defmodule Foo do
  import OK, only: ["~>>": 2]

  def double(a) do
    {:ok, 2 * a}
  end

  def foo(num) do
    # num = 3
    {:ok, num}
    ~>> double() # <<< complains on this line <<<<

    {:ok, 1}
    ~>> double() # <<< does NOT complain about this
  end
end