ListAndRecursion.exs

defmodule ListAndRecursion do
  def all?([head | tail], filter) do
    if filter.(head) == true do
      pall?(tail, filter, true)
    else
      false
    end
  end
  defp pall?([head | tail], filter, _previous) do
    if filter.(head) == true do
      pall?(tail, filter, true)
    else
      false
    end
  end
  defp pall?([], _filter, previous) do
    previous
  end
  def filter([head | tail], filterfn) do
    if filterfn.(head) == true do
      [head | filter(tail, filterfn)]
    else
      filter(tail, filterfn)
    end
  end
  def filter([], _filterfn) do
    []
  end
end