(* It seems that the ability of the Format and Printf modules to compose
   printing functions with "%a" format strings do not mix well with functions
   defined with optional arguments.

   In the following code, I think that "test2" should be accepted, but is not
   because of some bug int the format-string typing code. OCaml 3.12.0 fails
   with the message:

   File "...", line 23, characters 37-45:
   Error: This expression has type ?a:bool -> out_channel -> string -> unit
   but an expression was expected of type out_channel -> 'a -> unit
*)

module F = Printf (* (* same for *) Format *)

let with_opt ?(a = true) fmt x = F.fprintf fmt "%b %s" a x

(* Works *)
let ok f fmt x = F.fprintf fmt "%a" f x
let g = ok with_opt

(* Fails *)
let wrong fmt x = F.fprintf fmt "%a" with_opt x