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
}

func citiesFromJSON(json: [[String:Any]]) -> Promise<[City]> {
	return Promise{ fulfill, reject in
		fulfill(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
					}
				}
			}
			reject(NSError(domain: "domain", code: 666, userInfo: nil))
		})
	}
}