#money #currency #exchange

moneta

Currency-safe computations with money amounts

4 releases (2 breaking)

Uses new Rust 2021

0.3.1 Aug 3, 2022
0.3.0 May 29, 2022
0.2.0 May 26, 2022
0.1.1 May 23, 2022
0.1.0 May 23, 2022

#289 in Math

50 downloads per month

Custom license

76KB
2K SLoC

Currency-safe computations with money amounts.

Money is a special type of quantity. Its unit type is known as Currency.

Money differs from physical quantities mainly in two aspects:

  • Money amounts are discrete. For each currency there is a smallest fraction that can not be split further.

  • The relation between different currencies is not fixed, instead, it varies over time.

This package provides types to deal with these specifics. It is based an the package "quantities" with feature "fpdec" (aliasing "AmountT" to "Decimal").

Currency is an enumeration of all currencies defined in ISO 4217. In addition, for each currency there is a constant named after the 3-character ISO 4217 code.

Currency implements quantities::Unit, so all operations on units can be applied to Currency. Especially, a Currency instance can be multiplied with an AmountT to create a Money instance.

Example:

# use moneta::{Amnt, Currency, Dec, Decimal, EUR, Money, Quantity, Unit};
let amnt = Amnt!(17.95);
let eur_amnt = amnt * EUR;
assert_eq!(eur_amnt.amount(), amnt);
assert_eq!(eur_amnt.unit(), Currency::EUR);

Money implements trait quantities::Quantity, so all operations on quantities can also be applied to instances of Money. Because there is no fixed relation between currencies, there is no implicit conversion between money amounts of different currencies. Resulting values are always quantized to the smallest fraction defined with the currency.

Example:

# use moneta::{Amnt, Currency, Dec, Decimal, EUR, Money, Quantity, Unit};
let qty = Amnt!(3.2);
let price = Money::new(Amnt!(13.58), EUR);
let total = qty * price;
assert_eq!(total.to_string(), "43.46 EUR");

A conversion factor between two currencies can be defined by using the type ExchangeRate. It is given a unit currency (aka base currency), a unit multiple, a term currency (aka price currency) and a term amount, i.e. the amount in term currency equivalent to unit multiple in unit currency.

Multiplying an amount in some currency with an exchange rate with the same currency as unit currency results in the equivalent amount in term currency. Likewise, dividing an amount in some currency with an exchange rate with the same currency as term currency results in the equivalent amount in unit currency.

Examples:

# use moneta::{Dec, Decimal, ExchangeRate, EUR, HKD, USD};
let usd = Dec!(17.95) * USD;
let rate = ExchangeRate::new(USD, 1, EUR, Dec!(0.98078));
let eur = usd * rate;
assert_eq!(eur.to_string(), "17.61 EUR");
let rate = ExchangeRate::new(HKD, 10, EUR, Dec!(1.187253));
let hkd = eur / rate;
assert_eq!(hkd.to_string(), "148.33 HKD");

Dependencies

~0.7–1MB
~24K SLoC