[swift-evolution] Guaranteed closure execution

[Gwendal Roué]

Hello,

I’d like to discuss the opportunity to let functions declare that a closure argument is guaranteed to have been executed when the function has returned.

For example:

	func f(@noescape(executed) closure: () -> ()) {
	    closure()
	}

The expected advantage is that the compiler would know that a variable set inside the closure is guaranteed to be initialized, and that it can be used after the execution of the function, as below:

	let x: Int  // Not initialized
	f { x = 1 }
	print(x)    // Guaranteed to be initialized

Today developers have to write pessimistic code like below:

	var x: Int = 0 // `var` declaration, with some irrelevant value
	f { x = 1 }
	print(x)

As for a real world usage, I’d like to access a database in a safe (queued) way, and fetch values out of it:

	let items: [Item]
	let users: [User]
	dbQueue.inDatabase { db in
	    items = Item.all().fetchAll(db)
	    users = Item.all().fetchAll(db)
	}

Gwendal Roué

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