Using some parameters such as your net income, your expenses, your initial capital, and so on, we will create functions to calculate the following:

• Your future capital at retirement
• Your capital after a number of years in retirement
• How long you need to save to be able to retire

We will first use a fixed interest rate for these calculations. After that, we will load market data from .tsv files, then refactor the previous functions to simulate what could happen during the investment period.

The first thing you need to know when planning for retirement is how much capital you can get at your chosen retirement date. For now, we will assume that you invest your savings every month at a constant rate. To simplify things, we will ignore the effects of inflation, hence the capital calculated will be in today's money and the interest rate will be calculated as real rate = nominal interest rate - inflation rate.

We intentionally do not mention any currency in the rest of this chapter. You can consider that the amounts are in USD, EUR, or any other currency. It will not change the results as long as all the amounts are expressed in the same currency.

If you have tried to call simulatePlan from the Scala Console, you probably tried different values for nbOfMonths and observed the resulting capital at retirement and after death. It would be useful to have a function that finds the optimal nbOfMonths so that you have enough capital to never run out of money during your retirement.

As usual, let's start with a new unit test to clarify what we expect from this function:

"RetCalc.nbOfMonthsSaving" should {
  "calculate how long I need to save before I can retire" in {
    val actual = RetCalc.nbOfMonthsSaving(
      interestRate = 0.04 / 12, nbOfMonthsInRetirement = 40 * 12...

In our calculations, we have always assumed that the interest rate of return was constant, but the reality is more complex. It would be more accurate to use real rates from market data to gain more confidence with our retirement plan. For this, we first need to change our code to be able to perform the same calculations using variable interest rates. Then, we will load real market data to simulate regular investments in a fund by tracking the S & P 500 index.

In order to support variable rates, we need to change the signature of all functions that accept interestRate: Double. Instead of a double, we need a type that can represent either a constant rate or a sequence of...

We have now implemented some useful building blocks, and it is time to create an executable so that end users can use our calculator with their own parameters.

We are going to build a simple executable around RetCalc.simulatePlan. It will take a list of parameters separated by spaces, and print the results on the console.

The test we are going to write integrates several components together and will use a full market data set. As such, it is not really a unit test anymore; it is an integration test. For this reason, we suffixed it with IT instead of Spec.

First, copy sp500.tsv and cpi.tsv from https://github.com/PacktPublishing/Scala-Programming-Projects/blob/master...

We have covered how to create a small project in Scala from scratch to packaging. We used TDD along the way so that it is guaranteed that our code was well-designed and robust. It gives us confidence when we refactor code: as long as all tests pass, we know that our code stills works. We modeled our domain with immutable data structures and processed them using pure functions that have no side effects.

We used some basic features of the language that are used in most projects, and you should now be familiar with enough building blocks to implement a wide variety of projects.

As a further exercise, you could enhance this calculator with one or many of these features:

• Create an app for the function RetCalc.nbOfMonthsSaving, which calculates how long you need to save before you can retire.
• Create a function called RetCalc.annualizedTotalReturn, which calculates...