struct User {
	let id: Int
	let name: String
	let email: String?
}

extension User: JSONDecodable {
	static func create(id: Int, name: String, email: String?) -> User {
		return User(id: id, name: name, email: email)
	}

	static func decode(json: JSONValue) {
		return check(User.create, json["id"], json["name"], json["email"])
		// check() calls its fn only if the required arguments are non-nil
		// You could readily define check() as an infix operator that takes a tuple, e.g.:
		// return User.create?<(json["id"], json["name"], json["email"])
	}
}

protocol JSONDecodeable {
	class func decode(json: JSONValue) -> Self?
}

enum JSONValue {
	case JSONObject([String: JSONValue])
	case JSONArray([JSONValue])
	// etc

	subscript(key: String) -> Int {}
	subscript(key: String) -> Bool {}
	// etc
}

func check<A, B, C, R>(fn: (A,B,C) -> R, a: A?, b: B?, c: C?) -> R? {
	if a == nil || b == nil || c == nil {
		return nil
	} else {
		return fn(a!, b!, c!)
	}
}

func check<A, B, C, R>(fn: (A?,B,C) -> R, a: A?, b: B?, c: C?) -> R? {
	if b == nil || c == nil {
		return nil
	} else {
		return fn(a, b!, c!)
	}
}

func check<A, B, C, R>(fn: (A,B?,C) -> R, a: A?, b: B?, c: C?) -> R? {
	if a == nil || c == nil {
		return nil
	} else {
		return fn(a!, b, c!)
	}
}

func check<A, B, C, R>(fn: (A,B,C?) -> R, a: A?, b: B?, c: C?) -> R? {
	if a == nil || b == nil {
		return nil
	} else {
		return fn(a!, b!, c)
	}
}

func check<A, B, C, R>(fn: (A?,B?,C) -> R, a: A?, b: B?, c: C?) -> R? {
	if c == nil {
		return nil
	} else {
		return fn(a, b, c!)
	}
}

func check<A, B, C, R>(fn: (A?,B,C?) -> R, a: A?, b: B?, c: C?) -> R? {
	if b == nil {
		return nil
	} else {
		return fn(a, b!, c)
	}
}

func check<A, B, C, R>(fn: (A,B?,C?) -> R, a: A?, b: B?, c: C?) -> R? {
	if a == nil {
		return nil
	} else {
		return fn(a!, b, c)
	}
}

func check<A, B, C, R>(fn: (A?,B?,C?) -> R, a: A?, b: B?, c: C?) -> R? {
	return fn(a, b, c)
}

// etc.