mylist.exs

defmodule MyList do
  def square([]), do: []
  def square([ head | tail]), do: [ head * head | square(tail) ]

  def add_1([]), do: []
  def add_1([ head | tail ]), do: [ head + 1 | add_1(tail) ]

  def map([], _func),           do: []
  def map([head | tail], func), do: [ func.(head) | map(tail, func) ]

  def reduce([], value, _), do: value
  def reduce([head | tail], value, func) do
    reduce tail, func.(head, value), func
  end

  def mapsum(list, func) do
    sum map(list, func)
  end

  def max([single]), do: single
  def max([head | tail]), do: Kernel.max(head, max(tail))

  def min([single]), do: single
  def min([head | tail]), do: Kernel.min(head, min(tail))

  def caesar(list, n) do
    map list, fn char ->
      rem(char - ?a + n, (?z - ?a + 1)) + ?a
    end
  end

  # with accumulator, but hidden.
  def sum(list), do: _sum(list, 0)

  defp _sum([], total), do: total
  defp _sum([head | tail], total), do: _sum(tail, head + total)

  # without accumulator.
  def sum([]), do: 0
  def sum([head | tail]), do: head + sum(tail)
end