Elixir comprehension ideas

elixir_for_ideas.ex

# Idea 1: using $, $$...

{sections, _} =
  for section <- sections,
      let: [$section_counter: 1, $lesson_counter: 1] do
    $lesson_counter = if section["reset_lesson_position"], do: 1, else: $lesson_counter

    {lessons, $lesson_counter} =
      for lesson <- section["lessons"], let: [$$lesson_counter: $lesson_counter] do
        lesson = Map.put(lesson, "position", $$lesson_counter)
        $$lesson_counter = $$lesson_counter + 1
        lesson
      end

    section =
      section
      |> Map.put("lessons", lessons)
      |> Map.put("position", $section_counter)

    $section_counter = $section_counter + 1
    section
  end


# Idea 2: forcing to split as a separate function:

def get_sections do
  {sections, _} =
    for section <- sections,
        let: [$section_counter: 1, $lesson_counter: 1] do
      $lesson_counter = if section["reset_lesson_position"], do: 1, else: $lesson_counter

      {lessons, $lesson_counter} = get_lessons($lesson_counter)

      section =
        section
        |> Map.put("lessons", lessons)
        |> Map.put("position", $section_counter)

      $section_counter = $section_counter + 1
      section
    end

  sections
end

def get_lessons(lesson_counter) do
  for lesson <- section["lessons"], let: [$lesson_counter: lesson_counter] do
    lesson = Map.put(lesson, "position", $lesson_counter)
    $lesson_counter = $lesson_counter + 1
    lesson
  end
end


# Idea 3: compile error if forgot to re-bind $lesson_counter

{sections, _} =
  for section <- sections,
      let: [$section_counter: 1, $lesson_counter: 1] do
    $lesson_counter = if section["reset_lesson_position"], do: 1, else: $lesson_counter

    {lessons, $lesson_counter} =
      for lesson <- section["lessons"] do
        lesson = Map.put(lesson, "position", $lesson_counter)
        
        # COMPILE ERROR: $lesson_counter CANNOT BE REASSIGNED IN THIS SCOPE!
        # .. <explanation>
        $lesson_counter = $lesson_counter + 1
        
        lesson
      end

    section =
      section
      |> Map.put("lessons", lessons)
      |> Map.put("position", $section_counter)

    $section_counter = $section_counter + 1
    section
  end