Getting Started with Cldr

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Cldr is an Elixir library for the Unicode Consortium's Common Locale Data Repository (CLDR). The intentions of CLDR, and this library, is to simplify the locale specific formatting of numbers, lists, currencies, calendars, units of measure and dates/times. As of November 2020 and ex_cldr Version 2.18.0, Cldr is based upon CLDR version 38.0.0.

The first step is to define a module that will host the desired Cldr configuration and the functions that serve as the public API. This module is referred to in this documentation as a backend module. For example:

@doc """
Define a backend module that will host our
Cldr configuration and public API.

Most function calls in Cldr will be calls
to functions on this module.
defmodule MyApp.Cldr do
  use Cldr,
    locales: ["en", "fr", "zh", "th"],
    default_locale: "en"


This strategy means that different configurations can be defined and it also means that one Cldr implementation won't interfer with implementations in other, potentially dependent, applications.

The functions you are mostly likely to use are:

To access the raw Cldr data for a locale the Cldr.Config module is available. Note that the functions in Cldr.Config are typically used by library authors. The most useful function is:

  • Cldr.Config.get_locale/2 which returns a map of all the CLDR data known to Cldr. Since this data is read from a file, parsed and then formatted it is a function that should be used with care due to the material performance implications. Cldr uses this function during compilation to build functions that return the relevant data with higher performance and these functions are to be preferred over the use of Cldr.Config.get_locale/2.

Use this package when you have a requirement to...

  • Support multiple languages and locales in your application

  • Need to support formatting numbers, dates, times, date-times, units and lists in one language or many

  • Access the data maintained in the CLDR repository in a functional manner

  • Parse an Accept-Language http header or a language tag

It is highly likely that you will also want to install one or more of the dependent packages that provide localization and formatting for a particular data domain. See Additional Cldr Packages below.

Elixir Version Requirements

  • ex_cldr requires Elixir 1.6 or later.


Add ex_cldr and the JSON library of your choice as a dependencies to your mix project:

defp deps do
    {:ex_cldr, "~> 2.0"},
    # Posion or any other compatible json library
    # that implements `encode!/1` and `decode!/1`
    # :jason is recommended
    {:jason, "~> 1.0"}
    # {:poison, "~> 2.1 or ~> 3.0"}

then retrieve ex_cldr and the JSON library from hex:

mix deps.get
mix deps.compile

Additional Cldr Packages

ex_cldr includes only basic functions to maintain the CLDR data repository in an accessible manner and to manage locale definitions. Additional functionality is available by adding additional packages:

Each of these packages includes ex_cldr as a dependency so configuring any of these additional packages will automatically install ex_cldr.


Cldr attempts to maximise runtime performance at the expense of additional compile time. Where possible Cldr will create functions to encapsulate data at compile time. To perform these optimizations for all 541 locales known to Cldr wouldn't be an effective use of your time or your computer's. Therefore Cldr requires that you configure the locales you want to use.

The preferred way to configure Cldr is to define the configuration in your backend module. This removes any dependency on your mix.exs and therefore simplifies deployment as a release. However configuration can also be defined in other ways:

Global configuration.

In mix.exs a global configuration can be defined under the :ex_cldr key. Although any valid configuration keys can be used here, only the keys :json_library, :cacertfile and default_locale are considered valid. Other configuration keys may be used to aid migration from Cldr version 1.x but a deprecation message will be printed during compilation. Here's an example of global configuration:

config :ex_cldr,
  default_locale: "en",
  default_backend: MyApp.Cldr,
  json_library: Jason,
  cacertfile: "path/to/cacertfile"

Note that the :json_library key can only be defined at the global level since it is required during compilation before any backend module is compiled.

On most platforms other than Windows the :cacertfile will be automatically detected. Any configured :cacertfile will take precedence on all platforms.

If configuration beyond the keys :default_locale, :cacertfile or :json_library are defined a deprecation warning is printed at compile time noting that configuration should be moved to a backend module.

