What Is A GenServer?

1.exs

# test/account_test.exs
defmodule AccountTest do
  use ExUnit.Case

  test "fails" do
    assert true == false
  end
end

10.ex

defmodule Account do
  # ...

  def get_balance(pid) do
    
  end
end

11.ex

defmodule Account do
  # ...

  def get_balance(pid) do
    0
  end
end

12.ex

defmodule Account do
  # ...

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

  # ...
end

13.ex

defmodule Account do
   # ...

  def get_balance(pid) do
    GenServer.call(pid, :get_balance)
  end
end

14.ex

defmodule Account do
  # ...

  def handle_call(:get_balance, _from, state) do
    {:reply, Map.get(state, :balance), state} 
  end
end

15.exs

defmodule AccountTest do
  use ExUnit.Case

  # ...

  test 'depositing money changes the balance' do
    {:ok, pid} = Account.start_link

    Account.deposit(pid, 10.0)
  end
end

16.ex

defmodule Account
  def deposit(pid, amount) do

  end
end

17.exs

defmodule AccountTest do
  # ...

  test 'depositing money changes the balance' do
    {:ok, pid} = Account.start_link

    Account.deposit(pid, 10.0)

    assert 10.0 == Account.get_balance(pid)
  end
end

18.ex

defmodule Account do
  # ...

  def deposit(pid, amount) do
    GenServer.cast(pid, {:deposit, amount})
  end
end

19.ex

defmodule Account do
  # ...

  def handle_cast({:deposit, amount}, state) do

  end
end

20.ex

defmodule Account do
  # ...

  def handle_cast({:deposit, amount}, state) do
    {:noreply, state}
  end
end

21.ex

defmodule Account do
  # ...

  def handle_cast({:deposit, amount}, state) do
    balance = Map.get(state, :balance)
    {:noreply, Map.put(state, :balance, balance + amount)}
  end
end

22.ex

defmodule Account do
  # ...
  def handle_cast({:deposit, amount}, state) do
    {_value, balance_with_deposit} =
      Map.get_and_update(state, :balance, fn balance ->
        {balance, balance + amount}
      end)

    {:noreply, balance_with_deposit}
  end
end

23.exs

defmodule AccountTest do
  # ...

  test 'withdraw money reduces the balance' do
    {:ok, pid} = Account.start_link()

    Account.withdraw(pid, 52.34)
  end
end

24.ex

defmodule Account do
  # ...

  def withdraw(pid, amount) do
  end

  # ...
end

25.exs

defmodule AccountTest do
  # ...

  test 'withdraw money reduces the balance' do
    {:ok, pid} = Account.start_link()

    Account.withdraw(pid, 52.34)

    assert -52.34 == Account.get_balance(pid)
  end
end

26.ex

defmodule Account do
  # ...

  def withdraw(pid, amount) do
    GenServer.cast(pid, {:withdraw, amount}) 
  end

  # ...
end

27.ex

defmodule Account do
  # ...
  def handle_cast({:deposit, amount}, state) do
    # ...
  end
  def handle_cast({:withdraw, amount}, state) do
    {:noreply, state}
  end
end

28.ex

defmodule Account do
  # ...

  def handle_cast({:deposit, amount}, state) do
    {_value, balance_with_deposit} =
      Map.get_and_update(state, :balance, fn balance ->
        {balance, balance + amount}
      end)

    {:noreply, balance_with_deposit}
  end
  def handle_cast({:withdraw, amount}, state) do
    {_value, balance_with_deposit} =
      Map.get_and_update(state, :balance, fn balance ->
        {balance, balance - amount}
      end)

    {:noreply, balance_with_deposit}
  end
end

29.ex

defmodule Account do
  # ...

  def handle_cast({:deposit, amount}, state) do
    # Extracted anonymous function
    update_balance = fn balance, amount -> balance + amount end
    {_value, balance_with_deposit} =
      Map.get_and_update(state, :balance, fn balance ->
        # we now call the function instead of doing a calculation
        {balance, update_balance.(balance, amount)} 
      end)

    {:noreply, balance_with_deposit}
  end
  # similar changes were made to this function
  def handle_cast({:withdraw, amount}, state) do
    update_balance = fn balance, amount -> balance - amount end
    {_value, balance_with_deposit} =
      Map.get_and_update(state, :balance, fn balance ->
        {balance,  update_balance.(balance, amount)}
      end)

    {:noreply, balance_with_deposit}
  end
end

3.exs

defmodule AccountTest do
  use ExUnit.Case

  test "initial balance is zero" do
    {:ok, pid} = Account.start_link
  end
end

30.ex

defmodule AccountTest do
  use ExUnit.Case

  test "initial balance is zero" do
    {:ok, pid} = Account.start_link()

    assert 0 == Account.get_balance(pid)
  end

  test 'depositing money changes the balance' do
    {:ok, pid} = Account.start_link()

