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 rates.
Considering two types A and B, we previously saw how to define a type that can hold a value of type A and a value of type B. This is a product type, and we can define it using a tuple, such as ab: (A, B), or a case class, such as case class MyProduct(a: A, b: B).
On...