The Gettext module provides a gettext-based API for working with internationalized applications.

Using Gettext

To use Gettext, a module that calls use Gettext has to be defined:

defmodule MyApp.Gettext do
  use Gettext, otp_app: :my_app
end

This automatically defines some macros in the MyApp.Gettext module. Here are some examples:

import MyApp.Gettext

# Simple translation
gettext "Here is the string to translate"

# Plural translation
ngettext "Here is the string to translate",
         "Here are the strings to translate",
         3

# Domain-based translation
dgettext "errors", "Here is the error message to translate"

Translations are looked up from .po files. In the following sections we will explore exactly what are those files before we explore the “Gettext API” in detail.

Translations

Translations are stored inside PO (Portable Object) files, with a .po extension. For example, this is a snippet from a PO file:

# This is a comment
msgid "Hello world!"
msgstr "Ciao mondo!"

PO files containing translations for an application must be stored in a directory (by default it’s priv/gettext) that has the following struture:

gettext directory
└─ locale
   └─ LC_MESSAGES
      ├─ domain_1.po
      ├─ domain_2.po
      └─ domain_3.po

Here, locale is the locale of the translations (for example, en_US), LC_MESSAGES is a fixed directory, and domain_i.po are PO files containing domain-scoped translations. For more information on domains, check out the “Domains” section below.

A concrete example of such a directory structure could look like this:

priv/gettext
└─ en_US
|  └─ LC_MESSAGES
|     ├─ default.po
|     └─ errors.po
└─ it
   └─ LC_MESSAGES
      ├─ default.po
      └─ errors.po

By default, Gettext expects translations to be stored under the priv/gettext directory of an application. This behaviour can be changed by specifying a :priv option when using Gettext:

# Look for translations in my_app/priv/translations instead of
# my_app/priv/gettext
use Gettext, otp_app: :my_app, priv: "priv/translations"

The translations directory specified by the :priv option should be a directory inside priv/, otherwise some things (like mix compile.gettext) won’t work as expected.

Locale

At runtime, all gettext-related functions and macros that do not explicitely take a locale as an argument read the locale from Gettext.get_locale/1. The locale can be set with Gettext.put_locale/2. Locales are expressed as strings (like "en" or "fr"); they can be arbitrary strings as long as they match a directory name.

Gettext stores the locale per-process (in the process dictionary) and per Gettext module. This means that Gettext.put_locale/2 must be called in every new process in order to have the right locale available in that process. Pay attention to this behaviour, since not setting the locale with Gettext.put_locale/2 will not result in any errors when Gettext.get_locale/1 is called; the default locale will be returned instead.

Default locale

The default Gettext locale is "en". The value of the default locale for a given Gettext module can be set in the configuration for the :otp_app of that Gettext module. For example, in the config/config.exs file of the my_app application:

config :my_app, MyApp.Gettext, default_locale: "fr"

Gettext API

There are two ways to use gettext:

• using macros from your own gettext module, like MyApp.Gettext
• using functions from the Gettext module

These two approaches are different and each one has its own use case.

Using macros

Each module that calls use Gettext is usually referred to as a “Gettext backend”, as it implements the Gettext.Backend behaviour. When a module calls use Gettext, the following macros are automatically defined inside it:

• gettext/2
• dgettext/3
• ngettext/4
• dngettext/5

Supposing the caller module is MyApp.Gettext, the macros mentioned above behave as follows:

• gettext(msgid, bindings \\ %{}) - like Gettext.gettext(MyApp.Gettext, msgid, bindings)
• dgettext(domain, msgid, bindings \\ %{}) - like Gettext.dgettext(MyApp.Gettext, domain, msgid, bindings)
• ngettext(msgid, msgid_plural, n, bindings \\ %{}) - like Gettext.ngettext(MyApp.Gettext, msgid, msgid_plural, n, bindings)
• dngettext(domain, msgid, msgid_plural, n, bindings \\ %{}) - like Gettext.dngettext(MyApp.Gettext, domain, msgid, msgid_plural, n, bindings)

Using macros is preferred as gettext is able to automatically sync the translations in your code with PO files. This, however, imposes a constraint: arguments passed to any of these macros have to be strings at compile time. This means that they have to be string literals or somthing that expands to a string literal at compile time (e.g., a module attribute like @my_string "foo").

