A precise, type-safe representation of a monetary amount in a given currency.
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README.md

Money

A precise, type-safe representation of monetary amounts in a given currency.

This functionality is discussed in Chapter 3 of Flight School Guide to Swift Numbers.

Requirements

  • Swift 4.0+

Installation

Swift Package Manager

Add the Money package to your target dependencies in Package.swift:

import PackageDescription

let package = Package(
  name: "YourProject",
  dependencies: [
    .package(
        url: "https://github.com/Flight-School/Money",
        from: "1.0.0"
    ),
  ]
)

Then run the swift build command to build your project.

Carthage

To use Money in your Xcode project using Carthage, specify it in Cartfile:

github "Flight-School/Money" ~> 1.0

Then run the carthage update command to build the framework, and drag the built Money.framework into your Xcode project.

Usage

Creating Monetary Amounts

The Money type has a required associated Currency type. These currency types are named according to their three letter ISO 4217 currency code. You can initialize a monetary using a Decimal value:

let amount = Decimal(12)
let monetaryAmount = Money<USD>(amount)

You can also create monetary amounts using integer, floating-point, and string literals.

12 as Money<USD>
12.00 as Money<USD>
"12.00" as Money<USD>

This package provides all of the currencies defined by the ISO 4217 standard as of June 2018, with the exception of special codes, such as USN (US Dollar, Next day) and XBC (Bond Markets Unit European Unit of Account 9).

Important: Swift floating-point literals are currently initialized using binary floating-point number type, which cannot precisely express certain values. As a workaround, monetary amounts initialized from a floating-point literal are rounded to the number of places of the minor currency unit. If you want to express a smaller fractional monetary amount, initialize from a string literal or Decimal value instead.

let preciseAmount: Money<USD> = "123.4567"
let roundedAmount: Money<USD> = 123.4567

preciseAmount.amount // 123.4567
roundedAmount.amount // 123.46

For more information, see https://bugs.swift.org/browse/SR-920.

Comparing Monetary Amounts

You can compare two monetary amounts with the same currency:

let amountInWallet: Money<USD> = 60.00
let price: Money<USD> = 19.99

amountInWallet >= price // true

Attempting to compare monetary amounts with different currencies results in a compiler error:

let dollarAmount: Money<USD> = 123.45
let euroAmount: Money<EUR> = 4567.89

dollarAmount == euroAmount // Error: Binary operator '==' cannot be applied

Adding, Subtracting, and Multiplying Monetary Amounts

Monetary amounts can be added, subtracted, and multiplied using the standard binary arithmetic operators (+, -, *):

let prices: [Money<USD>] = [2.19, 5.39, 20.99, 2.99, 1.99, 1.99, 0.99]
let subtotal = prices.reduce(0.00, +) // "$36.53"
let tax = 0.08 * subtotal // "$2.92"
let total = subtotal + tax // "$39.45"

Important: Multiplying a monetary amount by a floating-point number results in an amount rounded to the number of places of the minor currency unit. If you want to produce a smaller fractional monetary amount, multiply by a Decimal value instead.

Supporting Multiple Currencies

Consider a Product structure with a price property. If you only support a single currency, such as US Dollars, you would define price to be of type Money<USD>:

struct Product {
    var price: Money<USD>
}

If you want to support multiple currencies, however, you can't specify an explicit currency type in the property declaration. Instead, the Product would have to be defined as a generic type:

struct Product<Currency: CurrencyType> {
    var price: Money<Currency>
}

Unfortunately, this approach is unwieldy, as each type that interacts with Product would also need to be generic, and so on, until the entire code base is generic over the currency type.

class ViewController<Currency: CurrencyType> : UIViewController { ... } // 😭

A better solution would be to define a new Price protocol with requirements that match the Money type:

protocol Price {
    var amount: Decimal { get }
    var currency: CurrencyType.Type { get }
}

extension Money: Price {}

Doing this allows prices to be defined in multiple currencies without making Product generic over the currency type:

struct Product {
    var price: Price
}

let product = Product(price: 12.00 as Money<USD>)
product.price // "$12.00"

If you want to support only certain currencies, such as US Dollars and Euros, you can define a SupportedCurrency protocol and add conformance to each currency type through an extension:

protocol SupportedCurrency: CurrencyType {}
extension USD: SupportedCurrency {}
extension EUR: SupportedCurrency {}

extension Money: Price where Currency: SupportedCurrency {}

Now, attempting to create a Product with a price in an unsupported currency results in a compiler error:

Product(price: 100.00 as Money<EUR>)
Product(price: 100.00 as Money<GBP>) // Error

Formatting Monetary Amounts

You can create a localized representation of a monetary amount using NumberFormatter. Set the currencyCode property of the formatter to the currency.code property of the Money value and pass the amount property to the formatter string(for:) method.

let allowance: Money<USD> = 10.00
let formatter = NumberFormatter()
formatter.numberStyle = .currency
formatter.locale = Locale(identifier: "fr-FR")
formatter.currencyCode = allowance.currency.code
formatter.string(for: allowance.amount) // "10,00 $US"

Adding Custom Currencies

You can create your own custom currency types by defining an enumeration that conforms to the CurrencyType protocol. For example, here's how you might represent Bitcoin (BTC):

enum BTC: CurrencyType {
    static var name: String { return "Bitcoin" }
    static var code: String { return "BTC" }
    static var minorUnit: Int { return 8 }
}

let satoshi: Money<BTC> = 0.00000001

NumberFormatter only supports currencies defined by ISO 4217, so you'll have to configure the symbol, currency code, and any other necessary parameters:

let formatter = NumberFormatter()
formatter.numberStyle = .currency
formatter.currencySymbol = ""
formatter.currencyCode = "BTC"
formatter.maximumFractionDigits = 8

formatter.string(for: satoshi.amount) // ₿0.00000001

Showing Off with Emoji

If you're the type of person who enjoys putting clip art in your source code, here's a trick that'll really impress your teammates:

typealias 💵 = Money<USD>
typealias 💴 = Money<JPY>
typealias 💶 = Money<EUR>
typealias 💷 = Money<GBP>

let tubeFare: 💷 = 2.40 // "£2.40"

Alternatives to Consider

A type-safe Money structure like the one provided by this package can reduce the likelihood of certain kinds of programming errors. However, you may find the cost of using this abstraction to outweigh the benefits it can provide in your code base.

If that's the case, you might consider implementing your own simple Money type with a nested Currency enumeration like this:

struct Money {
   enum Currency: String {
      case USD, EUR, GBP, CNY // supported currencies here
   }

   var amount: Decimal
   var currency: Currency
}

It's ultimately up to you to decide what kind of abstraction is best for your particular use case. Whatever you choose, just make sure to represent monetary amounts using a Decimal type with an explicit currency.

License

MIT

Contact

Mattt (@mattt)