Introduction to Money

Money implements a set of functions to store, retrieve and perform arithmetic on a %Money{} type that is composed of a currency code and a currency amount.

Money is opinionated in the interests of serving as a dependable library that can underpin accounting and financial applications. In its initial release it can be expected that this contract may not be fully met.

How is this opinion expressed?

  1. Money must always have both a amount and a currency code.

  2. The currency code must always be a valid ISO4217 code.

  3. Money arithmetic can only be performed when both operands are of the same currency.

  4. Money amounts are represented as a Decimal.

  5. Money can be serialised to the database as a composite Postgres type that includes both the amount and the currency. Therefore for Ecto serialization Postgres is assumed as the data store. Serialization is entirely optional.

  6. All arithmetic functions work on a Decimal. No rounding occurs automatically (unless expressly called out for a function, as is the case for Money.split/2).

  7. Explicit rounding obeys the rounding rules for a given currency. The rounding rules are defined by the Unicode consortium in its CLDR repository as implemented by the hex package ex_cldr. These rules define the number of fractional digits for a currency and the rounding increment where appropriate.

  8. Money output string formatting output using the hex package ex_cldr that correctly rounds to the appropriate number of fractional digits and to the correct rounding increment for currencies that have minimum cash increments (like the Swiss Franc and Australian Dollar)

Exchange rates and currency conversion

Money includes a process to retrieve exchange rates on a periodic basis. These exchange rates can then be used to support currency conversion. This service is started by default and will attempt to retrieve exchange rates every 5 minutes.

By default, exchange rates are retrieved from Open Exchange Rates however any module that conforms to the Money.ExchangeRates behaviour can be configured.

An optional callback module can be defined. This module defines a rates_retrieved/2 function that is invoked upon every successful retrieval of exchange rates.

Configuration

Money provides a set of configuration keys to customize behaviour. The default configuration is:

config :ex_money,
  exchange_rate_service: true,
  exchange_rates_retrieve_every: 360_000,
  api_module: Money.ExchangeRates.OpenExchangeRates,
  open_exchange_rates_app_id: nil,
  callback_module: Money.ExchangeRates.Callback
  log_failure: :warn,
  log_success: nil

These keys are are defined as follows:

  • exchange_rate_service is a boolean that determines whether to start the exchange rate retrieval service. The default it true.

  • exchange_rates_retrieve_every defines how often the exchange rates are retrieved in milliseconds. The default is 5 minutes (360,000 milliseconds)

  • api_module identifies the module that does the retrieval of exchange rates. This is any module that implements the Money.ExchangeRates behaviour. The default is Money.ExchangeRates.OpenExchangeRates

  • open_exchange_rates_app_id defines the app_id that is required to use the Open Exchange Rates api

  • callback_module defines a module that follows the Money.ExchangeRates.Callback behaviour whereby the function rates_retrieved/2 is invoked after every successful retrieval of exchange rates. The default is Money.ExchangeRates.Callback.

  • log_failure defines the log level at which api retrieval errors are logged. The default is :warn

  • log_success defines the log level at which successful api retrieval notifications are logged. The default is nil which means no logging.

Keys can also be configured to retrieve values from environment variables. This lookup is done at runtime to facilitate deployment strategies. If the value of a configuration key is {:system, "some_string"} then "some_string" is interpreted as an environment variable name which is passed to System.get_env/2. An example configuration might be:

config :ex_money,
  exchange_rate_service: {:system, "RATE_SERVICE"},
  exchange_rates_retrieve_every: {:system, "RETRIEVE_EVERY"},
  open_exchange_rates_app_id: {:system, "OPEN_EXCHANGE_RATES_APP_ID"}

Why yet another Money package?

  • Fully localized formatting and rounding using ex_cldr

  • Provides serialization to Postgres using a composite type that keeps both the currency code and the amount together removing a source of potential error

  • Uses the Decimal type in Elixir and the Postgres numeric type to preserve precision

Examples

Creating a %Money{} struct

iex> Money.new(:USD, 100)
 #Money<:USD, 100>

 iex> Money.new("CHF", 130.02)
 #Money<:CHF, 130.02>

 iex> Money.new("thb", 11)
 #Money<:THB, 11>

The canonical representation of a currency code is an atom that is a valid ISO4217 currency code. The amount of a %Money{} is represented by a Decimal.

