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Async/Await for Swift

Proposal: SE-XXXX
Authors: Chris Lattner, Joe Groff

Introduction

Modern Cocoa development involves a lot of asynchronous programming using closures and completion handlers, but these APIs are hard to use. This gets particularly problematic when many asynchronous operations are used, error handling is required, or control flow between asynchronous calls gets complicated. This proposal describes a language extension to make this a lot more natural and less error prone.

This paper introduces a first class Coroutine model to Swift. Functions can opt into to being async, allowing the programmer to compose complex logic involving asynchronous operations, leaving the compiler in charge of producing the necessary closures and state machines to implement that logic.

	/// A repository of functionality depending on the conformances of `Model`.
	///
	/// Conditional conformances provide implementation functions that take a
	/// generic argument type with the safe assumption that the argument's concrete
	/// type is `Model`.
	struct Dispatch<Model> {
	/// Returns `f(a as! Model) as! R1`
	///
	/// Used by implementation functions to avoid the clutter of casting
	/// explicitly.

	/// - returns: `true` when dynamic type is `Equatable` and `==` returns `true`, otherwise `false`.
	func areEquatablyEqual(_ lhs: Any, _ rhs: Any) -> Bool {
	func receiveLHS<LHS>(_ typedLHS: LHS) -> Bool {
	guard
	let rhsAsLHS = rhs as? LHS
	else { return false }
	return areEquatablyEqual(typedLHS, rhsAsLHS)
	}
	return _openExistential(lhs, do: receiveLHS)
	}