All permutations

Permutations module

defmodule Permutations do
  def all(list) do
    1..length(list)
    |> Enum.reduce([], fn n, acc ->
      acc ++ all(list, n)
    end)
  end

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

  def all(list, n) do
    for x <- list,
        rest = list -- [x],
        perm <- all(rest, n - 1),
        do: [x | perm]
  end
end

Usage example

list = [1, 2, 3, 4]

Permutations.all(list, 2)

Permutations.all(list, 3)

Permutations.all(list)