a.swift

import PerfectCURL

let urls = ["http://localhost:8080/a", "http://localhost:8080/b", "http://localhost:8080/c", "http://localhost:8080/d"]

func fetchURLS(_ urls: [String], process: (_ url: String, _ headers: String?, _ body: String?) -> ()) {
	
	struct curlTrack {
		let url: String
		let curl: CURL
		var done = false
		var headers = [UInt8]()
		var body = [UInt8]()
		init(url: String) {
			self.url = url
			self.curl = CURL(url: url)
		}
	}
	
	var tracker = urls.map { curlTrack(url: $0) }
	
	while !tracker.isEmpty {
		tracker = tracker.map {
			var ret = $0
			let (go, _, h, b) = $0.curl.perform()
			ret.headers.append(contentsOf: h ?? [])
			ret.body.append(contentsOf: b ?? [])
			ret.done = !go
			return ret
		}.filter {
			guard $0.done else {
				return true
			}
			let head = ($0.headers + [0 as UInt8]).map { Int8($0) }
			let body = ($0.body + [0 as UInt8]).map { Int8($0) }
			
			process($0.url, String(validatingUTF8: head), String(validatingUTF8: body))
			return false
		}
	}
}

fetchURLS(urls) {
	url, headers, body in
	print("Completed URL \(url) \(body ?? "")")
}

print("done")