replacing command pattern with functions in Elixir

command.exs

defmodule CashRegister do
  def new do
    Agent.start_link(fn -> 0 end)
  end
  
  def add(pid, amount) do
    Agent.update(pid, fn(register) -> register + amount end)
  end
  
  def reset(pid) do
    Agent.update(pid, fn(register) -> 0 end)
  end
  
  def get(pid) do
    Agent.get(pid, fn(register) -> register end)
  end
end

ExUnit.start

defmodule CashRegisterTest do
  use ExUnit.Case
  test "cashier" do
    {:ok, pid} = CashRegister.new
    assert CashRegister.get(pid) == 0
    CashRegister.add(pid, 100)
    assert CashRegister.get(pid)== 100
    CashRegister.add(pid, 20)
    assert CashRegister.get(pid)== 120
    CashRegister.reset(pid)
    assert CashRegister.get(pid)== 0
  end
end