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")

# Context-based translation
pgettext("email", "Email text to translate")

# All of the above
dpngettext(
  "errors",
  "context",
  "Here is the string to translate",
  "Here are the strings to translate",
  3
)

The arguments for the Gettext macros and their order can be derived from their names. For dpgettext/4 the arguments are: domain, context, msgid, bindings (default to %{}).

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 structure:

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 explicitly take a locale as an argument read the locale from the backend locale and fallbacks to Gettext's locale.

Gettext.put_locale/1 can be used to change the locale of all backends for the current Elixir process. That's the preferred mechanism for setting the locale at runtime. Gettext.put_locale/2 can be used when you want to set the locale of one specific Gettext backend without affecting other Gettext backends.

Similarly, Gettext.get_locale/0 gets the locale for all backends in the current process. Gettext.get_locale/1 gets the locale of a specific backend for the current process. Check their documentation for more information.

Locales are expressed as strings (like "en" or "fr"); they can be arbitrary strings as long as they match a directory name. As mentioned above, the locale is stored per-process (in the process dictionary): this means that the locale must be set in every new process in order to have the right locale available for that process. Pay attention to this behaviour, since not setting the locale will not result in any errors when Gettext.get_locale/0 or Gettext.get_locale/1 are called; the default locale will be returned instead.

To decide which locale to use, each gettext-related function in a given backend follows these steps:

• if there is a backend-specific locale for the given backend for this process (see put_locale/2), use that, otherwise
• if there is a global locale for this process (see put_locale/1), use that, otherwise
• if there is a backend-specific default locale in the configuration for that backend's :otp_app (see the "Default locale" section below), use that, otherwise
• use the default global Gettext locale (see the "Default locale" section below)

Default locale

The global Gettext default locale can be configured through the :default_locale key of the :gettext application:

config :gettext, :default_locale, "fr"

By default the global locale is "en". See also get_locale/0 and put_locale/1.

If for some reason a backend requires a different :default_locale than all other backends, you can set the :default_locale inside the backend configuration, but this approach is generally discouraged as it makes it hard to track which locale each backend is using:

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

Default Domain

Each backend can be configured with a specific :default_domain that replaces "default" in gettext/2, pgettext/3, and ngettext/4 for that backend.

defmodule MyApp.Gettext do
  use Gettext, otp_app: :my_app, default_domain: "messages"
end

config :my_app, MyApp.Gettext, default_domain: "translations"

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
• pgettext/3
• dpgettext/4
• ngettext/4
• dngettext/6
• pngettext/6
• dpngettext/6
• all macros above with a _noop suffix (and without accepting bindings), for example pgettext_noop/2

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)

• pgettext(msgctxt, msgid, bindings \\ %{}) - like Gettext.pgettext(MyApp.Gettext, msgctxt, msgid, bindings)

• dpgettext(domain, msgctxt, msgid, bindings \\ %{}) - like Gettext.dpgettext(MyApp.Gettext, domain, msgctxt, 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)

• pngettext(msgctxt, msgid, msgid_plural, n, bindings \\ %{}) - like Gettext.pngettext(MyApp.Gettext, msgctxt, msgid, msgid_plural, n, bindings)

• dpngettext(domain, msgctxt, msgid, msgid_plural, n, bindings \\ %{}) - like Gettext.dpngettext(MyApp.Gettext, domain, msgctxt, msgid, msgid_plural, n, bindings)

• *_noop family of functions - used to mark translations for extraction without translating them. See the documentation for these macros in Gettext.Backend

See also the Gettext.Backend behaviour for more detailed documentation about these macros.

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 something that expands to a string literal at compile time (for example, 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 macros raise an ArgumentError exception if they receive a domain, msgctxt, msgid, or 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 backend gettext, ngettext, or pgettext are used, the backend's default domain is used (which defaults to "default"). The Gettext functions accepting a backend (gettext/3, ngettext/5, and pgettext/4) always use a domain of "default".

Contexts

The GNU Gettext implementation supports contexts, which are a way to contextualize translations. For example, in English, the word "file" could be used both as a noun as well as a verb. Contexts can be used to solve similar problems: you could have a imperative_verbs context and a nouns context as to avoid ambiguity. The functions that handle contexts have a p in their name (to match the GNU Gettext API), and are pgettext, dpgettext, pngettext, and dpngettext. The "p" stands for "particular".

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 each Gettext backend.

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

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/pgettext/dpgettext, the msgid argument is returned as is;
• in case of calls to ngettext/dngettext/pngettext/dpngettext, 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

Configuration

:gettext configuration

The :gettext application supports the following configuration options:

• :default_locale - a string which specifies the default global Gettext locale to use for all backends. See the "Locale" section for more information on backend-specific, global, and default locales.

Backend configuration

A Gettext backend supports some options to be configured. These options can be configured in two ways: either by passing them to use Gettext (hence at compile time):

defmodule MyApp.Gettext do
  use Gettext, options
end

or by using Mix configuration, configuring the key corresponding to the backend in the configuration for your application:

# For example, in config/config.exs
config :my_app, MyApp.Gettext, options

Note that the :otp_app option (an atom representing an OTP application) has to always be present and has to be passed to use Gettext because it's used to determine the application to read the configuration of (:my_app in the example above); for this reason, :otp_app can't be configured via the Mix configuration. This option is also used to determine the application's directory where to search translations in.

The following is a comprehensive list of supported options:

• :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. By default it's "priv/gettext".

• :plural_forms - a module which will act as a "pluralizer". For more information, look at the documentation for Gettext.Plural.

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

• :split_module_by - instead of bundling all locales into a single module, this option makes Gettext build internal modules per locale, per domain, or both. This reduces compilation times and beam file sizes for large projects. For example: split_module_by: [:locale, :domain].

• :split_module_compilation - control if compilation of split modules should happen in :parallel (the default) or :serial.

• :allowed_locales - a list of locales to bundle in the backend. Defaults to all the locales discovered in the :priv directory. This option can be useful in development to reduce compile-time by compiling only a subset of all available locales.

• :interpolation - the name of a module that implements the Gettext.Interpolation behaviour. Default: Gettext.Interpolation.Default

Mix tasks configuration

You can configure Gettext Mix tasks under the :gettext key in the configuration returned by project/0 in mix.exs:

def project() do
  [app: :my_app,
   # ...
   gettext: [...]]
end

The following is a list of the supported configuration options:

• :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.

• :compiler_po_wildcard - a binary that specifies the wildcard that the :gettext compiler will use to find changed PO files in order to recompile their respective Gettext backends. This wildcard has to be relative to the "priv" directory of your application. Defaults to "gettext/*/LC_MESSAGES/*.po".

• :write_reference_comments - a boolean that specifies whether reference comments should be written when outputting PO(T) files. If this is false, reference comments will not be written when extracting translations or merging translations, and the ones already found in files will be discarded.

• :sort_by_msgid - a boolean that modifies the sorting behavior. By default, the order of existing translations in a POT file is kept and new translations are appended to the file. If :sort_by_msgid is set to true, existing and new translations will be mixed and sorted alphabetically by msgid.