exlist.exs

defmodule ExList do

  def span(from, to) when from < to do
    _span([], from, to)
  end

  defp _span(list, from, to) when from > to do
    list
  end

  defp _span(list, from, to) when from <= to do
    _span(list ++ [from], from + 1, to)
  end

  def max(list) do
    _max(list, 0)
  end

  defp _max([], highest) do
    highest
  end

  defp _max([head|tail], highest) when highest >= head do
    _max(tail, highest)
  end

  defp _max([head|tail], highest) when head > highest do
    _max(tail, head)
  end
  
  def mapsum(list, fun) do
    _mapsum(list, fun, 0)
  end

  defp _mapsum([], _, total) do
    total
  end

  defp _mapsum([head | tail], fun, total) do
    _mapsum(tail, fun, fun.(head) + total)
  end

end