Typesafe representations of monetary values in Scala

Representing monetary values, which are usually fixed-point decimal values, is a common requirement for many commercial projects, but can involve a variety of small challenges involving different currencies, rounding errors, showing monetary amounts, and handling sales taxes. Plutocracy exists to make these mundane tasks easier.

• provides simple representations of currencies and monetary values
• monetary values are represented precisely as fixed-point decimals
• currencies are stored in memory as Longs using opaque types
• new currencies are easy to add
• implementations of the 16 most traded currencies are included
• introduces another type representing taxed prices
• distinct Money and Price types avoids common mistakes relating to taxation

Money values can be constructed by applying a Double value to a currency object, such as, Eur, Usd or Jpy:

import plutocracy.*
val fivePounds = Gbp(5.00)
val sixCents = Usd(0.06)

Each value will have the type Money, parameterized on the singleton type of its currency, thus:

val fivePounds: Money[Gbp.type] = Gbp(5.00)
val sixCents: Money[Usd.type] = Usd(0.06)

Values are represented internally as Longs, with a fixed decimal point.

A currency is trivial to define, as an object extending Currency which has four parameters:

• a three-letter currency code, such as t"USD"
• a currency symbol, often (but not always) a single character, such as t" £" or t"kr"
• the currency name
• an integer number of currency subunits in each currency unit, e.g. cents in a euro; usually 100

For example,

object Nok extends Currency(t"NOK", t"kr", t"Norwegian Krone", 100)

Addition and subtraction of values of Money is possible, provided they have the same currency. If not, a compile error will be produced.

Multiplication and division are also possible, either by another Money value of the same currency, or by a Double value, yielding a Double or a Money respectively.

When a multiplication or division by a double value results in an inexact monetary value, rounding must occur. Since Money values are represented as integers, this happens eagerly. Rounding is "half away from zero", which is not (currently) configurable.

In addition to division, a split method is provided on Money values which will divide a Money value into an integer number of parts, with the property that their sum is equal to the original amount, while sacrificing the invariant that every part is equal, if the division is not exact: in order to maintain the total amount, some parts may be rounded up, while others may be rounded down. Each part will differ by at most 1 currency subunit.

As monetary values are opaque Long instances, they share Long's toString implementation, which will just display the raw number, without a currency or a decimal point.

Instead, to display a Money, the show method should be used. This will use the information in the Money's Currency value to print the value correctly, however it requires a contextual CurrencyStyle value, which will format the currency appropriately. A choice of two is provided:

• plutocrat.currencyStyles.generic, for currencies in the style, 3.01 EUR
• plutocrat.currencyStyles.local, for currencies in the style, €3.01

Different implementations may be provided, if necessary.

It's common to need to work with prices which represent an amount which has some tax applied to it. For example, in many countries a sales tax of about 20% is applied to items bought by consumers.

For the seller, the untaxed amount is ususally the most important; for the buyer, the tax-inclusive amount is the only relevant one; and the government is probably most interested in the tax amount. If all values are represented in code by the same type, it's possible to inadvertently mix them up, for example by applying a sales tax twice.

There are multiple different ways to protect against this sort of mistake, so Plutocracy provides one possible solution: a Price type which comprises of a principal amount (the tax-exclusive amount) and a tax amount, both of which are Money instances. Price, like Money is parameterized with a currency type.

A Price may be constructed from a Money with the tax method, which takes a tax rate, a Double, as its parameter. For example,

val price: Price[Eur.type] = Eur(2.99).tax(0.18)

would apply an 18% tax to an amount costing €2.99.

Prices may be added, subtracted and negated, with the principal and tax amounts combined independently of each other.

However, there is, quite deliberately, no Show instance for Price. In order to show a price, its principal and tax Money amounts, or the inclusive Money amount should be accessed explicitly, and can be shown appropriately for the context.

Plutocrat is classified as fledgling. For reference, Soundness projects are categorized into one of the following five stability levels:

• embryonic: for experimental or demonstrative purposes only, without any guarantees of longevity
• fledgling: of proven utility, seeking contributions, but liable to significant redesigns
• maturescent: major design decisions broady settled, seeking probatory adoption and refinement
• dependable: production-ready, subject to controlled ongoing maintenance and enhancement; tagged as version 1.0.0 or later
• adamantine: proven, reliable and production-ready, with no further breaking changes ever anticipated

Projects at any stability level, even embryonic projects, can still be used, as long as caution is taken to avoid a mismatch between the project's stability level and the required stability and maintainability of your own project.

Plutocrat is designed to be small. Its entire source code currently consists of 200 lines of code.

Plutocrat will ultimately be built by Fury, when it is published. In the meantime, two possibilities are offered, however they are acknowledged to be fragile, inadequately tested, and unsuitable for anything more than experimentation. They are provided only for the necessity of providing some answer to the question, "how can I try Plutocrat?".

1. Copy the sources into your own project

   Read the fury file in the repository root to understand Plutocrat's build structure, dependencies and source location; the file format should be short and quite intuitive. Copy the sources into a source directory in your own project, then repeat (recursively) for each of the dependencies.

   The sources are compiled against the latest nightly release of Scala 3. There should be no problem to compile the project together with all of its dependencies in a single compilation.

2. Build with Wrath

   Wrath is a bootstrapping script for building Plutocrat and other projects in the absence of a fully-featured build tool. It is designed to read the fury file in the project directory, and produce a collection of JAR files which can be added to a classpath, by compiling the project and all of its dependencies, including the Scala compiler itself.

   Download the latest version of wrath, make it executable, and add it to your path, for example by copying it to /usr/local/bin/.

   Clone this repository inside an empty directory, so that the build can safely make clones of repositories it depends on as peers of plutocrat. Run wrath -F in the repository root. This will download and compile the latest version of Scala, as well as all of Plutocrat's dependencies.

   If the build was successful, the compiled JAR files can be found in the .wrath/dist directory.

Contributors to Plutocrat are welcome and encouraged. New contributors may like to look for issues marked beginner.

We suggest that all contributors read the Contributing Guide to make the process of contributing to Plutocrat easier.

Please do not contact project maintainers privately with questions unless there is a good reason to keep them private. While it can be tempting to repsond to such questions, private answers cannot be shared with a wider audience, and it can result in duplication of effort.

Plutocrat was designed and developed by Jon Pretty, and commercial support and training on all aspects of Scala 3 is available from Propensive OÜ.

Plutocracy is power through money, which this library provides; at least a representation thereof.

In general, Soundness project names are always chosen with some rationale, however it is usually frivolous. Each name is chosen for more for its uniqueness and intrigue than its concision or catchiness, and there is no bias towards names with positive or "nice" meanings—since many of the libraries perform some quite unpleasant tasks.

Names should be English words, though many are obscure or archaic, and it should be noted how willingly English adopts foreign words. Names are generally of Greek or Latin origin, and have often arrived in English via a romance language.

The logo shows an imagined currency symbol based on a lambda (λ).

Plutocrat is copyright © 2025 Jon Pretty & Propensive OÜ, and is made available under the Apache 2.0 License.