let f (fn : (?foo:bool -> unit -> unit)) : (?foo:bool -> unit -> unit) = fun ?foo () -> fn ?foo () ;;
val f : (?foo:bool -> unit -> unit) -> ?foo:bool -> unit -> unit = <fun>

let g (fn : (unit -> unit)) : (?foo:bool -> unit -> unit) = fun ?foo:_ () -> fn () ;;
val g : (unit -> unit) -> ?foo:bool -> unit -> unit = <fun>

let h = fun ?(foo=true) () -> if foo then () else () ;;
val h : ?foo:bool -> unit -> unit = <fun>

f h ;;
- : ?foo:bool -> unit -> unit = <fun>

g h ;;
- : ?foo:bool -> unit -> unit = <fun>