How can I get permutations of a list?

Question

How can I get permutations of a list in Elixir?

Eg, for [\"a\", \"b\", \"c\"], I would expect:

# [[\"a\", \"b\", \"c\"], [\"a\", \"c\",          


        

Answer 1

Here's a version without comprehensions:

defmodule Permute do
  def permute(_chars, building, 0) do
    [building]
  end

  def permute(chars, building, dec) do
    Stream.map(chars, fn char -> building ++ [char] end)
    |> Enum.flat_map(fn building -> permute(chars, building, dec - 1) end)
  end
end

Useful to have a helper function to allow the input as a string too:

def permute(str) do
  permute(String.split(str, "", trim: true), [], String.length(str))
end