10-8.tax_rate.exs

defmodule TaxRate do
  @tax_rates [ NC: 0.075, TX: 0.08 ]

  def calc(list) do
    for x <- list do
      total = x[:net_amount] * (1 + (@tax_rates[x[:ship_to]] || 0))
      x ++ [total_amount: total]
    end
  end
end

orders = [
  [ id: 123, ship_to: :NC, net_amount: 100.00 ],
  [ id: 124, ship_to: :OK, net_amount: 35.50 ],
  [ id: 125, ship_to: :TX, net_amount: 24.00 ],
  [ id: 126, ship_to: :TX, net_amount: 44.80 ],
  [ id: 127, ship_to: :NC, net_amount: 25.00 ],
  [ id: 128, ship_to: :MA, net_amount: 10.00 ],
  [ id: 129, ship_to: :CA, net_amount: 102.00 ],
  [ id: 130, ship_to: :NC, net_amount: 50.00 ]
]

IO.inspect(TaxRate.calc(orders))