module type SHOW =
sig
  type t
  val show : t -> string
end

let print (implicit Show : SHOW) x = Show.show x

implicit module ShowInt = struct type t = int let show = string_of_int end
implicit module ShowFloat = struct type t = float let show = string_of_float end

implicit functor ShowPair (Show1 : SHOW) (Show2 : SHOW) =
struct
  type t = Show1.t * Show2.t
  let show (x, y) = "(" ^ Show1.show x ^ ", " ^ Show2.show y ^ ")"
end

(* none of these work *)

(* this works *)
let rec grow_pairs : 'a. (implicit Show : SHOW with type t = 'a) -> Show.t -> 'b =
  fun (type a) (implicit Show : SHOW with type t = a) x ->
  let module Show2 = ShowPair (Show) (Show) in
  grow_pairs (implicit Show2) (x, x)

(* but this doesn't, and the error message is strange *)
let rec grow_pairs2 : 'a. (implicit Show : SHOW with type t = 'a) -> 'a -> 'b =
  fun (type a) (implicit Show : SHOW with type t = a) x ->
  let module Show2 = ShowPair (Show) (Show) in
  grow_pairs2 (implicit Show2) (x, x)
(*
Error: This expression has type
         (implicit Show : SHOW with type t = 'a) -> Show.t -> 'b
       but an expression was expected of type
         (implicit Show : SHOW with type t = 'a) -> 'a -> 'b
       The type variable 'a occurs inside 'a
 *)