correct_macro.ex

defmodule Hygiene do
  defmacro interference do
    quote do: var!(a) + var!(b) + var!(c)
  end
end

defmodule HygieneTest do
  def go do
    require Hygiene
    a = 10
    b = 20
    c = 30
    Hygiene.interference
  end
end

error_macro.ex

defmodule Hygiene do
  defmacro no_interference do
    quote do: a + b + c
  end
end

defmodule HygieneTest do
  def go do
    require Hygiene
    a = 10
    b = 20
    c = 30
    Hygiene.no_interference
  end
end

log.ex

$ iex test1.ex
Interactive Elixir (0.9.0.dev) - press Ctrl+C to exit
Erlang R15B01 (erts-5.9.1) [source] [64-bit] [smp:8:8] [async-threads:0] [hipe] [kernel-poll:false]

iex> HygieneTest.go
60
iex> require Hygiene
[]
iex> Hygiene.interference
** (UndefinedFunctionError) undefined function: Elixir.IEx.a/0
    Elixir.IEx.a()
    erl_eval.erl:572: :erl_eval.do_apply/6
    erl_eval.erl:398: :erl_eval.expr/5
    src/elixir.erl:95: :elixir.eval_forms/3
    /private/tmp/homebrew-elixir-HEAD-DzdP/lib/elixir/iex.ex:63: Elixir.IEx.do_loop/1
iex> a = 1
1
iex> b = 2
2
iex> c = 3
3
iex> Hygiene.interference
6
iex> a = 10
10
iex> Hygiene.interference
15  # is 10 + 2 + 3
iex>
BREAK: