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https://github.com/propensive/cosmopolite

Typesafe internationalization of strings in Scala
https://github.com/propensive/cosmopolite

i18n internationalization scala

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Typesafe internationalization of strings in Scala

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# Cosmopolite



__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.



## Features



- provides a representation for multilingual strings

- provides a representation of a language

- interpolated string constructors for common languages

- support for all [ISO 639-1](https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes) 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



## Availability Plan



Cosmopolite has not yet been published. The medium-term plan is to build Cosmopolite

with [Fury](https://github.com/propensive/fury) and to publish it as a source build on

[Vent](https://github.com/propensive/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](#building).



## Getting Started



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.



### Language types



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,

```scala

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.



### Multilingual strings



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]` and `Messages[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.



#### Defining constructors for other languages



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,

```scala

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""`.



### The `Language` type



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,

```scala

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

```

would return `Some(Language[Fr])` while,

```scala

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

```

would return `None`.



### Type Aliases



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,

```scala

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,

```scala

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.



### Abstract Languages



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.



## Status



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.



## Building



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](https://github.com/propensive/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`](https://github.com/propensive/wrath/releases/latest), 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.



## Contributing



Contributors to Cosmopolite are welcome and encouraged. New contributors may like

to look for issues marked

[beginner](https://github.com/propensive/cosmopolite/labels/beginner).



We suggest that all contributors read the [Contributing

Guide](/contributing.md) 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.



## Author



Cosmopolite was designed and developed by Jon Pretty, and commercial support and

training on all aspects of Scala 3 is available from [Propensive

OÜ](https://propensive.com/).



## Name



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.



## Logo



The logo shows two overlapping speech bubbles, highlighting Cosmopolite's focus on communication (in particular, between groups speaking different languages).



## License



Cosmopolite is copyright © 2024 Jon Pretty & Propensive OÜ, and

is made available under the [Apache 2.0 License](/license.md).