Latinum
Latinum is a library for resource and currency calculations. It provides immutable Resource
objects for dealing with quantities of named resources with an arbitrary number of decimal places, and Bank
objects for converting resources and formatting them for output. Latinum doesn't include any global state by default and thus is ideal for integration with other frameworks/libraries.
Motivation
I was originally planning on using the Money gem, but it's dependency on global state makes it hard to use if you want to deal with money as an immutable value type.
Additionally, I wanted to support BitCoin, Japanese Yen, etc. The money gem was heavily biased towards decimal currency. It had (~2012) fields like dollars
and cents
which don't really make sense and don't really align with the real world. These days they have fixed parts of the API, but it's a bit of a mess now, supporting both decimal and non-decimal values.
Another problem I had at the time was the concept of zero. It should be possible to have an additive (e.g. 0) and multiplicative identity (e.g. 1) do the right thing. In fact, in Latinum, you can multiply Latinum::Resource
instances by a scalar and get a useful result (e.g. for computing discounts).
Finally, because of the above problem, it was not obvious at the time how to sum up a collection of money instances correctly. In fact, this is still a problem and a separate gem, based on the Latinum::Collection
concept, was made. However, this all fits together in a rather haphazard way.
Latinum addresses all these issues. It has an immutable value type Latinum::Resource
which has a robust definition: A value (e.g. 5.0025) and a resource name (USD). The semantics of resources are well defined without the need for "Currency" state like the symbol, how many decimal places, etc. So, it suits well for serialization into a database, and for formatting to the user, there is Latinum::Bank
which gives you the choice of how you decide to format things or exchange them, whether you want to round something off, etc.
Installation
Add this line to your application's Gemfile:
gem 'latinum'
And then execute:
$ bundle
Or install it yourself as:
$ gem install latinum
Usage
Latinum has several core concepts:
- A
Resource
represents an immutable value with a specific face name (e.g.'USD'
). - A
Resource
can only be combined with resources with the same face name. - A
Bank
is responsible for managing currencies and formatting options. - A
Bank
can exchange currencies explicitly with a given set of exchange rates. - A
Collection
is responsible for adding currencies together and is completely deterministic.
Resources and Collections
To create a new resource, use a string for accuracy:
> ten = Latinum::Resource.new("10.00", "NZD")
=> 10.0 NZD
> ten.amount == "10.00".to_d
=> true
You can add resources of different values but with the same name:
> ten + ten
=> 20.0 NZD
But, you can't add resources of different names together:
> twenty = Latinum::Resource.new("20.00", "AUD")
=> 20.0 AUD
> ten + twenty
DifferentResourceNameError: Cannot operate on different currencies!
To add multiple currencies together, use a collection:
> collection = Latinum::Collection.new
> collection << [ten, twenty]
> collection.collect(&:to_s)
=> [10.0 NZD, 20.0 AUD]
Calculating Totals
The Latinum::Collection
is the correct way to sum up a list of transactions or other items with an
associated Latinum::Resource
. Here is an example:
<table class="listing transactions" data-model="Transaction">
<thead>
<tr>
<th class="name">Name</th>
<th class="date">Date</th>
<th class="price">Price</th>
<th class="quantity">Quantity</th>
<th class="subtotal">Sub-total</th>
<th class="tax_rate">Tax</th>
<th class="total">Total</th>
</tr>
</thead>
<tbody>
<?r
currencies = Set.new
summary = {
:subtotal => Latinum::Collection.new(currencies),
:tax => Latinum::Collection.new(currencies),
:total => Latinum::Collection.new(currencies)
