lists.exs

defmodule Lists do
  def len([]), do: 0
  def len([ _head | tail ]), do: 1 + len(tail)

  def sum([]), do: 0
  def sum([h | t]), do: h + sum(t)

  def double([]), do: []
  def double([head | tail ]), do: [2*head | double(tail)]

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

  def sum_pairs([]), do: []
  def sum_pairs([ h1, h2 | t]), do: [ h1 + h2 | sum_pairs(t) ]

  def even_length?([]), do: true
  def even_length?([ _, _ | [_]]), do: false
  def even_length?([ _, _ | _]), do: true
end