Mastering the Art of Programming in Elixir/

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Macros Are Hygienic

Macros Are Hygienic

Learn more about macros and their scope in Elixir.

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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:

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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,
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