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!
Execute the tests with:
$ mix test
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!"
end
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.
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}
end
# @tag :pending
test "initial balance is 0", %{account: account} do
assert BankAccount.balance(account) == 0
end
@tag :pending
test "incrementing and checking balance", %{account: account} do
assert BankAccount.balance(account) == 0
BankAccount.update(account, 10)
assert BankAccount.balance(account) == 10
end
@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
end
@tag :pending
test "closing account rejects further inquiries", %{account: account} do
assert BankAccount.balance(account) == 0
BankAccount.close_bank(account)
assert BankAccount.balance(account) == {:error, :account_closed}
assert BankAccount.update(account, 10) == {:error, :account_closed}
end
@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)
end)
receive do
:continue -> :ok
after
1000 -> flunk("Timeout")
end
assert BankAccount.balance(account) == 20
end
@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)
end
end
ExUnit.start()
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)
pid
end
@spec close_bank(account) :: none
def close_bank(account) do
GenServer.cast(account, :close)
end
@spec balance(account) :: integer | {:error, :account_closed}
def balance(account) do
GenServer.call(account, :balance)
end
@spec update(account, integer) :: integer | {:error, :account_closed}
def update(account, amount) do
GenServer.call(account, {:update, amount})
end
def init(:ok) do
{:ok, %{balance: 0}}
end
def handle_cast(:close, _) do
{:noreply, %{}}
end
def handle_call(:balance, _from, bank) do
{:reply, get_balance(bank), bank}
end
def handle_call({:update, amount}, _from, bank) do
{:ok, bank} = update_balance(bank, amount)
{:reply, get_balance(bank), bank}
end
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, %{}}
end
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