Behaviour and default implementation for finding plural forms in given locales.

This module both defines the Gettext.Plural behaviour and provides a default implementation for it.

Plural forms

For a given language, there is a grammatical rule on how to change words depending on the number qualifying the word. Different languages can have different rules. [source]

Such grammatical rules define a number of plural forms. For example, English has two plural forms: one for when there is just one element (the singular) and another one for when there are zero or more than one elements (the plural). There are languages which only have one plural form and there are languages which have more than two.

In GNU Gettext (and in Gettext for Elixir), plural forms are represented by increasing 0-indexed integers. For example, in English 0 means singular and 1 means plural.

The goal of this module is to determine, given a locale:

• how many plural forms exist in that locale (nplurals/1);
• to what plural form a given number of elements belongs to in that locale (plural/2).

Default implementation

Gettext.Plural provides a default implementation of a plural module. Most languages used on Earth should be covered by this default implementation. If custom pluralization rules are needed (for example, to add additional languages) a different plural module can be specified when creating a Gettext backend. For example, pluralization rules for the Elvish language could be added as follows:

defmodule MyApp.Plural do
  @behaviour Gettext.Plural

  def nplurals("elv"), do: 3

  def plural("elv", 0), do: 0
  def plural("elv", 1), do: 1
  def plural("elv", _), do: 2

  # Fallback to Gettext.Plural
  def nplurals(locale), do: Gettext.Plural.nplurals(locale)
  def plural(locale, n), do: Gettext.Plural.plural(locale, n)
end

The mathematical expressions used in this module to determine the plural form of a given number of elements are taken from this page as well as from Mozilla's guide on "Localization and plurals".

Now that we have defined our custom plural forms, we can use them in two ways. You can set it for all :gettext backends in your config files:

config :gettext, :plural_forms, MyApp.Plural

Or to each specific backend:

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

Note: set :plural_forms in your config/config.exs and not in config/runtime.exs, as this configuration is read when compiling your backends.

Notice that tasks such as mix gettext.merge use the plural backend configured under the :gettext application, so generally speaking the first format is preferred.

Note some tasks also allow the number of plural forms to be given explicitly, for example:

mix gettext.merge priv/gettext --locale=gsw_CH --plural-forms=2

Unknown locales

Trying to call Gettext.Plural functions with unknown locales will result in a Gettext.Plural.UnknownLocaleError exception.

Language and territory

Often, a locale is composed as a language and territory couple, such as en_US. The default implementation for Gettext.Plural handles xx_YY by forwarding it to xx (except for just Brazilian Portuguese, pt_BR, which is not forwarded to pt as pluralization rules slightly differ). We treat the underscore as a separator according to ISO 15897. Sometimes, a dash - is used as a separator (for example BCP47 locales use this as in en-US): this is not forwarded to en in the default Gettext.Plural (and it will raise an Gettext.Plural.UnknownLocaleError exception if there are no translations for en-US).

Examples

An example of the plural form of a given number of elements in the Polish language:

iex> Plural.plural("pl", 1)
0
iex> Plural.plural("pl", 2)
1
iex> Plural.plural("pl", 5)
2
iex> Plural.plural("pl", 112)
2

As expected, nplurals/1 returns the possible number of plural forms:

iex> Plural.nplurals("pl")
3