Set.playground

precedencegroup ForwardApplication {
  associativity: left
}
infix operator |>: ForwardApplication
public func |> <A, B>(x: A, f: (A) -> B) -> B {
  return f(x)
}

precedencegroup SingleTypeComposition {
  associativity: right
  higherThan: ForwardApplication
}
infix operator <>: SingleTypeComposition
public func <> <A: AnyObject>(f: @escaping (A) -> Void, g: @escaping (A) -> Void) -> (A) -> Void {
  return { a in
    f(a)
    g(a)
  }
}

func prop<Root, Value>(_ keyPath: WritableKeyPath<Root, Value>)
  -> (@escaping (Value) -> Value)
  -> (Root)
  -> Void {
    return { update in
      return { root in
        var copy = root
        copy[keyPath: keyPath] = update(copy[keyPath: keyPath])
      }
    }
}

func const<A, B>(_ x: A) -> (B) -> (A) {
  return { _ in x }
}

public func set<Root, Value>(_ keyPath: WritableKeyPath<Root, Value>, _ value: Value) -> (Root) -> Void {
  return const(value) |> prop(keyPath)
}

let label = UILabel()
let font = UIFont.systemFont(ofSize: 12)
label |> set(\UILabel.font, font) <> set(\UILabel.textColor, .blue)