Phantom types

Phantom types

import UIKit

protocol Currency {
    static var code: String { get }
    static var factor: NSDecimalNumber { get }
}
enum USD: Currency {
    static let code = "USD"
    static let factor: NSDecimalNumber = 1.0
}
enum RUB: Currency {
    static let code = "RUB"
    static let factor: NSDecimalNumber = 65.74
}
enum UAH: Currency {
    static let code = "UAH"
    static let factor: NSDecimalNumber = 26.95
}

struct Money<Cur: Currency>: CustomStringConvertible, ExpressibleByFloatLiteral, ExpressibleByIntegerLiteral {
    let amount: NSDecimalNumber
    
    var description: String {
        let f = NumberFormatter()
        f.numberStyle = .currency
        f.currencyCode = Cur.code
        return f.string(from: self.amount)!
    }
    
    init(floatLiteral value: Double) {
        self.amount = NSDecimalNumber(value: value)
    }
    
    init(integerLiteral value: Int) {
        self.amount = NSDecimalNumber(value: value)
    }
    
    init(_ amount: NSDecimalNumber) {
        self.amount = amount
    }
    
    func convertTo<C: Currency>() -> Money<C> {
        let baseAmount = amount.multiplying(by: Cur.factor)
        let convertedAmount = baseAmount.multiplying(by: C.factor)
        return Money<C>(convertedAmount)
    }
}

extension Money {
    var usd: Money<USD> { return convertTo() }
    var rub: Money<RUB> { return convertTo() }
    var uah: Money<UAH> { return convertTo() }
}

// Usage
let cocaCola: Money<USD> = 2.44

print(cocaCola.usd)
print(cocaCola.rub)
print(cocaCola.uah)