costaraphael

defmodule Person do
  defstruct [:name, :age]

  def with do
    {Person.Builder, %{}}
  end

  defmodule Builder do
    def unquote(:'$handle_undefined_function')(:and_with, [{__MODULE__, _acc} = tuple]) do
      tuple

costaraphael

defmodule Permutations do
  def permutate([]), do: [[]]
  def permutate(list) do
    for n <- list,
        p <- permutate(list -- [n]),
        do: [n | p]
  end
end