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to Bank Account in the Elixir Track

Published at Dec 19 2019 · 0 comments
Test suite

Simulate a bank account supporting opening/closing, withdrawals, and deposits of money. Watch out for concurrent transactions!

A bank account can be accessed in multiple ways. Clients can make deposits and withdrawals using the internet, mobile phones, etc. Shops can charge against the account.

Create an account that can be accessed from multiple threads/processes (terminology depends on your programming language).

It should be possible to close an account; operations against a closed account must fail.


Run the test file, and fix each of the errors in turn. When you get the first test to pass, go to the first pending or skipped test, and make that pass as well. When all of the tests are passing, feel free to submit.

Remember that passing code is just the first step. The goal is to work towards a solution that is as readable and expressive as you can make it.

Have fun!

Running tests

Execute the tests with:

$ mix test

Pending tests

In the test suites, all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by commenting out the relevant @tag :pending with a # symbol.

For example:

# @tag :pending
test "shouting" do
  assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"

Or, you can enable all the tests by commenting out the ExUnit.configure line in the test suite.

# ExUnit.configure exclude: :pending, trace: true

If you're stuck on something, it may help to look at some of the available resources out there where answers might be found.

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


defmodule BankAccountTest do
  use ExUnit.Case

  setup do
    account = BankAccount.open_bank()
    {:ok, account: account}

  # @tag :pending
  test "initial balance is 0", %{account: account} do
    assert BankAccount.balance(account) == 0

  @tag :pending
  test "incrementing and checking balance", %{account: account} do
    assert BankAccount.balance(account) == 0
    BankAccount.update(account, 10)
    assert BankAccount.balance(account) == 10

  @tag :pending
  test "amount is added to balance", %{account: account} do
    assert BankAccount.balance(account) == 0
    BankAccount.update(account, 10)
    BankAccount.update(account, 10)
    assert BankAccount.balance(account) == 20

  @tag :pending
  test "closing account rejects further inquiries", %{account: account} do
    assert BankAccount.balance(account) == 0
    assert BankAccount.balance(account) == {:error, :account_closed}
    assert BankAccount.update(account, 10) == {:error, :account_closed}

  @tag :pending
  test "incrementing balance from another process then checking it from test process", %{
    account: account
  } do
    assert BankAccount.balance(account) == 0
    this = self()

    spawn(fn ->
      BankAccount.update(account, 20)
      send(this, :continue)

    receive do
      :continue -> :ok
      1000 -> flunk("Timeout")

    assert BankAccount.balance(account) == 20

  @tag :pending
  test "implementation for multiple account support", %{account: account} do
    assert is_pid(account)

    account_two = BankAccount.open_bank()
    assert is_pid(account_two)

    assert account != account_two

    BankAccount.update(account, 20)
    assert BankAccount.balance(account) != BankAccount.balance(account_two)


ExUnit.configure(exclude: :pending, trace: true)
defmodule BankAccount do
  use GenServer

  @opaque account :: pid

  @spec open_bank() :: account
  def open_bank() do
    {:ok, pid} = GenServer.start_link(__MODULE__, :ok)

  @spec close_bank(account) :: none
  def close_bank(account) do
    GenServer.cast(account, :close)

  @spec balance(account) :: integer | {:error, :account_closed}
  def balance(account) do
    GenServer.call(account, :balance)

  @spec update(account, integer) :: integer | {:error, :account_closed}
  def update(account, amount) do
    GenServer.call(account, {:update, amount})

  def init(:ok) do
    {:ok, %{balance: 0}}

  def handle_cast(:close, _) do
    {:noreply, %{}}

  def handle_call(:balance, _from, bank) do
    {:reply, get_balance(bank), bank}

  def handle_call({:update, amount}, _from, bank) do
    {:ok, bank} = update_balance(bank, amount)
    {:reply, get_balance(bank), bank}

  defp get_balance(%{balance: balance}), do: balance
  defp get_balance(_), do: {:error, :account_closed}

  defp update_balance(%{balance: balance} = bank, amount),
    do: {:ok, %{bank | balance: balance + amount}}
  defp update_balance(_, _),
    do: {:ok, %{}}

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