(* compile with ppx_implicits 0.2.0 *)
(* ocamlfind ocamlc -c -package ppx_implicits implicit_read.ml *)
module Read = struct
  type 'v t = (string -> 'v option, [%imp Instances]) Ppx_implicits.t

  let read ?(d:'v t option) x = Ppx_implicits.imp ?d x
end

module Instances = struct
  let _int : string -> int option = fun s ->
    try
      Some (int_of_string s)
    with
      _ -> None
  let _a : string -> [`A] option = function
    | "[`A]" -> Some(`A)
    | _ -> None
end

let _ =
  ignore @@ (Read.read "1" : int option);    (* ok *)
  ignore @@ (Read.read "[`A]" : [`A] option) (* error *)

(* File "_none_", line 1: *)
(* Error: File "implicit_read.ml", line 22, characters 13-22: *)
(* The experssion has type string -> [ `A ] option which is too ambiguous to resolve this implicit. *)
(* The following instances may cause infinite loop of the resolution: *)
(*   Instances._a *)

(* File "implicit_read.ml", line 1: *)
(* Error: Error while running external preprocessor *)
(* Command line: /Users/keigoi/.opam/4.04.2/lib/ppx_implicits/./ppx_implicits '/var/folders/kl/bbg2vr1173j8xrmpc3xp9b3h0000gn/T/camlppx6ed0ad' '/var/folders/kl/bbg2vr1173j8xrmpc3xp9b3h0000gn/T/camlppx1a7f20' *)