Typesafe internationalization for Scala

Cosmopolite provides a typesafe representation for working with multilingual strings, Messages, with convenient constructors which guarantee through their types that translations for a specific set of languages exist in each instance. Furthermore, a Language type provides a coproduct representation of a language from the same set which can safely select one string from the set.

• provides a representation for multilingual strings
• provides a representation of a language
• interpolated string constructors for common languages
• support for all ISO 639-1 languages
• users choose their exact set of languages with a union type
• support for all language guaranteed by type for all static strings
• additions to the language set produce compile errors for every incomplete multilingual string
• checks for duplicate languages in construction of multilingual strings

Cosmopolite has not yet been published. The medium-term plan is to build Cosmopolite with Fury and to publish it as a source build on Vent. This will enable ordinary users to write and build software which depends on Cosmopolite.

Subsequently, Cosmopolite will also be made available as a binary in the Maven Central repository. This will enable users of other build tools to use it.

For the overeager, curious and impatient, see building.

When working with front-end applications that must be presented to different users in different languages, it's common that the vast majority of the code which provides the user interface will be identical for every language, with the exception of the strings which provide the text to be used in that user interface.

It is therefore useful to abstract over just the parts which differ, and to minimise repetition of the parts which are the same.

Languages are represented by types named after their ISO 639-1 code, for example En (English), La (Latin) or De (German). These are found in the cosmopolite.languages object, and each of these would typically be imported for use in a particular project, for example,

import language.{En, La, De}

Additionally, ten "common" languages are also made available in the cosmopolite.languages.common object, which can be conveniently imported with a wildcard import. These languages are:

• German (De)
• English (En)
• Spanish (Es)
• French (Fr)
• Italian (It)
• Japanese (Ja)
• Polish (Pl)
• Portuguese (Pt)
• Russian (Ru)
• Chinese (Zh)

Each of these is an alias for the language type defined in the cosmopolite.languages object.

Where more than one language needs to be specified in Cosmopolite, this is done as a union of language types, for example, the languages English, Polish and Russian would be represented as En | Pl | Ru.

The order of the language types in the union is not significant, as Scala will consider Pl | En | Ru an identical type to En | Pl | Ru, or any other permutation.

A multilingual string is represented by an instance of Messages[L] where L is the set of languages that instance has, as a union type, and which is guaranteed by its construction.

Constructors exist for single-language Messages instances of all common languages as prefixed strings, for example, en"English text" or fr"Texte français" will create instances of Messages[En]andMessages[Fr]` respectively.

Any pair of messages may be combined with the & operator to construct a Messages instance of all their languages combined. For example, en"English text" & fr"Texte français" will create an instance of Messages[En | Fr].

Using these construction methods makes it possible to define instances of Messages whose type is guaranteed to reflect their content.

The & combinator adds the further guarantee that every additional language combined with an existing multilingual string must not already be defined, so it should be impossible to accidentally overwrite a string for one language with another when constructing a Messages instance.

Convenient constructors for languages that are not in the common object can be provided as extensions on Scala's built-in StringContext type, and follow this pattern,

extension (ctx: StringContext)
  def la(msgs: Messages[La]*): Messages[La] = Messages(ctx.parts, msgs)

which defines a constructor for the La language (Latin) on strings starting with the prefix la"".

In addition to representing product types for a collection of languages, we need a type that represents a single language chosen from a collection of languages.

This Language instance will typically be constructed from a value that is only known at runtime, as it will be used to pick one particular language from a multilingual string. To construct a new Language[L] instance from a set of languages, L, we can parse it from a String with the Languages.parse method.

As long as L is concretely known, Cosmopolite will build a parser that checks its input string against each of languages in the union, and returns an Option[Language[L]] instance.

For example,

Language.parse[En | Fr]("fr")

would return Some(Language[Fr]) while,

Language.parse[En | Fr]("es")

would return None.

It's likely that an application will use the same set of languages globally, so it's useful to define a global type alias for these languages, for example,

type Langs = En | Fr | De | Es

and to use the Langs type everywhere instead of any specific language types. The type will always dealias to the union of language types, and the Scala compiler can continue to perform all necessary checks as before.

Consequently, adding a language to such an alias can be a useful way to statically identify every multilingual string that needs to be adapted to include the messages.

For example, changing the Langs definition above to,

type Langs = En | Fr | De | Es | It

would suddenly result in multiple compile errors: one for each usage of a multilingual string that fails to provide an Italian string.

This offers an additional level of safety when developing for multilingual applications, as it becomes impossible to compile code which does not provide language strings for every language required.

It is also largely possible to define methods and classes which are language-independent. It is possible to define a method which takes both Messages[L] instances and a Language[L] instance, and combines them without ever concretely knowing the language L.

As long as the type parameter of the Messages instance is a supertype of the type parameter of the Language instance—and most likely the types would be equal—they may be combined to produce a string.

As one example use case, we could write a language-independent login form (for whatever framework we like) that takes field and button labels as multilingual strings, and a corresponding Language instance. In combining the Language coproduct with each of the Messages products, their type parameter would be eliminated.

Note, however, that the Language.parse method, however, can only be invoked on a concrete type, as it needs to build a concrete parser for the languages in the union type.

Cosmopolite is classified as fledgling. For reference, Scala One 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.

Cosmopolite is designed to be small. Its entire source code currently consists of 488 lines of code.

Cosmopolite 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 Cosmopolite?".

1. Copy the sources into your own project

   Read the fury file in the repository root to understand Cosmopolite'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 Cosmopolite 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 cosmopolite. Run wrath -F in the repository root. This will download and compile the latest version of Scala, as well as all of Cosmopolite's dependencies.

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

Contributors to Cosmopolite 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 Cosmopolite 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.

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

Cosmopolite is so-named because it provides multilingual strings, which are free to travel, and are not associated with any particular nation.

In general, Scala One 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 two overlapping speech bubbles, highlighting Cosmopolite's focus on communication (in particular, between groups speaking different languages).

Cosmopolite is copyright © 2024 Jon Pretty & Propensive OÜ, and is made available under the Apache 2.0 License.