$handle_undefined_function in Elixir/Erlang

I was searching for ways to implement Ruby's method_missing in Elixir the other day. Then I ran into this package: andrewvy/method_missing: Elixir Library for dynamic code execution. And it implements exactly what I want: a method_missing function to dynamically generate function body for what's not defined yet.

defmodule Dog do
  use MethodMissing

  def method_missing(func, _args) do
    func_name = Atom.to_string(func)

    cond do
      Regex.match?(~r/bark|woof/, func_name) -> "WOOF"
      true -> "?"

# > "WOOF"
# > "WOOF"
# > "?"

I was curious about how it was implemented in Elixir, so I looked into the lib directory. Surprisingly, it's fairly simple:

defmodule MethodMissing do
  defmacro __using__(_opts) do
    quote do
      def method_missing(_func, _args) do

      def unquote(:"$handle_undefined_function")(func, args), do: method_missing(func, args)

      defoverridable method_missing: 2

From this implementation, we can know that we don't even need to use this package to get method_missing in Elixir. (The name method_missing is not very functional either.)

All we need to do is to define $handle_undefined_function/2. And just like method_missing, it will receive two arguments, one for the function name, and one for the list of arguments.

Again, I want to praise Elixir/Erlang when I discovered this feature. Almost every time I miss something in Ruby, Elixir already has a solution for that.

Although, I doubt that $handle_undefined_function will be as popular in the Elixir community as method_missing in the Ruby community.

  1. Elixir is a language that emphasize on explicity.

    Looking for behaviors that are hidden under $handle_undefined_function is a long trip.

    This kind of meta-programming would always be the last resort for an Elixir developer.

  2. $handle_undefined_function might not be as efficient as method_missing

    When we use method_missing, we usually use define_method to speed up future method calls.

    But Elixir, as a compiled language, doesn't have a function like define_method to generate new functions. So it may be much slower.

After all, I'm very glad that Elixir/Erlang provide us an option to do this kind of meta-programming. And I can't wait to see more surprises Elixir/Erlang bring to me in the future.