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Mastering the Art of Programming in Elixir

Macros Are Hygienic

Explore how hygienic macros in Elixir maintain distinct scopes that prevent variable conflicts during code expansion. Learn to use macros safely without overwriting variables in modules and functions.

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Introduction

It’s 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:

Elixir
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
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