1.swift

protocol Shape {
    func accept<V: ShapeVisitor>(_ visitor: V) -> V.Result
}

struct Rectangle: Shape {
    let width: Float
    let height: Float

    func accept<V: ShapeVisitor>(_ visitor: V) -> V.Result {
        return visitor.visit(self)
    }
}

struct Circle: Shape {
    let radius: Float

    func accept<V: ShapeVisitor>(_ visitor: V) -> V.Result {
        return visitor.visit(self)
    }
}