How to generate permutations from different sets of elements?

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12

Did you challenged a senior developer? :smiling_imp:

defmodule Example do
  def sample(list, opts \\ [skip: true])

  # uncomment if empty list is could be passed as an input
  def sample([], _opts), do: []

  # uncomment if empty list could be a first element
  def sample([[] | tail], opts), do: sample(tail, opts)

  # uncomment if list could contain only one element list
  def sample([list], _opts), do: Enum.map(list, &List.wrap/1)

  # in any other case
  def sample(list, opts) do
    if opts[:skip], do: sample_with_skip(list), else: sample_without_skip(list)
  end

  # skip empty elements in root list
  def sample_with_skip(list) do
    for sub_list <- list, reduce: [] do
      # skip empty elements in root list
      data when sub_list == [] ->
        data

      # passing a first element of root list as data
      [] ->
        sub_list

      data ->
        for sub_data <- data, element <- sub_list do
          [element | List.wrap(sub_data)]
        end
    end
    |> Enum.map(&Enum.reverse/1)
  end

  # alternatively do not skip empty elements in root list
  def sample_without_skip(list) do
    for sub_list <- list, reduce: [] do
      # when element is empty list set data to nil
      _data when sub_list == [] ->
        nil

      # no matter what happens later if data is nil keep it as is
      nil ->
        nil

      # passing a first element of root list as data
      [] ->
        sub_list

      data ->
        for sub_data <- data, element <- sub_list do
          [element | List.wrap(sub_data)]
        end
    end
    |> case do
      nil -> []
      list -> Enum.map(list, &Enum.reverse/1)
    end
  end
end

Pretty much the same as Enum.reduce/3 version. Simply pattern-matching here is not only in function clause, but also in for …, reduce: … clauseend notation.

However this looks like an art for art's sake. Just look how simple is raw pattern-matching comparing to 2 other versions of my solution. :smiley:

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