Backend Module Configuration

The preferred configuration method is to define the configuration in the backend module. Using the backend configuration in config.exs is discouraged and will result in a warning at compile time. The configuration keys are the same so the preferred way to achieve the same configuration as defined in the global example is:

defmodule MyApp.Cldr do
  use Cldr,
    default_locale: "en",
    locales: ["fr", "en", "bs", "si", "ak", "th"],
    add_fallback_locales: false,
    gettext: MyApp.Gettext,
    data_dir: "./priv/cldr",
    otp_app: :my_app,
    precompile_number_formats: ["¤¤#,##0.##"],
    precompile_transliterations: [{:latn, :arab}, {:thai, :latn}],
    providers: [Cldr.Number],
    generate_docs: true,
    force_locale_download: false

Otp App Configuration

In the backend configuration example above the :otp_app key has been defined. This means that configuration for Cldr has been defined in mix.exs under the key :my_app with the sub-key MyApp.Cldr. For example:

defmodule MyApp.Cldr do
  use Cldr, otp_app: :my_app
# In mix.exs
config :my_app, MyApp.Cldr,
  default_locale: "en",
  locales: ["fr", "en", "bs", "si", "ak", "th"],
  gettext: MyApp.Gettext,
  data_dir: "./priv/cldr",
  precompile_number_formats: ["¤¤#,##0.##"],
  precompile_transliterations: [{:latn, :arab}, {:thai, :latn}]

Multiple backends can be configured under a single :otp_app if required.

Configuration Priority

When building the consolidated configuration the following priority applies:

  • Consider the global configuration
  • Merge the otp_app configuration over the top of the global configuration
  • Merge the backend module configuration over the top

Backend Configuration Keys

The configuration keys available for Cldr are:

  • default_locale specifies the default locale to be used for this backend. The default locale in case no other locale has been set is "en-001". The default locale calculated as follows:

    • If set by the :default_locale key, then this is the priority
    • If no :default_locale key, then a configured Gettext default locale for this backend is chosen
    • If no :default_locale key is specified and no Gettext module is configured, or is configured but has no default set, use Cldr.default_locale/0 which returns either the default locale configurated in mix.exs under the ex_cldr key or then the system default locale will is currently en-001
  • locales: Defines what locales will be configured in Cldr. Only these locales will be available and an exception Cldr.UnknownLocaleError will be raised if there is an attempt to use an unknown locale. This is the same behaviour as Gettext. Locales are configured as a list of binaries (strings). For convenince it is possible to use wildcard matching of locales which is particulalry helpful when there are many regional variances of a single language locale. For example, there are over 100 regional variants of the "en" locale in CLDR. A wildcard locale is detected by the presence of ., [, * and + in the locale string. This locale is then matched using the pattern as a regex to match against all available locales. The example below will configure all locales that start with en- and the locale fr.

use Cldr,
  default_locale: "en",
  locales: ["en-*", "fr"]
  • There is one additional setting which is :all which will configure all 541 locales. This is highly discouraged since it will take many minutes to compile your project and will consume more memory than you really want. This setting is there to aid in running the test suite. Really, don't use this setting.

  • :add_fallback_locales is a boolean key which when true results in the fallback locales being added for each of the configured locales. The default is false. The reason to set this option to true is that some data such as rules based number formats and subdivision data are inherited from their language roots. For example, the locale en-001 is inherited from the locale en. Locale en-001 does not have any rules based number formats or subdivision data defined for it. However locale en does. Including the fallback locales maximises the opportunity to resolve localised data.

  • :gettext: configures Cldr to use a Gettext module as an additional source of locales you want to configure. Since Gettext uses the Posix locale name format (locales with an '_' in them) and Cldr uses the Unicode format (a '-' as the subtag separator), Cldr will transliterate locale names from Gettext into the Cldr canonical form.

  • :data_dir: indicates where downloaded locale files will be stored. The default is :code.priv_dir(otp_app) where otp_app is the app defined under the :otp_app configuration key. If that key is not specified then the :ex_cldr app is used. It is recommended that an :otp_app key is specified in your backend module configuration.

  • :precompile_number_formats: provides a means to have user-defined format strings precompiled at application compile time. This has a performance benefit since precompiled formats execute approximately twice as fast as formats that are not precompiled.

  • :precompile_transliterations: defines those transliterations between the digits of two different number systems that will be precompiled. The is a list of 2-tuples where each tuple is of the form {from_number_system, to_number_system} where each number system is expressed as an atom. The available number systems is returned by Cldr.Number.System.systems_with_digits/0. The default is the empty list [].

  • :precompile_date_time_formats: provides a means to have user-defined date, time and date time format strings precompiled at application compile time. This has a performance benefit since precompiled formats execute approximately twice as fast as formats that are not precompiled. These formats are used by ex_cldr_date_times.