These are all valid uses of the gettext macros:

Gettext.put_locale MyApp.Gettext, "it"

MyApp.Gettext.gettext "Hello world"
#=> "Ciao mondo"

@msgid "Hello world"
MyApp.Gettext.gettext @msgid
#=> "Ciao mondo"

The gettext/dgettext/ngettext/dngettext macros raise an ArgumentError exception if they receive a msgid or a msgid_plural that doesn’t expand to a string at compile time:

msgid = "Hello world"
MyApp.Gettext.gettext msgid
#=> ** (ArgumentError) msgid must be a string literal

Using compile-time strings isn’t always possible. For this reason, the Gettext module provides a set of functions as well.

Using functions

If compile-time strings cannot be used, the solution is to use the functions in the Gettext module instead of the macros described above. These functions perfectly mirror the macro API, but they all expect a module name as the first argument. This module has to be a module which calls use Gettext. For example:

defmodule MyApp.Gettext do
  use Gettext, otp_app: :my_app
end

Gettext.put_locale MyApp.Gettext, "pt_BR"

msgid = "Hello world"
Gettext.gettext(MyApp.Gettext, msgid)
#=> "Olá mundo"

While using functions from the Gettext module yields the same results as using macros (with the added benefit of dynamic arguments), all the compile-time features mentioned in the previous section are lost.

Domains

The dgettext and dngettext functions/macros also accept a domain as one of the arguments. The domain of a translation is determined by the name of the PO file that contains that translation. For example, the domain of translations in the it/LC_MESSAGES/errors.po file is "errors", so those translations would need to be retrieved with dgettext or dngettext:

MyApp.Gettext.dgettext "errors", "Error!"
#=> "Errore!"

When gettext or ngettext are used, the "default" domain is used.

Interpolation

All *gettext functions and macros provided by gettext support interpolation. Interpolation keys can be placed in msgids or msgid_plurals with by enclosing them in %{ and }, like this:

"This is an %{interpolated} string"

Interpolation bindings can be passed as an argument to all of the *gettext functions/macros. For example, given the following PO file for the "it" locale:

msgid "Hello, %{name}!"
msgstr "Ciao, %{name}!"

interpolation can be done like follows:

Gettext.put_locale MyApp.Gettext, "it"
MyApp.Gettext.gettext "Hello, %{name}!", name: "Meg"
#=> "Ciao, Meg!"

Interpolation keys that are in a string but not in the provided bindings result in a Gettext.Error exception:

MyApp.Gettext.gettext "Hello, %{name}!"
#=> ** (Gettext.Error) missing interpolation keys: name

Keys that are in the interpolation bindings but that don’t occur in the string are ignored. Interpolations in gettext are often expanded at compile time, ensuring a low performance cost when running them at runtime.

Pluralization

Pluralization in gettext for Elixir works very similar to how pluralization works in GNU gettext. The *ngettext functions/macros accept a msgid, a msgid_plural and a count of elements; the right translation is chosen based on the pluralization rule for the given locale.

For example, given the following snippet of PO file for the "it" locale:

msgid "One error"
msgid_plural "%{count} errors"
msgstr[0] "Un errore"
msgstr[1] "%{count} errori"

the ngettext macro can be used like this:

Gettext.put_locale MyApp.Gettext, "it"
MyApp.Gettext.ngettext "One error", "%{count} errors", 3
#=> "3 errori"

The %{count} interpolation key is a special key since it gets replaced by the number of elements argument passed to *ngettext, like if the count: 3 key-value pair were in the interpolation bindings. Hence, never pass the count key in the bindings:

# `count: 4` is ignored here
MyApp.Gettext.ngettext "One error", "%{count} errors", 3, count: 4
#=> "3 errori"

You can specify a “pluralizer” module via the :plural_forms option in the configuration for the :gettext application.