    Account.deposit(pid, 10.0)

    assert 10.0 == Account.get_balance(pid)
  end

  test 'withdraw money reduces the balance' do
    {:ok, pid} = Account.start_link()

    Account.withdraw(pid, 52.34)

    assert -52.34 == Account.get_balance(pid)
  end
end


defmodule Account do
  # ...

  def handle_cast({:deposit, amount}, state) do
    update_balance = fn balance, amount -> balance + amount end
    # variable 'update_balance' changed to 'updated_balance'
    {_value, updated_balance} =
      Map.get_and_update(state, :balance, fn balance ->
        {balance, update_balance.(balance, amount)}
      end)

    # variable name changed 
    {:noreply, updated_balance}
  end
  def handle_cast({:withdraw, amount}, state) do
    update_balance = fn balance, amount -> balance - amount end
    # variable name changed 
    {_value, updated_balance} =
      Map.get_and_update(state, :balance, fn balance ->
        {balance,  update_balance.(balance, amount)}
      end)

    # variable name changed
    {:noreply, updated_balance}
  end
end

31.ex

defmodule Account do
  # ...
  def handle_cast({:withdraw, amount}, state) do
    update_balance = fn balance, amount -> balance - amount end
    {:noreply, change_balance(state, amount, update_balance)}
  end

  # extracted function
  defp change_balance(state, amount, update_balance) do
    {_value, updated_balance} =
      Map.get_and_update(state, :balance, fn balance ->
        {balance,  update_balance.(balance, amount)}
      end)
    updated_balance
  end
end

32.ex

defmodule Account do
  # ...
  def handle_cast({:withdraw, amount}, state) do
    {:noreply, change_balance(state, amount, &(&1 - &2))}
  end
end

33.ex

defmodule Account do
  # ...
  defp change_balance(state, amount, calculate_balance) do
    {_value, updated_balance} =
      Map.get_and_update(state, :balance, fn balance ->
        {balance,  calculate_balance.(balance, amount)}
      end)
    updated_balance
  end
end

34.ex

defmodule Account do
  # ...
  def handle_cast({:withdraw, amount}, state) do
    {:noreply, get_updated_balance(state, amount, &(&1 - &2))}
  end

  defp get_updated_balance(state, amount, calculate_balance) do
    {_value, updated_balance} =
      Map.get_and_update(state, :balance, fn balance ->
        {balance,  calculate_balance.(balance, amount)}
      end)
    updated_balance
  end
end

35.ex

defmodule Account do
  # ...

  # notice the only different is in the anonymous function 
  def handle_cast({:deposit, amount}, state) do
    {:noreply, get_updated_balance(state, amount, &(&1 + &2))}
  end
  def handle_cast({:withdraw, amount}, state) do
    {:noreply, get_updated_balance(state, amount, &(&1 - &2))}
  end

  #...
end

36.ex

defmodule Account do
  use GenServer

  def start_link do
    GenServer.start_link(__MODULE__, :ok)
  end

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

  # API
  def get_balance(pid) do
    GenServer.call(pid, :get_balance)
  end

  def deposit(pid, amount) do
    GenServer.cast(pid, {:deposit, amount})
  end

  def withdraw(pid, amount) do
    GenServer.cast(pid, {:withdraw, amount})
  end

  # Callbacks
  def handle_call(:get_balance, _from, state) do
    {:reply, Map.get(state, :balance), state}
  end

  def handle_cast({:deposit, amount}, state) do
    {:noreply, get_updated_balance(state, amount, &(&1 + &2))}
  end
  def handle_cast({:withdraw, amount}, state) do
    {:noreply, get_updated_balance(state, amount, &(&1 - &2))}
  end

  # private
  defp get_updated_balance(state, amount, calculate_balance) do
    {_value, updated_balance} =
      Map.get_and_update(state, :balance, fn balance ->
        {balance,  calculate_balance.(balance, amount)}
      end)
    updated_balance
  end
end

4.ex

# bank/lib/account.ex
defmodule Account
end

5.ex

# lib\account.ex
defmodule Account do
  def start_link do
  end
end

6.ex

defmodule Account do
  use GenServer

  def start_link do
    GenServer.start_link(__MODULE__, :ok)
  end
end

7.ex

defmodule Account do
  # ...

  def init(:ok) do
    
  end
end

8.ex

defmodule Account do
  # ...

  def init(:ok) do
    {:ok, %{}}
  end
end

9.exs

# test\account_test.exs
defmodule AccountTest do
  use ExUnit.Case

  test "initial balance is zero" do
    {:ok, pid} = Account.start_link

    assert 0 == Account.get_balance(pid)
  end
end

Intro

This is a list of code samples for my post about writing GenServers with TDD.