blaketan / money

Elixir implementation of Money with Currency

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. Before release 1.0 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. For MySQL, money is serialized into a json column with the amount converted to a string to preserve precision since json does not have a decimal type. 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)

Prerequisities

• ex_money is supported on Elixir 1.5 and later only

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 not started by default. If started it will attempt to retrieve exchange rates every 5 minutes by default.

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 also be defined. This module defines a rates_retrieved/2 function that is invoked upon every successful retrieval of exchange rates. This might be used to serialize exchange rate to a data store or to stream rates to other applications or systems.

Configuration

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

config :ex_money,
  exchange_rate_service: false,
  exchange_rates_retrieve_every: 300_000,
  delay_before_first_retrieval: 100,
  api_module: Money.ExchangeRates.OpenExchangeRates,
  callback_module: Money.ExchangeRates.Callback,
  retriever_options: nil
  log_failure: :warn,
  log_info: :info,
  log_success: nil

Configuration key definitions

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

• :exchange_rates_retrieve_every defines how often the exchange rates are retrieved in milliseconds. The default is 5 minutes (300,000 milliseconds)

• :delay_before_first_retrieval defines how quickly the retrieval service makes its first request for exchange rates. The default is 100 milliseconds. Any value that is not a positive integer means that no first retrieval is made. Retrieval will continue on interval defined by :retrieve_every

• :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

• 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.

• log_info defines the log level at which service startup messages are logged. The default is info.

• :retriever_options is available for exchange rate retriever module developers as a place to add retriever-specific configuration information. This information should be added in the init/1 callback in the retriever module. See Money.ExchangeRates.OpenExchangeRates.init/1 for an example.

Open Exchange Rates configuration

If you plan to use the provided Open Exchange Rates module to retrieve exchange rates then you should also provide the addition configuration key for app_id:

  config :ex_money,
    open_exchange_rates_app_id: "your_app_id"

or configure it via environment variable, for example:

  config :ex_money,
    open_exchange_rates_app_id: {:system, "OPEN_EXCHANGE_RATES_APP_ID"}

The default exchange rate retrieval module is provided in Money.ExchangeRates.OpenExchangeRates which can be used as a example to implement your own retrieval module for other services.

Managing the configuration at runtime

During exchange rate service startup, the function init/1 is called on the configuration exchange rate retrieval module. This module is expected to return an updated configuration allowing a develop to customise how the configuration is to be managed. See the implementation at Money.ExchangeRates.OpenExchangeRates.init/1 for an example.

Using Environment Variables in the configuration

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"}

The Exchange rates service process supervision and startup

If the exchange rate service is configured to automatically start up (because the config key exchange_rate_service is set to true) then a supervisor process named Money.ExchangeRates.Supervisor is started which in turns starts a child GenServer called Money.ExchangeRates.Retriever. It is Money.ExchangeRates.Retriever which will call the configured api_module to retrieve the rates. It is also responsible for calling the configured callback_module after a successfull retrieval.

                                     +-----------------+
                                     |                 |
+-------------+    +-----------+     |   api_module    |-> External Service
|             |    |           |---> |                 |
| Supervisor  |--->| Retriever |     +-----------------+
|             |    |           |---> +-----------------+
+-------------+    +-----------+     |                 |
                                     | callback_module |
                                     |                 |
                                     +-----------------+

On application start (or manual start if :exchange_rate_service is set to false), Money.ExchangeRates.Retriever will schedule the first retrieval to be executed after :delay_before_first_retrieval milliseconds and then each :exchange_rates_retrieve_every milliseconds thereafter.

Using Ecto or other applications from within the callback module

If you provide your own callback module and that module depends on some other applications, like Ecto, already being started then automatically starting Money.ExchangeRates.Supervisor may not work since your Ecto.Repo is unlikely to have already been started.

In this situation the appropriate way to configure the exchange rates retrieval service is the following:

1. Set the configuration key exchange_rate_service to false to prevent automatic startup of the service.

2. Configure your api_module, callback_module and any other required configuration as appropriate

3. In your client application code, add the Money.ExchangeRates.Supervisor to the children configuration of your application. For example, in an application that uses Ecto and where your callback_module is designed to save exchange rates to a database, your application may would look something like:

defmodule Application do
  use Application

  def start(_type, _args) do
    import Supervisor.Spec

    children = [

      # Start your repo first so that it is running before your
      # exchange rates callback module is called
      supervisor(MoneyTest.Repo, []),

      # Include the Money.ExchangeRates.Supervisor in your application's
      # supervision tree.  This supervisor will start the child process
      # Money.ExchangeRates.Retriever
      supervisor(Money.ExchangeRates.Supervisor, [])
    ]

    opts = [strategy: :one_for_one, name: Application.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

API Usage Examples

Creating a %Money{} struct

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

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

Note that the arguments to Money.new/2 can be supplied in either order.

