type IDuck =
  abstract member Quack: unit -> unit
  abstract member Quack: string -> unit

let inline (|IsDuck|) x =
  let quackDefault = fun () -> (^a: (member Quack: unit -> unit) x)
  let quack = fun s -> (^a: (member Quack: string -> unit) x, s)

  { new IDuck with
      member this.Quack() = quackDefault()
      member this.Quack(s: string) = quack s }

type NominalDuck() =
  interface IDuck with
    member this.Quack() = printfn "Quack!"
    member this.Quack(s: string) = printfn "Quack, %s!" s

type StructuralDuck() =
  member this.Quack() = printfn "Кря!"
  member this.Quack(s: string) = printfn "Кря, %s!" s

let inline quack (IsDuck x) = x.Quack()

let nominalDuck = NominalDuck()

let structDuck = StructuralDuck()

quack (nominalDuck :> IDuck) |> ignore
quack structDuck |> ignore