Open Radar

 
When implementing SPTask (a Promise class for Swift) and its wrap functionality (convert callback-based function into promise-based function), i ran into the problem of having to copy-paste its implementation for a fixed number of parameters.

Here’s the function, hard-coded for two parameters:

func taskize<R1, P1, P2> (
	asyncFunc: (
		p1: P1,
		p2: P2,
		callback: (
			r1: R1,
			err: NSError?
		) -> Void
	) -> Void
) -> (P1, P2) -> Task<R1>
{
	let source = TaskCompletionSource<R1>()
	
	return { (p1: P1, p2: P2) -> Task<R1> in
		asyncFunc(p1: p1, p2: p2, { (r1: R1, error: NSError?) -> Void in
			source.completeWithValue(r1)
		})
		return source.task
	}
}

If it was generic over a variadic number of parameters, it could be written something like:

func taskize<R1, P…> (
	asyncFunc: (
		P p…,
		callback: (
			r1: R1,
			err: NSError?
		) -> Void
	) -> Void
) -> (P…) -> Task<R1>
{
	let source = TaskCompletionSource<R1>()
	
	return { (p…) -> Task<R1> in
		asyncFunc(p…, { (r1: R1, error: NSError?) -> Void in
			source.completeWithValue(r1)
		})
		return source.task
	}
}

I’ve just copied the C++ style of doing variadics, but I’m not a language designer, I’m sure there are much better ways of doing it.