *)
let rec encoder: type b. (b, string) term_list -> string -> b = function
  | Int :: xs ->
    fun acc v -> encoder xs (Printf.sprintf "%s %d" acc v)
  | String :: xs ->
    fun acc v -> encoder xs (Printf.sprintf "%s '%s'" acc v)
  | Const s :: xs  ->
    fun acc -> encoder xs (Printf.sprintf "%s %s" acc s)
  | [] -> fun x -> x

let expr = Const "SELECT a FROM x WHERE a =" :: Int :: Const "AND x.y =" :: String :: Const "ORDER BY a" :: []
let expr_func = encoder expr ""

let () = Printf.printf "Result: %s\n" (expr_func 5 "value")