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)
{:ok, "THB11.00"}

iex> Money.to_string Money.new("USD", 234.467)
{:ok, "$234.47"}

iex> Money.to_string Money.new("USD", 234.467), format: :long
{:ok, "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_current_local/0. Its default value is "en". 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)
{:ok, #Money<:USD, 300>}

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

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

iex> Money.add Money.new(:USD, 200), Money.new(:AUD, 100)
{:error, {ArgumentError, "Cannot add monies with different currencies. Received :USD and :AUD."}}

# 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 or Money.to_currency!/3. For example:

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

iex> Money.to_currency Money.new(:USD, 100) , :AUDD, %{USD: Decimal.new(1), AUD: Decimal.new(0.7345)}
{:error, {Cldr.UnknownCurrencyError, "Currency :AUDD is not known"}}

iex> 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> 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 basic financial functions are available in the module Money.Financial. These functions are:

• Present value: Money.Financial.present_value/3
• Future value: Money.Financial.future_value/3
• Interest rate: Money.Financial.interest_rate/3
• Number of periods: Money.Financial.periods/3
• Payment amount: Money.Financial.payment/3
• Net Present Value of a set of cash flows: Money.Financial.net_present_value/2
• Internal rate of return: Money.Financial.internal_rate_of_return/1

For more detail see Money.Financial.

Serializing to a Postgres database with Ecto

First generate the migration to create the custom type:

mix money.gen.postgres.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 database migration with the new type (don't forget to mix ecto.migrate as well):

defmodule MoneyTest.Repo.Migrations.CreateLedger do
  use Ecto.Migration

  def change do
    create table(:ledgers) do
      add :amount, :money_with_currency
      timestamps()
    end
  end
end

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

defmodule Ledger do
  use Ecto.Schema

  schema "ledgers" do
    field :amount, Money.Ecto.Composite.Type

    timestamps()
  end
end

Insert into the database:

iex> 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:

iex> 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: ~N[2017-02-21 00:15:40.979576],
  updated_at: ~N[2017-02-21 00:15:40.991391]}]

Serializing to a MySQL (or other non-Postgres) database with Ecto

Since MySQL does not support composite types, the :map type is used which in MySQL is implemented as a JSON column. The currency code and amount are serialised into this column.

defmodule MoneyTest.Repo.Migrations.CreateLedger do
  use Ecto.Migration

  def change do
    create table(:ledgers) do
      add :amount, :map
      timestamps()
    end
  end
end

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

defmodule Ledger do
  use Ecto.Schema

  schema "ledgers" do
    field :amount, Money.Ecto.Map.Type

    timestamps()
  end
end

Insert into the database:

iex> Repo.insert %Ledger{amount_map: Money.new(:USD, 100)}
[debug] QUERY OK db=25.8ms
INSERT INTO "ledgers" ("amount_map","inserted_at","updated_at") VALUES ($1,$2,$3)
RETURNING "id" [%{amount: "100", currency: "USD"},
{{2017, 2, 21}, {0, 15, 40, 979576}}, {{2017, 2, 21}, {0, 15, 40, 991391}}]

{:ok,
 %MoneyTest.Thing{__meta__: #Ecto.Schema.Metadata<:loaded, "ledgers">,
  amount: nil, amount_map: #Money<:USD, 100>, id: 3,
  inserted_at: ~N[2017-02-21 00:15:40.979576],
  updated_at: ~N[2017-02-21 00:15:40.991391]}}

Retrieve from the database:

iex> Repo.all Ledger
[debug] QUERY OK source="ledgers" db=16.1ms decode=0.1ms
SELECT t0."id", t0."amount_map", t0."inserted_at", t0."updated_at" FROM "ledgers" AS t0 []
[%Ledger{__meta__: #Ecto.Schema.Metadata<:loaded, "ledgers">,
  amount_map: #Money<:USD, 100>, id: 3,
  inserted_at: ~N[2017-02-21 00:15:40.979576],
  updated_at: ~N[2017-02-21 00:15:40.991391]}]

Notes:

1. In order to preserve precision of the decimal amount, the amount part of the %Money{} struct is serialised as a string. This is done because JSON serializes numeric values as either integer or float, neither of which would preserve precision of a decimal value.

2. The precision of the serialized string value of amount is affected by the setting of Decimal.get_context. The default is 28 digits which should cater for your requirements.

3. Serializing the amount as a string means that SQL query arithmetic and equality operators will not work as expected. You may find that CASTing the string value will restore some of that functionality. For example:

    CAST(JSON_EXTRACT(amount_map, '$.amount') AS DECIMAL(20, 8)) AS amount;

Installation

ex_money can be installed by:

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

  def deps do
    [{:ex_money, "~> 0.8.4"}]
  end

Why yet another Money package?

• Fully localized formatting and rounding using ex_cldr

• Provides serialization to Postgres using a composite type and MySQL using a JSON 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. For MySQL the amount is serialised as a string to preserve precision that might otherwise be lost if stored as a JSON numeric type (which is either an integer or a float)

• Includes a set of financial calculations (arithmetic and cash flow calculations) that follow solid rounding rules