Existentials in Swift

Existentials.swift

protocol Shape {
  associatedtype NumericType

  func area() -> NumericType
}

struct Rectangle: Shape {
  typealias NumericType = Double

  var width: Double
  var length: Double

  func area() -> Double {
    width * length
  }
}

struct IntSquare: Shape {
  typealias NumericType = Int

  var side: Int

  func area() -> Int {
    side * side
  }
}

var shape: any Shape = Rectangle(width: 3, length: 4)
print(shape.area())
shape = IntSquare(side: 5)
print(shape.area())