mixins.swift

// Feature 1: @mixin extension
struct Pizza {
  var containsCheese: Bool
  init(cheese: Bool) { ... }
  func prepare() { print(self) }
}

struct Margherita {
  var pizza: Pizza
  init() { pizza = Pizza(cheese: true) }
}

@mixin(pizza) extension Margherita {
  func prepare()
}

// Usage
let margherita = Margherita()
margherita.prepare()             //=> Pizza(containsCheese: true)
mergherita.pizza.prepare()       //=> Pizza(containsCheese: true)
margherita.containsCheese        // error
margherita.pizza.containsCheese  //=> true
margherita as Pizza  // error


// Feature 2: @mixin structs
protocol Orderable {
  func makeOrder()
  func meetCourier()
}

// Mixin structs are not really types; they have requirements to be met in outer type
@mixin struct Pizza : Orderable {
  func prepare() { makeOrder(); meetCourier() }
}

struct Margherita {
  var pizza: Pizza
  func makeOrder() { ... }
  func meetCourier() { ... }
}

@mixin(pizza) extension Margherita {
  func prepare()
}


// Example: diamond pattern, split base
struct A { ... }

struct B { var a: A }
@mixin(a) extension B { ... }

struct C { var a: A }
@mixin(a) extension C { ... }

struct D { var b: B; var c: C }
@mixin(b) extension D { ... }
@mixin(c) extension D { ... }

// Example: diamond pattern, shared base
struct A { ... }
protocol HasA { var a: A { get set } }

struct B : HasA { ... }
struct C : HasA { ... }

struct D { var a: A; var b: B; var c: C }
@mixin(a) extension D { ... }
@mixin(b) extension D { ... }
@mixin(c) extension D { ... }