My attempt on mixing functional programming and good ol' OO in Ruby.

functional_cart.rb

Item = Struct.new(:amount, :price, :name) do
  def total_price
    amount * price
  end
end

Cart = Struct.new(:items) do
  # total amount is finding sum of item amount
  def total_amount
    # a.k.a finding sum by item.amount function
    sum_by_fuction(->(item) { item.amount })
  end

  # total price is finding sum of each item total price
  def total_price
    # a.k.a finding sum by item.total_price function
    sum_by_fuction(->(item) { item.total_price })
  end

  def average_price
    total_price / total_amount.to_f
  end

  # finding sum by function
  def sum_by_fuction(fn)
    items.reduce(0) { |sum, item| sum + fn.(item) }
  end
end

def main
  items = [Item.new(2, 20, 'apple'), Item.new(1, 30, 'orange')]
  cart = Cart.new(items)
  puts cart.items
  puts cart.total_amount
  puts cart.total_price
  puts cart.average_price
end

main