Optional ~M sigil

An optional sigil module is available to aid in creating %Money{} structs. It needs to be imported before use:

import Money.Sigil

~M[100]USD
#> #Money<:USD, 100>

Localised String formatting

See also Money.to_string/2 and Cldr.Number.to_string/2):

iex> Money.to_string Money.new("thb", 11)
"THB11.00"

iex> Money.to_string Money.new("USD", 234.467)
"$234.47"

iex> Money.to_string Money.new("USD", 234.467), format: :long
"234.47 US dollars"

Note that the output is influenced by the locale in effect. By default the localed used is that returned by Cldr.get_local/0. Its default value is β€œen”. Additional locales can be configured, see Cldr. The formatting options are defined in Cldr.Number.to_string/2.

Arithmetic Functions

See also the module Money.Arithmetic:

iex> m1 = Money.new(:USD, 100)
#Money<:USD, 100>

iex> m2 = Money.new(:USD, 200)
#Money<:USD, 200>

iex> Money.add(m1, m2)
#Money<:USD, 300>

iex> m3 = Money.new(:AUD, 300)
#Money<:AUD, 300>

iex(11)> Money.add(m1, m3)
** (ArgumentError) Cannot add two %Money{} with different currencies. Received :USD and :AUD.
    (ex_money) lib/money.ex:46: Money.add/2

# Split a %Money{} returning the a dividend and a remainder. All
# operations respect the number of fractional digits defined for a currency
iex> m1 = Money.new(:USD, 100)
#Money<:USD, 100>

iex> Money.split(m1, 3)
{#Money<:USD, 33.33>, #Money<:USD, 0.01>}

# Rounding applies the currency definitions of CLDR as implemented in
# the hex package [ex_cldr](https://hex.pm/packages/ex_cldr)
iex> Money.round Money.new(:USD, 100.678)
#Money<:USD, 100.68>

iex> Money.round Money.new(:JPY, 100.678)
#Money<:JPY, 101>

Currency Conversion

A %Money{} struct can be converted to another currency using Money.to_currency/3. For example:

iex> Money.to_currency Money.new(:USD,100), :AUD
#Money<:AUD, 136.4300>

iex> Money.to_currency Money.new(:USD,100), :ZIP
** (Money.UnknownCurrencyError) The currency code :ZIP is not known

iex(1)> Money.to_currency Money.new(:USD,100), :XXX
** (Money.ExchangeRateError) No exchange rate is available for currency :XXX

A user-defined map of exchange rates can also be supplied:

iex(2)> Money.to_currency Money.new(:USD,100), :AUD, %{USD: Decimal.new(1.0), AUD: Decimal.new(1.3)}
#Money<:AUD, 130>

Financial Functions

A set of financial functions are available in the module Money.Financial. These are used in the Money module. See Money for the available functions.

Serializing to a Postgres database with Ecto

First generate the migration to create the custom type:

mix money.gen.migration
* creating priv/repo/migrations
* creating priv/repo/migrations/20161007234652_add_money_with_currency_type_to_postgres.exs

Then migrate the database:

mix ecto.migrate
07:09:28.637 [info]  == Running MoneyTest.Repo.Migrations.AddMoneyWithCurrencyTypeToPostgres.up/0 forward
07:09:28.640 [info]  execute "CREATE TYPE public.money_with_currency AS (currency_code char(3), amount numeric(20,8))"
07:09:28.647 [info]  == Migrated in 0.0s

Create your schema using the Money.Ecto.Type ecto type:

defmodule Ledger do
  use Ecto.Schema

  @primary_key false
  schema "ledgers" do
    field :amount, Money.Ecto.Type

    timestamps()
  end
end

Insert into the database:

Repo.insert %Ledger{amount: Money.new(:USD, 100)}
[debug] QUERY OK db=4.5ms
INSERT INTO "ledgers" ("amount","inserted_at","updated_at") VALUES ($1,$2,$3) [{"USD", #Decimal<100>}, {{2016, 10, 7}, {23, 12, 13, 0}}, {{2016, 10, 7}, {23, 12, 13, 0}}]

Retrieve from the database:

Repo.all Ledger
[debug] QUERY OK source="ledgers" db=5.3ms decode=0.1ms queue=0.1ms
SELECT l0."amount", l0."inserted_at", l0."updated_at" FROM "ledgers" AS l0 []
[%Ledger{__meta__: #Ecto.Schema.Metadata<:loaded, "ledgers">, amount: #<:USD, 100.00000000>,
  inserted_at: #Ecto.DateTime<2016-10-07 23:12:13>,
  updated_at: #Ecto.DateTime<2016-10-07 23:12:13>}]

Roadmap

The next phase of development will focus on additional financial functions.

Installation

ex_money can be installed by:

  1. Adding ex_money to your list of dependencies in mix.exs:

    def deps do
      [{:ex_money, "~> 0.0.9"}]
    end
  2. Ensuring ex_money is started before your application:

    def application do
      [applications: [:ex_money]]
    end