View Source Getting Started with Cldr

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If you use Cldr.Plug.PutLocale

Please note that as of ex_cldr version 2.29.9, the CLDR-based plugs have been extracted to their own ex_cldr_plugs library. you will need to add that dependency to your application. For example:

# in mix.exs
def deps do
    {:ex_cldr, "~> 2.37"},
    {:ex_cldr_plugs, "~> 1.2"},



ex_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 and parsing of numbers, lists, currencies, calendars, units of measure and dates/times. As of April 28th 2023 and ex_cldr Version 2.37.0, ex_cldr is based upon CLDR version 43.0.

The first step is to define a module that will host the desired ex_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:

defmodule MyApp.Cldr do
  @moduledoc """
  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.
  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:


Use Case

Use this library if you need to:

  • Support multiple languages and locales in your application

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

  • Need to 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 1.11 or later.



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

defp deps do
    {:ex_cldr, "~> 2.37"},
    # Poison 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.


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 Cldr will look for additional configuration, defined under the key :my_app with the sub-key MyApp.Cldr. For example:

# cldr.ex
defmodule MyApp.Cldr do
  use Cldr,
    otp_app: :my_app,
    default_locale: "en",
    gettext: MyApp.Gettext,
    json_library: Jason,
    data_dir: "./priv/cldr",
    precompile_number_formats: ["¤¤#,##0.##"],
    providers: [Cldr.Number]
# config/config.exs
config :my_app, MyApp.Cldr,
  # a single locale, for fast compilation in dev / test
  locales: ["en"]
# config/production.exs
config :my_app, MyApp.Cldr,
  # these will take a while to compile
  locales: ["fr", "en", "bs", "si", "ak", "th"],
  precompile_transliterations: [{:latn, :arab}, {:thai, :latn}]

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


Global configuration.

In config.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, :default_locale, :default_backend, :cacertfile, :data_dir, :force_locale_download 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.


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 convenience 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: specifies the name of a Gettext module that informs Cldr to use that 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. For example:

 use Cldr,
   default_locale: "en",
   gettext: MyApp.Gettext,
   locales: ["en-*", "fr"]
  • :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.

  • :default_currency_format determines whether Cldr.Number.to_string/2 will use :currency or :accounting if no format is specified but a currency is. The default is nil which means that the format will be derived from the locale.

  • :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.

  • :suppress_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 reproducible 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_currencyCldr.CurrencyCurrency definitions and localizations
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_cldr_locale_displayCldr.LocaleDisplayLocalising locale names
ex_cldr_routesCldr.RouteLocalized routes and route helpers
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 Locales

Cldr can be installed from either github or from hex.

  • If installed from github then all 571 locales are installed when the repo is cloned into your application deps.

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


Localizing 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 formatting 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

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 Pluralization

gettext allows for user-defined plural forms modules to be configured for a gettext backend.

To define a plural forms module that uses CLDR plural rules create a new module and then use Cldr.Gettext.Plural. For example:

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

This module can then be used in the configuration of a gettext backend. For example:

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

Note that MyApp.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. See Cldr.Gettext.Plural for more information.


About Language Tags

Note that ex_cldr defines locale strings according to the IETF standard as defined in RFC5646. ex_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
  • All other subtags are lower-cased



As of ex_cldr version 2.23.0, a sigil is available to simplify creating t:Cldr.LanguageTag structs. Usage is:

iex> import Cldr.LanguageTag.Sigil

# Returns a locale that is valid and known to
# the default backend module
iex> ~l(en-US)
#Cldr.LanguageTag<en-US [validated]>

# Same, but specifying the backend module
# MyApp.Cldr specifically
iex> ~l(en-US|MyApp.Cldr)
#Cldr.LanguageTag<en-US [validated]>

# The `u` flag will parse and validate
# the language tag but it may not be known
# as a configured locale
iex> ~l(zh)u
#Cldr.LanguageTag<zh [canonical]>

# Language tags can convey a lot more information
# than might be initially expected!
iex> ~l(en-u-ca-ethiopic-cu-aud-sd-gbsct-t-d0-lower-k0-extended-m0-ungegn-x-ux)
#Cldr.LanguageTag<en-t-d0-lower-k0-extended-m0-ungegn-u-ca-ethiopic-cu-aud-sd-gbsct-x-ux [validated]>


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_variants: nil,
   locale: %Cldr.LanguageTag.U{cf: :account, 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 ISO-3166 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 Portuguese as used in Portugal.
  • In ex_cldr a locale name is always a binary and never an atom. Internally a locale is parsed and stored as a t:Cldr.LanguageTag struct.
  • The locales known to ex_cldr can be retrieved by Cldr.known_locale_names/1 to get the locales known to this configuration of ex_cldr and Cldr.all_locale_names/0 to get the locales available in the CLDR data repository.


Developing ex_cldr

See the file in the github repository.



Tests cover the full ~700 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 1.11 and later. ex_cldr is not supported on Elixir versions before 1.11.