Introducing Elixir

Chapter 14. Using Macros to Extend Elixir

You have now learned enough Elixir to write interesting and fairly powerful programs. Sometimes, though, you need to extend the language itself in order to make your code easier to read or to implement some new functionality. Elixir’s macro feature lets you do this.

Functions versus Macros

On the surface, macros look a lot like functions, except that they begin with defmacro instead of def. However, macros work very differently than functions. The best way to explain the difference is to show you Example 14-1, which is in directory ch13/ex1-difference.

Example 14-1. Showing the difference between function and macro calls

defmodule Difference do

  defmacro m_test(x) do
    IO.puts("#{inspect(x)}")
    x
  end

  def f_test(x) do
    IO.puts("#{inspect(x)}")
    x
  end

end

In order to use a macro, you must require the module that it’s in. Type the following in the shell:

iex(1)> require Difference
Difference
iex(2)> Difference.f_test(1 + 3)
4
4
iex(3)> Difference.m_test(1 + 3)
{:+, [line: 3], [1, 3]}
4

Line 2 gives you exactly what you’d expect—Elixir evaluates 1 + 3 and passes it on to the f_test function, which prints the number 4 and returns the number 4 as its result.

Line 3 may be something of a surprise. Instead of an evaluated expression, the argument is a tuple that is the internal representation of the code before it is executed. The macro returns the tuple (in Elixir terms, the macro has been expanded) and then that tuple is passed on to Elixir ...