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

I’d put this code here for the sake of discoverability.

defmodule P do
  defmacro permutations(l, n) do
    clause =
      fn i -> {:<-, [], [{:"i#{i}", [], Elixir}, l]} end
    return =
      Enum.map(1..n, fn i -> {:"i#{i}", [], Elixir} end)
    Enum.reduce(1..n, return, fn i, acc ->
      {:for, [], [clause.(i), [do: acc]]}
    end)
  end
end

defmodule T do
  require P

  def permute3(list), do: P.permutations(list, 3)
end

It uses plain AST to return the nested list comprehensions.

T.permute3 ~w|a b|  
#⇒ [
#    [[["a", "a", "a"], ["b", "a", "a"]],
#     [["a", "b", "a"], ["b", "b", "a"]]],
#    [[["a", "a", "b"], ["b", "a", "b"]],
#     [["a", "b", "b"], ["b", "b", "b"]]]
#  ]

It would require more effort to make it possible to pass n through as a parameter, because of early Range expansion, but it’s still doable.