type x = Bool of bool | Int of int;;

let rec to_formatter ff o =
  let pf = Format.fprintf ff in
  let _ = match o with
    | Bool b -> pf "%s" (string_of_bool b)
    | Int i -> pf "%d" i
  in ();;

(* 
Error: This expression has type (int -> 'a, 'b, 'c, 'd, 'd, 'a) format6
       but an expression was expected of type
         (string -> unit, Format.formatter, unit) format =
           (string -> unit, Format.formatter, unit, unit, unit, unit) format6
*)