# config/config.exs
config :gettext, plural_forms: MyApp.Plural

To learn more about pluralization rules, plural forms and what they mean to Gettext check the documentation for Gettext.Plural.

Missing translations

When a translation is missing in the specified locale (both with functions as well as with macros), the argument is returned:

• in case of calls to gettext/dgettext, the msgid argument is returned as is;
• in case of calls to ngettext/dngettext, the msgid argument is returned in case of a singular value and the msgid_plural is returned in case of a plural value (following the English pluralization rule).

For example:

Gettext.put_locale MyApp.Gettext, "foo"
MyApp.Gettext.gettext "Hey there"
#=> "Hey there"
MyApp.Gettext.ngettext "One error", "%{count} errors", 3
#=> "3 errors"

Empty translations

When a msgstr is empty (""), the translation is considered missing and the behaviour described above for missing translation is applied. A plural translation is considered to have an empty msgstr if at least one translation in the msgstr is empty.

Compile-time features

As mentioned above, using the gettext macros (as opposed to functions) allows gettext to operate on those translations at compile-time. This can be used to extract translations from the source code into POT files automatically (instead of having to manually add translations to POT files when they’re added to the source code). The gettext.extract does exactly this: whenever there are new translations in the source code, running gettext.extract syncs the existing POT files with the changed code base. Read the documentation for Mix.Tasks.Gettext.Extract for more information on the extraction process.

POT files are just template files and the translations in them do not actually contain translated strings. A POT file looks like this:

# The msgstr is empty
msgid "hello, world"
msgstr ""

Whenever a POT file changes, it’s likely that developers (or translators) will want to update the corresponding PO files for each locale. To do that, gettext provides the gettext.merge Mix task. For example, running:

mix gettext.merge priv/gettext --locale pt_BR

will update all the PO files in priv/gettext/pt_BR/LC_MESSAGES with the new version of the POT files in priv/gettext. Read more about the merging process in the documentation for Mix.Tasks.Gettext.Merge.

Finally, gettext is able to recompile modules that call use Gettext whenever PO files change. To enable this feature, the :gettext compiler needs to be added to the list of Mix compilers. In mix.exs:

def project do
  [compilers: [:gettext] ++ Mix.compilers]
end

Options

The following is a comprehensive list of options that can be passed to use Gettext.

defmodule MyApp.Gettext do
  use Gettext, # options
end

• :otp_app (required) - an atom representing an OTP application. Translations will be searched in directories inside this application’s directory (priv/gettext by default, see the :priv option).
• :priv - a string representing a directory where translations will be searched. The directory is relative to the directory of the application specified by the :otp_app option. It is recommended to always have this directory inside "priv", otherwise some features like the “mix compile.gettext” won’t work as expected.
• :plural_forms - a module which will act as a “pluralizer”. For more information, look at the documentation for Gettext.Plural.

Configuration

You can configure :gettext like any other application:

# config/config.exs
config :gettext,
  ...

The following is a list of the configuration options that :gettext supports:

• :fuzzy_threshold - the default threshold for the Jaro distance measuring the similarity of translations. Look at the documentation for the mix gettext.merge task (Mix.Tasks.Gettext.Merge) for more information on fuzzy translations.

• :excluded_refs_from_purging - a regex that is matched against translation references. Gettext will preserve all translations in all POT files that have a matching reference. You can use this pattern to prevent Gettext from removing translations that you have extracted using another tool.

Backend configuration

A Gettext backend such as MyApp.Gettext can be configured under the MyApp.Gettext key of the application specified in the :otp_app option for that backend. For example, the following backend:

defmodule MyApp.Gettext do
  use Gettext, otp_app: :my_app
end

can be configured like this:

# config/config.exs
config :my_app, MyApp.Gettext,
  default_locale: "fr"

The following is a list of the configuration options supported by each backend:

• :default_locale - a binary which specifies the default locale to use for the given backend.