let foo opt x fmt =
  match opt with
  | None -> assert false
  | Some f -> ksprintf (f x) fmt

val foo : ('a -> string -> 'b) option
       -> 'a
       -> ('c, unit, string, 'b) format4
       -> 'c = <fun>

# foo (Some (fun x s -> s)) "foo" "%s" "blah";;
- : string = "blah"

let bar opt x fmt =
  match opt with
  | None -> ()
  | Some f -> ksprintf (f x) fmt

val bar : ('a -> string -> 'b) option
       -> 'a
       -> (unit, unit, string, 'b) format4
       -> unit = <fun>

# bar (Some (fun x s -> s)) "bar" "%s" "blah";;
Error: This function is applied to too many arguments;