defmodule Map do
  def map([], _), do: []
  def map([head|tail], fun), do: [fun.(head) | map(tail, fun)]

  def reduce([], acumulator, _), do: acumulator
  def reduce([head|tail], acumulator, fun), do: reduce(tail, fun.(head, acumulator), fun)

  def mapsum(list, fun) do
    map(list, fun)
    |> reduce(0, &(&1 + &2))
  end

  def max([head|tail]) do
    compare = fn
      a,b when a > b -> a
      a,b when b >= a -> b
    end

    reduce([head|tail], head, compare)
  end
end

list = [1,2,3]
squares = Map.map(list, &(&1 * &1))
IO.puts Map.mapsum(squares, &(&1 + 1))
IO.puts Map.max(list)