  • :precompile_interval_formats: provides a means to have user-defined interval format strings precompiled at application compile time. This has a performance benefit since precompiled formats execute approximately twice as fast as formats that are not precompiled. These formats are used by ex_cldr_date_times.

  • :providers: a list of modules that provide Cldr functionality to be compiled into the backend module. See the providers section below.

  • :generate_docs defines whether or not to generate documentation for the modules built as part of the backend. Since these modules represent the public API for ex_cldr, the default is true. Setting this key to false (the atom false, not a falsy value) which prevent the generation of docs for this backend.

  • :supress_warnings defines whether warnings are logged when a provider module is configured but not available. It also controls whether warnings are logged when a number format is compiled at runtime. Its purpose is to help identify those formats which might best be added to the :precompile_number_formats configuration. The default is false. Warning are not logged when set to true.

  • :force_locale_download determines whether to always download locale files during compilation. Locale data is ex_cldr version dependent. When a new version of ex_cldr is installed, no locales are installed and therefore locales are downloaded at compilation time as required. This ensures that the right version of the locale data is always associated with the right version of ex_cldr. However if locale data is being cached in CI/CD there is some possibility that there can be a version mismatch. Since reproducable builds are important, setting the force_locale_download: true in a backend or in global configuration adds additional certainty. The default setting is false thereby retaining compatibility with existing behaviour. The configuration can also be made dependent on mix environment as shown in this example:

defmodule MyApp.Cldr do
  use Cldr,
    locales: ["en", "fr"],
    default_locale: "en",
    force_locale_download: Mix.env() == :prod


The data maintained by CLDR is quite large and not all capabilities are required by all applications. Hence Cldr has additional optional functionality that can be provided through additional hex packages. In order to support compile-time additions to a configured backend, any package can define a provider that will be called at compile time.

The currently known providers and their hex package names are:

Hex PackageProvider ModuleComment
ex_cldr_numbersCldr.NumberFormatting of numbers, currencies
ex_cldr_listsCldr.ListFormatting of lists
ex_cldr_unitsCldr.UnitFormatting of SI and Imperial units
ex_cldr_territoriesCldr.TerritoryFormatting of territory (country) data
ex_cldr_languagesCldr.LanguageFormatting of language information
ex_cldr_dates_timesCldr.DateTimeFormatting of dates, times & datetimes
ex_moneyMoneyOperations on and formatting of a money type
ex_messagesCldr.MessageFormatting of ICU-formatted messages

Any library author can create a provider module by exposing a function called cldr_backend_provider/1 that takes a Cldr.Config struct as a single parameter. The function should return an AST that is inserted into the backend module being compiled.

Providers are configured on each backend module under the :providers key. It must be a list of provider modules. For example:

defmodule MyApp.Cldr do
  use Cldr,
    locales: ["en", "zh"],
    default_locale: "en",
    providers: [Cldr.Number, Cldr.List]

If :providers is nil (the default), Cldr will attempt to configure all of the providers described above if they have been installed as deps. If you don't wish to invoke any providers, use the empty list [].

Migrating from Cldr 1.x

  1. Create a backend module by following the configuration instructions
  2. Delete any duplicated global configuration in any config.exs files. Only the keys :default_locale and :json_library are supported in the global configuration
  3. Update any plugs to configure the desired backend
  4. Adjust any API calls from Cldr.some_function to MyApp.Cldr.some_function. Or better still, alias your backend module where required. ie. alias MyApp.Cldr, as: Cldr

Downloading Configured Locales

Cldr can be installed from either github or from hex.

  • If installed from github then all 541 locales are installed when the repo is cloned into your application deps.

  • If installed from hex then only the locales "en", "en-001" and "root" are installed. When you configure additional locales these will be downloaded during application compilation.

Localizing and Formatting Numbers

The Cldr.Number module implemented in the ex_cldr_numbers package provides number formatting. The public API for number formatting is MyApp.Cldr.Number.to_string/2. Some examples:

iex> MyApp.Cldr.Number.to_string 12345

iex> MyApp.Cldr.Number.to_string 12345, locale: "fr"
"12 345"

iex> MyApp.Cldr.Number.to_string 12345, locale: "fr", currency: "USD"
"12 345,00 $US"

iex> MyApp.Cldr.Number.to_string 12345, format: "#E0"

iex(> MyApp.Cldr.Number.to_string 1234, format: :roman

iex> MyApp.Cldr.Number.to_string 1234, format: :ordinal

iex> MyApp.Cldr.Number.to_string 1234, format: :spellout
"one thousand two hundred thirty-four"

See h MyApp.Cldr.Number and h MyApp.Cldr.Number.to_string in iex for further information.

