Programming Elixir ≥ 1.6

Macros Are Hygienic

It is tempting to think of macros as some kind of textual substitution—a macro’s body is expanded as text and then compiled at the point of call. But that’s not the case. Consider this example:

macros/hygiene.ex

defmodule Scope do
  defmacro update_local(val) do
    local = "some value"
    result = quote do
      local = unquote(val)
      IO.puts "End of macro body, local = #{local}"
    end
    IO.puts "In macro definition, local = #{local}"
    result
  end
end

defmodule Test do
  require Scope

  local = 123
  Scope.update_local("cat")
  IO.puts "On return, local = #{local}"
end

Here’s the result of running that code:

In macro definition, local = some value
End of ...