}
invoice.transactions.each do |transaction|
subtotal = transaction.subtotal
summary[:subtotal] << subtotal
summary[:tax] << subtotal * transaction.tax_rate.to_d
summary[:total] << transaction.total
?>
<tr data-id="#{transaction.id}" data-rev="#{transaction.rev}">
<th class="name">#{f.text transaction.name}</th>
<td class="date">#{f.text transaction.date}</td>
<td class="price">#{f.text transaction.price}</td>
<td class="quantity">#{f.quantity transaction}</td>
<td class="subtotal">#{f.text subtotal}</td>
<td class="tax_rate">#{f.tax transaction}</td>
<td class="total">#{f.text transaction.total}</td>
</tr>
<?r end ?>
</tbody>
<tfoot>
<?r currencies.each do |currency| ?>
<tr>
<td colspan="5">#{currency} Summary:</td>
<td class="subtotal">#{f.text summary[:subtotal][currency]}</td>
<td class="tax_rate">#{f.text summary[:tax][currency]}</td>
<td class="total">#{f.text summary[:total][currency]}</td>
<td></td>
</tr>
<?r end ?>
</tfoot>
</table>
Banks and Exchange Rates
The bank is responsible for formatting and exchange rates:
require 'latinum/bank'
require 'latinum/currencies/global'
> bank = Latinum::Bank.new(Latinum::Currencies::Global)
> bank << Latinum::ExchangeRate.new("NZD", "AUD", "0.5")
> nzd = Latinum::Resource.new("10", "NZD")
=> 10.0 NZD
> aud = bank.exchange nzd, "AUD"
=> 5.0 AUD
Formatting an amount is typically required for presentation to the end user:
> bank.format(nzd)
=> "$10.00 NZD"
> bank.format(aud, :format => :compact)
=> "$5.00"
The bank can also be used to parse currency, which will depend on the priority of currencies if a symbol that matches multiple currencies is supplied:
> bank.parse("$5")
=> 5.0 USD
> bank.parse("€5")
=> 5.0 EUR
Currency codes take priority over symbols if specified:
> bank.parse("€5 NZD")
=> 5.0 NZD
Conversion To and From Integers
For storage in traditional databases, you may prefer to use integers. Based on the precision of the currency, you can store integer representations:
> resource = Latinum::Resource.new("1.12345678", "BTC")
> 112345678 == bank.to_integral(resource)
true
> resource == bank.from_integral(112345678, "BTC")
true
As BitCoin has 8 decimal places, it requires an integer representation with at least 10^8.
ActiveRecord Serialization
Latinum can be easily used in a ActiveRecord model simply by declaring a serialized data-type for a string or text column, e.g.
class Transaction < ActiveRecord::Base
serialize :total, Latinum::Resource
end
It can be used like so:
> transaction = Transaction.new(:total => "10 NZD")
> transaction.total * 2
=> "20.0 NZD"
To format the output, use a Latinum::Bank
, e.g. assuming the bank is set up correctly:
> bank.format(transaction.total)
=> "$20.00 NZD"
> bank.format(transaction.total, name: nil)
=> "$20.00"
> bank.format(transaction.total, symbol: nil)
=> "20.00 NZD"
Relaxo Serialization
Latinum is natively supported by Relaxo (CouchDB) and as such can be used in Relaxo models easily.
require 'latinum'
require 'relaxo/model'
require 'relaxo/model/properties/latinum'
class Transaction
include Relaxo::Model
property :name
property :price, Attribute[Latinum::Resource]
end
db = Relaxo.connect('test')
db.create!
t = Transaction.create(db, price: Latinum::Resource.load("50 NZD"))
t.price
# => <Latinum::Resource "50.0 NZD">
# Save and reload from database server:
t.save
t = Transaction.fetch(db, t.id)
t.price
# => <Latinum::Resource "50.0 NZD">
It gets stored in the database like so:
{
"_id": "740f4728fc9a571d826688db2f004771",
"_rev": "1-45a29c63311cfa0d5a765707184b2b3b",
"type": "transaction",
"price": [
"50.0",
"NZD"
]
}
Contributing
- Fork it
- Create your feature branch (
git checkout -b my-new-feature
) - Commit your changes (
git commit -am 'Add some feature'
) - Push to the branch (
git push origin my-new-feature
) - Create new Pull Request
License
Copyright, 2015, by Samuel G. D. Williams.
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.