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

Basic Overview

When you use Gettext, you replace hardcoded user-facing text like this:

"Hello world"

with calls like this:

gettext("Hello world")

Here, the string "Hello world" serves two purposes:

1. It's displayed by default (if no translation is specified in the current language). This means that, at the very least, switching from a hardcoded string to a Gettext call is harmless.

2. It serves as the message ID to which translations will be mapped.

An example translation workflow is as follows.

First, call mix gettext.extract to extract gettext() calls to .pot (Portable Object Template) files, which are the base for all translations. These files are templates, which means they only contain message IDs, and not actual translated strings. POT files have entries like this:

#: lib/myapp_web/live/hello_live.html.heex:2
#, elixir-autogen, elixir-format
msgid "Hello world"
msgstr ""

Then, call mix gettext.merge priv/gettext to update all locale-specific .po (Portable Object) files so that they include this message ID. Entries in PO files contain translations for their specific locale. For example, in a PO file for Italian, the entry above would look like this:

#: lib/myapp_web/live/hello_live.html.heex:2
#, elixir-autogen, elixir-format
msgid "Hello world"
msgstr "Ciao mondo"

The English string is the msgid which is used to look up the correct Italian string. That's handy, because unlike a generic key like site.greeting (as some translations systems use), the message ID tells exactly what needs to be translated. This is easier to work with for translators, for example.

But it raises a question: what if you change the original English string in the code? Does that break all translations, requiring manual edits everywhere? Not necessarily. After you run mix gettext.extract again, the next mix gettext.merge can do fuzzy matching. So, if you change "Hello world" to "Hello world!", Gettext will see that the new message ID is similar to an existing msgid, and will do two things:

1. It will update the msgid in all .po files to match the new text.

2. It will mark those entries as "fuzzy"; this hints that a (probably human) translator should check whether the Italian translation of this string needs an update.

The resulting change in the .po file is this (note the "fuzzy" annotation):

#: lib/myapp_web/live/hello_live.html.heex:2
#, elixir-autogen, elixir-format, fuzzy
msgid "Hello world!"
msgstr "Ciao mondo"

This "fuzzy matching" behavior can be configured or disabled, but its existence makes updating translations to match changes in the base text easier.

The rest of the documentation will cover the Gettext API in detail.

Gettext API

To use Gettext, a module that calls use Gettext (referred to below as a "backend") has to be defined:

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

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

import MyApp.Gettext

# Simple message
gettext("Here is the string to translate")

# Plural message
ngettext(
  "Here is the string to translate",
  "Here are the strings to translate",
  3
)

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

# Context-based message
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 %{}).

Messages 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.

Messages

Messages 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 messages 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 messages (for example, en_US), LC_MESSAGES is a fixed directory, and domain_i.po are PO files containing domain-scoped messages. 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 messages 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 messages in my_app/priv/messages instead of
# my_app/priv/gettext
use Gettext, otp_app: :my_app, priv: "priv/messages"

The messages directory specified by the :priv option should be a directory inside priv/, otherwise some things 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 and fall back to Gettext's default 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"

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 messages 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 messages 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 message is determined by the name of the PO file that contains that message. For example, the domain of messages in the it/LC_MESSAGES/errors.po file is "errors", so those messages 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".

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: "messages"

Contexts

The GNU Gettext implementation supports contexts, which are a way to contextualize messages. 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 an exception:

MyApp.Gettext.gettext("Hello, %{name}!")
#=> ** (Gettext.MissingBindingsError) ...

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 message 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 messages

When a message 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 messages

When a msgstr is empty (""), the message is considered missing and the behaviour described above for missing message is applied. A plural message is considered to have an empty msgstr if at least one message 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 messages at compile-time. This can be used to extract messages from the source code into POT (Portable Object Template) files automatically (instead of having to manually add messages to POT files when they're added to the source code). The gettext.extract does exactly this: whenever there are new messages 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 messages 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.

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 messages in.

The following is a comprehensive list of supported options:

• :priv - a string representing a directory where messages 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 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 messages. Look at the documentation for the mix gettext.merge task (Mix.Tasks.Gettext.Merge) for more information on fuzzy messages.

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

• :custom_flags_to_keep - a list of custom flags that will be kept for existing messages during a merge. Gettext always keeps the fuzzy flag. If you want to keep the elixir-format flag, which is also commonly used by Gettext, add it to this list. Available since v0.23.0.

• :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 messages or merging messages, and the ones already found in files will be discarded.

• :write_reference_line_numbers - a boolean that specifies whether file reference comments include line numbers when outputting PO(T) files. Defaults to true.

• :sort_by_msgid - modifies the sorting behavior. Can be either nil (the default), :case_sensitive, or :case_insensitive. By default or if nil, the order of existing messages in a POT file is kept and new messages are appended to the file. If :sort_by_msgid is set to :case_sensitive, existing and new messages will be mixed and sorted alphabetically by msgid. If set to :case_insensitive, the same applies but the sorting is case insensitive. Note: this option also supports true and false for backwards compatibility, but these values are deprecated as of v0.21.0.

• :on_obsolete - controls what happens when obsolete messages are found. If :mark_as_obsolete, messages are kept and marked as obsolete. If :delete, obsolete messages are deleted. Defaults to :delete.

• :store_previous_message_on_fuzzy_match - a boolean that controls whether to store the previous message text in case of a fuzzy match. Defaults to false.