PromiseKit + YOLO?

// Trying to write a function that does some select/map magic with easy error handling

let cities = [[
	"name": "Geneva",
	"population": 184538
],[
	"name": "Bern",
	"population": 123154
], [
	"name": "Zurich",
	"population": 366765
], [
	"name": "Rubish",
	"population": -500
]]

struct City {
	let name: String
	let population: Int
	init?(name: String, population: Int) {
		if population > 0 {
			self.name = name
			self.population = population
		} else {
			return nil
		}
	}
}

// 1. No error handling
// -> Easy but crash prone

func citiesFromJSON(json: [[String:Any]]) -> [City] {
	return json.map({ (dictionary: [String:Any]) -> City in
		return City(name: dictionary["name"] as String, population: dictionary["population"] as Int)!
	})
}

// 2. for loop NSError solution
// -> Crash resistent but not very functional

func citiesFromJSON(json: [[String:Any]], error: NSErrorPointer) -> [City] {
	var cities: [City] = []
	
	for dictionary in json {
		if let name = dictionary["name"] as? String {
			if let population = dictionary["population"] as? Int {
				if let city = City(name: name, population: population) {
					cities.append(city)
					continue
				}
			}
		}
		
		error.memory = NSError(domain: "domain", code: 666, userInfo: nil)
		return []
	}
	
	return cities
}

// 2. Functional Result<T> solution
// -> Crash resistent but loops through everything before detecting errors and difficult to write

enum Result<T> {
	case Success(@autoclosure () -> T)
	case Failure(NSError)
}

func citiesFromJSON(json: [[String:Any]]) -> Result<[City]> {
	return json.map({ dictionary -> Result<City> in
		if let name = dictionary["name"] as? String {
			if let population = dictionary["population"] as? Int {
				if let city = City(name: name, population: population) {
					return .Success(city)
				}
			}
		}
		
		return .Failure(NSError(domain: "domain", code: 666, userInfo: nil))
	}).reduce(Result.Success([]), combine: { (result, cityResult) -> Result<[City]> in
		switch (result, cityResult) {
		case (.Failure, _):
			return result
		case (_, .Failure(let error)):
			return .Failure(error)
		case (.Success(let cities), .Success(let city)):
			return .Success(cities() + [city()])
		default:
			assertionFailure("should never happen")
		}
	})
}

// 3. PromiseKit maps??
// -> Perfect!

func citiesFromJSON(json: [[String:Any]]) -> Result<[City]> {
	return json.map({ dictionary in
		if let name = dictionary["name"] as? String {
			if let population = dictionary["population"] as? Int {
				if let city = City(name: name, population: population) {
					return city
				}
			}
		}
		
		return NSError(domain: "domain", code: 666, userInfo: nil)
	}).catch { error in
		return .Failure(error)
	}
}