How to write efficient combination algorithm with streams in Elixir?

Questions / Help
1

This is a function using comprehensions

  def combinations_of(_, 0), do: [[]]
  def combinations_of([], _), do: []

  def combinations_of([x | xt], n) do
    for(y <- combinations_of(xt, n - 1), do: [x | y]) ++ combinations_of(xt, n)
  end

but it eats a lot of memory (in a case of something like collection |> combinations_of(6)).

Replacing it with

  def combinations_of(_, 0), do: [[]]
  def combinations_of([], _), do: []

  def combinations_of([x | xt], n) do
    Stream.map(combinations_of(xt, n - 1), fn y ->
      [x | y]
    end)
    |> Stream.concat(combinations_of(xt, n))
  end

doesn’t help and eats even more memory (obviously, because of recursion).

How to solve this problem?