This seems to be doing the trick:

Enum.reduce(list, fn acc, sublist ->
  for a <- sublist, b <- acc do
    List.flatten([a, b])
  end
end)

[
  [:a, 1, {4, 5}],
  [:a, 1, {5, 6}],
  [:a, 2, {4, 5}],
  [:a, 2, {5, 6}],
  [:a, 3, {4, 5}],
  [:a, 3, {5, 6}],
  [:b, 1, {4, 5}],
  [:b, 1, {5, 6}],
  [:b, 2, {4, 5}],
  [:b, 2, {5, 6}],
  [:b, 3, {4, 5}],
  [:b, 3, {5, 6}]
]

Unless the elements of some of the sublists are lists themselves, cause in that case they would be flattened.

UPDATE: This one below should cover all cases, just like @Eiji’s solution

def magic_function([]), do: []

def magic_function(list) do
  list
  |> Enum.reduce(fn sublist, acc ->
    for a <- acc, b <- sublist do
      [b | List.wrap(a)]
    end
  end)
  |> Enum.map(&Enum.reverse/1)
end

Ah, good catch! Thanks.

I beg to differ. What about this one:

def magic_function([]), do: []

def magic_function(list) do
  list
  |> Enum.reduce([nil], fn sublist, acc ->
    for a <- acc, b <- sublist do
      [b | List.wrap(a)]
    end
  end)
  |> Enum.map(&Enum.reverse/1)
end

I think it covers all cases now. The initial accumulator of [nil] for the reduce handles the case when there’s only one sublist: the sublist’s elements are prepended to List.wrap(nil), which is an empty list.

So it appears recursion is not necessary after all

Did you challenged a senior developer?

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.