Swift: combining varargs and autoclosure

gistfile1.swift

// This compiles:
func immediateOr(args:Bool...) -> Bool {
	for arg in args {
		if arg {
			return true
		}
	}
	return false
}

// A. This doesn't compile:
func lazyOr(args: @autoclosure() -> Bool...) -> Bool {
	for arg in args { // "Error: Function produces expected type '[@autoclosure() -> Bool]'"
		if arg() {
			return true
		}
	}
	return false
}

// B. This doesn't compile (typealias):
typealias LazyPredicate = @autoclosure() -> Bool
func lazyOr(args: LazyPredicate...) -> Bool {
	for arg in args { // "Error: Function produces expected type '[@autoclosure() -> Bool]'"
		if arg() {
			return true
		}
	}
	return false
}

// C. This doesn't compile (parenthesize):
func lazyOr(args: (@autoclosure() -> Bool)...) -> Bool {
	for arg in args { // "Error: Function produces expected type '[@autoclosure() -> Bool]'"
		if arg() {
			return true
		}
	}
	return false
}

// D. This "works"...
func lazyOr(args: (@autoclosure() -> Bool, Int)...) -> Bool {
	for arg in args {
		if arg.0() {
			return true
		}
	}
	return false
}
lazyOr((expensiveA(), 42), (expensiveB(), 42))

Have you considered using an array parameter instead of a variadic parameter?

According to Apple's Documentation variadic parameters are made available to a function as a constant array anyway.

You might have some luck pinpointing exactly why you're getting compiler errors by taking a look at Swift's Grammar Specification.

Hope this helps!

func expensive() -> Bool {
    sleep(1);
    println("expensive");
    return true;
}
func lazyOr(args: [@autoclosure () -> Bool] ) -> Bool {
    println("lazyOr");
        // for in syntax does not work here
    for (var i: Int=0; i<args.count; i++) {
        if (args[i]()) {
            return true;
        }
    }

    return false
}

lazyOr([ expensive(), expensive() , expensive() ]);

Prints:

lazyOr
expensive

I was excited when I saw the comment by @vivekseth, but I've now tried it and it looks like it doesn't work with Swift 3.