[Concurrency] Structured concurrency

[Dante-Broggi]

@John_McCall

This proposal introduces an easy way to create child tasks with async let :

func makeDinner() async throws -> Meal {
  async let veggies = try chopVegetables()
  async let meat = marinateMeat()
  async let oven = try preheatOven(temperature: 350)

  let dish = Dish(ingredients: await [veggies, meat])
  return await try oven.cook(dish, duration: .hours(3))
}

async let is similar to a let , in that it defines a local constant that is initialized by the expression on the right-hand side of the = . However, it differs in that the initializer expression is evaluated in a separate, concurrently-executing child task. On completion, the child task will initialize the variables in the async let and complete.

While this makes sense, it seems to me like another option would be to have async replace await directly in an expression, instead of modifying let.

That is why was a syntax like the following not considered:

func makeDinner() async throws -> Meal {
 let veggies = try async chopVegetables()
 let meat = async marinateMeat()
 let oven = try async preheatOven(temperature: 350)

  let dish = Dish(ingredients: await [veggies, meat])
  return await try oven.cook(dish, duration: .hours(3))
}