Behaviour for custom MF2 formatting functions.

Implement this behaviour to add domain-specific formatting functions to Localize.Message.format/3. Custom functions can be registered in two ways:

Per-call registration

Pass a :functions map in the options to Localize.Message.format/3:

Localize.Message.format(
  "{$name :personName format=long}",
  %{"name" => person},
  locale: :en,
  functions: %{"personName" => MyApp.PersonNameFunction}
)

Application-level registration

# config/config.exs
config :localize, :mf2_functions, %{
  "personName" => MyApp.PersonNameFunction,
  "money"      => MyApp.MoneyFunction
}

Application-level functions are available to all Localize.Message.format/3 calls without passing :functions on every call. Per-call functions take precedence over application-level functions of the same name, which in turn take precedence over built-in functions.

Implementing a custom function

defmodule MyApp.PersonNameFunction do
  @behaviour Localize.Message.Function

  @impl true
  def format(value, func_opts, options) do
    locale = Keyword.get(options, :locale)
    format = func_opts["format"] || "medium"
    MyApp.PersonName.to_string(value, locale: locale, format: String.to_atom(format))
  end
end

The format/3 callback receives:

• value — the resolved operand from the MF2 expression.

• func_opts — a map of MF2 function options parsed from the expression (e.g. %{"format" => "long", "formality" => "formal"}). Keys and values are strings.

• options — the interpreter's keyword list, which includes at least :locale and :bindings.