Monet

Handle currency conversion and common operations (addition, subtraction, multiplication, division).

How it works

It defines some base types:

• CurrencyAmount is i128 and, as the name says, is used to store amounts.
• CurrencyCode is a wrapper for [u8; 3] that can be created from / converted into &strs.
• Money is the most important type. Money holds both a currency_code and an amount. It's used to store money and to perform operations. It can be converted into another currency code by providing Rates.
• Rates is a wrapper for a HashMap. It can be constructed from such pre-defined map or populated from an external source such as websites (Not yet, TODO: Implement).
• Exponent exists because there are no floats involved here. It has two fields: amount and exponent. Its decimal value is amount / (10).pow(exponent).

Dangers

Even this library isn't safe from precision losses. For example, an Exponent's amount could be cut out by its exponent. Also errors when converting money could occurr.

Examples

Summing two Moneys of the same type

use monet::{Money, CurrencyAmount, Rates, Operation};
use std::convert::TryInto;

// Custom rates.
let map = vec![("USD", 1_000_000)].into_iter()
    .map(|(code, worth)| (code.try_into().unwrap(), worth.into())
    .collect();
let rates = Rates::with_rates(map);

let money_owned = Money::with_str_code(CurrencyAmount::with_unit(2), "USD").unwrap();
let money_paid = Money::with_str_code(CurrencyAmount::with_unit(1), "USD").unwrap();

let remaining = (money_owned - money_paid).execute(&rates);

assert_eq!(remaining, Money::with_str_code(CurrencyAmount::with_unit(1), "USD"));
assert_eq!(remaining, Money::with_str_code(1_000_000.into(), "USD"));

Summing two Moneys of different type

use monet::{Money, CurrencyAmount, Rates, Operation};
use std::convert::TryInto;

// Custom rates.
let map = vec![
    ("USD", 1_000_000),
    ("CHF", 1_100_000),
].into_iter()
    .map(|(code, worth)| (code.try_into().unwrap(), worth.into())
    .collect();

let rates = Rates::with_rates(map);

let money_one = Money::with_str_code(1_000_000.into(), "CHF").unwrap();
let money_two = Money::with_str_code(1_100_000.into(), "USD").unwrap();

// Note: sum has currency code "CHF": when summing,
// the currency code of the first money is used
let sum = (money_one + money_two).execute(&rates);

assert_eq!(remaining, Money::with_str_code(2_000_000.into(), "CHF"));

Chaining operations

use monet::{Money, CurrencyAmount, Rates, Operation};
use std::convert::TryInto;

// Custom rates.
let map = vec![
    ("USD", 1_000_000),
    ("CHF", 1_100_000),
].into_iter()
    .map(|(code, worth)| (code.try_into().unwrap(), worth.into())
    .collect();

let rates = Rates::with_rates(map);

let money_one = Money::with_str_code(1_000_000.into(), "CHF").unwrap();
let money_two = Money::with_str_code(1_100_000.into(), "USD").unwrap();
let money_three = Money::with_str_code(2_000_000.into(), "CHF").unwrap();

// Note: sum has currency code "CHF"
let sum = (money_one + money_two - money_three).execute(&rates);

assert_eq!(remaining, Money::with_str_code(0.into(), "CHF"));