Early exit example with multiple clause function (for Thailand Developers Elixir slack channel)

account.ex

defmodule Account do
  defstruct id: nil, balance: 0

  def withdraw(account_id, amount) do
    account_id
    |> find
    |> make_withdrawal(amount)
  end

  defp find(account_id) do
    _fetch_account(db_query(account_id))
  end
  defp _fetch_account(nil) do
    raise ArgumentError, message: "Account does not exist!"
  end
  defp _fetch_account(account), do: account

  defp db_query(account_id) do
    if account_id == 541 do
      %Account{id: 541, balance: 1000}
    else
      nil
    end
  end

  defp make_withdrawal(account, amount) do
    if account.balance > amount do
      new_balance = account.balance - amount
      "Your new balance is #{new_balance}"
    else
      "Insufficient funds!"
    end
  end
end

account_test.exs

defmodule AccountTest do
  use ExUnit.Case, async: true

  test "withdraw from account with sufficient funds" do
    assert Account.withdraw(541, 500) == "Your new balance is 500"
  end

  test "withdrawal from account with insuffucient funds" do
    assert Account.withdraw(541, 1000) == "Insufficient funds!"
  end

  test "non-existing account raises error" do
    assert_raise ArgumentError, "Account does not exist!", fn ->
      Account.withdraw(2, 500)
    end
  end
end