Localizing Lists

The Cldr.List module provides list formatting and is implemented in the ex_cldr_lists package. The public API for list formating is Cldr.List.to_string/2. Some examples:

iex> MyApp.Cldr.List.to_string(["a", "b", "c"], locale: "en")
"a, b, and c"

iex> MyApp.Cldr.List.to_string(["a", "b", "c"], locale: "en", format: :unit_narrow)
"a b c"

iex> MyApp.Cldr.List.to_string(["a", "b", "c"], locale: "fr")
"a, b et c"

See h MyApp.Cldr.List and h MyApp.Cldr.List.to_string in iex for further information.

Localizing Units

The Cldr.Unit module provides unit localization and is implemented in the ex_cldr_units package. The public API for unit localization is Cldr.Unit.to_string/3. Some examples:

iex> MyApp.Cldr.Unit.to_string 123, :gallon
"123 gallons"

iex> MyApp.Cldr.Unit.to_string 1234, :gallon, format: :long
"1 thousand gallons"

iex> MyApp.Cldr.Unit.to_string 1234, :gallon, format: :short
"1K gallons"

iex> MyApp.Cldr.Unit.to_string 1234, :megahertz
"1,234 megahertz"

iex> MyApp.Cldr.Unit.available_units
[:acre, :acre_foot, :ampere, :arc_minute, :arc_second, :astronomical_unit, :bit,
 :bushel, :byte, :calorie, :carat, :celsius, :centiliter, :centimeter, :century,
 :cubic_centimeter, :cubic_foot, :cubic_inch, :cubic_kilometer, :cubic_meter,
 :cubic_mile, :cubic_yard, :cup, :cup_metric, :day, :deciliter, :decimeter,
 :degree, :fahrenheit, :fathom, :fluid_ounce, :foodcalorie, :foot, :furlong,
 :g_force, :gallon, :gallon_imperial, :generic, :gigabit, :gigabyte, :gigahertz,
 :gigawatt, :gram, :hectare, :hectoliter, :hectopascal, :hertz, :horsepower,
 :hour, :inch, ...]

See h MyApp.Cldr.Unit and h MyApp.Cldr.Unit.to_string in iex for further information.

Localizing Dates, Times and DateTimes

Formatting of relative dates and date times is supported in the Cldr.DateTime.Relative module implemented in the ex_cldr_dates_times package. The public API is MyApp.Cldr.DateTime.to_string/2 and MyApp.Cldr.DateTime.Relative.to_string/2. Some examples:

iex> MyApp.Cldr.Date.to_string Date.utc_today()
{:ok, "Aug 18, 2017"}

iex> MyApp.Cldr.Time.to_string Time.utc_now
{:ok, "11:38:55 AM"}

iex> MyApp.Cldr.DateTime.to_string DateTime.utc_now
{:ok, "Aug 18, 2017, 11:39:08 AM"}

iex> MyApp.Cldr.DateTime.Relative.to_string 1, unit: :day, format: :narrow
{:ok, "tomorrow"}

iex> MyApp.Cldr.DateTime.Relative.to_string(1, unit: :day, locale: "fr")

iex> MyApp.Cldr.DateTime.Relative.to_string(1, unit: :day, format: :narrow)

iex> MyApp.Cldr.DateTime.Relative.to_string(1234, unit: :year)
"in 1,234 years"

iex> MyApp.Cldr.DateTime.Relative.to_string(1234, unit: :year, locale: "fr")
"dans 1 234 ans"

Gettext Backend Pluralization Support

There is an experimental plurals module for Gettext called Cldr.Gettext.Plural. It is configured in Gettext by:

    defmodule MyApp.Gettext do
      use Gettext, plural_forms: Cldr.Gettext.Plural

Cldr.Gettext.Plural will fall back to Gettext pluralisation if the locale is not known to Cldr. This module is only compiled if Gettext is configured as a dependency in your project.

Note that Cldr.Gettext.Plural does not guarantee to return the same plural index as Gettext's own pluralization engine which can introduce some compatibility issues if you plan to mix plural engines.


Cldr provides two plugs to aid integration into an HTTP workflow. These two plugs are:

See Cldr.Plug.SetLocale for a description of how to configure the plug.

In addition, note that when migrating from ex_cldr 1.x versions, a backend needs to be configured for both plugs. In the simplest case an example would be:

plug Cldr.Plug.SetLocale,
  apps:    [:cldr],
  cldr:    MyApp.Cldr

plug Cldr.Plug.AcceptLanguage,
  cldr_backend: MyApp.Cldr

Using Cldr.Plug.SetLocale without Phoenix

If you are using Cldr.Plug.SetLocale without Phoenix and you plan to use :path_param to identify the locale of a request then Cldr.Plug.SetLocale must be configured after plug :match and before plug :dispatch. For example:

defmodule MyRouter do
  use Plug.Router

  plug :match

  plug Cldr.Plug.SetLocale,
    apps: [:cldr, :gettext],
    from: [:path, :query],
    gettext: MyApp.Gettext,
    cldr: MyApp.Cldr

  plug :dispatch

  get "/hello/:locale" do
    send_resp(conn, 200, "world")

Using Cldr.Plug.SetLocale with Phoenix

If you are using Cldr.Plug.SetLocale with Phoenix and you plan to use the :path_param to identify the locale of a request then Cldr.Plug.SetLocale must be configured in the router module, not in the endpoint module. This is because conn.path_params has not yet been populated in the endpoint. For example:

defmodule MyAppWeb.Router do
  use MyAppWeb, :router

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
    plug Cldr.Plug.SetLocale,
        apps: [:cldr, :gettext],
        from: [:path, :query],
        gettext: MyApp.Gettext,
        cldr: MyApp.Cldr
    plug :fetch_flash
    plug :protect_from_forgery
    plug :put_secure_browser_headers

  scope "/:locale", HelloWeb do
    pipe_through :browser

    get "/", PageController, :index


About Language Tags and Locale strings

Note that Cldr defines locale strings according to the IETF standard as defined in RFC5646. Cldr also implements the u extension as defined in RFC6067 and the t extension defined in RFC6497. This is also the standard used by W3C.

The IETF standard is slightly different to the ISO/IEC 15897 standard used by Posix-based systems; primarily in that ISO 15897 uses a "_" separator whereas IETF and W3C use "-".

Locale string are case insensitive but there are common conventions:

  • Language codes are lower-cased
  • Territory codes are upper-cased
  • Script names are capital-cased

Locale extensions

Unicode defines the U extension which support defining the requested treatment of CLDR data formats. For example, a locale name can configure the requested:

  • calendar to be used for dates
  • collation
  • currency
  • currency format
  • number system
  • first day of the week
  • 12-hour or 24-hour time
  • time zone
  • and many other items

For example, the following locale name will request the use of the timezone Australia/Sydney, and request the use of accounting format when formatting currencies:

iex> MyApp.Cldr.validate_locale "en-AU-u-tz-ausyd-cf-account"
   canonical_locale_name: "en-Latn-AU",
   cldr_locale_name: "en-AU",
   extensions: %{},
   gettext_locale_name: "en",
   language: "en",
   language_subtags: [],
   language_variant: nil,
   locale: %{currency_format: :accounting, timezone: "Australia/Sydney"},
   private_use: [],
   rbnf_locale_name: "en",
   requested_locale_name: "en-AU",
   script: "Latn",
   territory: "AU",
   transform: %{}

The implementation of these extensions is governed by each library in the ex_cldr family. As of January 2020, ex_cldr_numbers version 2.10 implements the following U extension keys:

  • cf (currency format)
  • cu (currency)
  • nu (number system)

Other libraries in the family will progressively implement other extension keys.


  • A language code is an ISO3166 language code.
  • Potentially one or more modifiers separated by - (dash), not a _. (underscore). If you configure a Gettext module then Cldr will transliterate Gettext's _ into - for compatibility.
  • Typically the modifier is a territory code. This is commonly a two-letter uppercase combination. For example pt-PT is the locale referring to Portugese as used in Portugal.
  • In Cldr a locale name is always a binary and never an atom. Internally a locale is parsed and stored as a Cldr.LanguageTag struct.
  • The locales known to Cldr can be retrieved by Cldr.known_locale_names/1 to get the locales known to this configuration of Cldr and Cldr.all_locale_names/0 to get the locales available in the CLDR data repository.


Tests cover the full 566 locales defined in CLDR. Since Cldr attempts to maximize the work done at compile time in order to minimize runtime execution, the compilation phase for tests is several minutes.

Tests are run on Elixir 1.6 and later. Cldr will not run on Elixir versions before 1.6.

Developing ex_cldr

See the file