Here are some questions to test your knowledge:

• What type can you use to represent an optional value?
• What type(s) can you use to represent the possibility of an error?
• What is referential transparency?
• Is it good practice to throw exceptions?

Here are some exercises:

• Write unit tests for SimulatePlanApp
• Use RetCalcResult instead of Either[RetCalcError, X] in RetCalc.scala
• Change VariableReturns.fromUntil to return an error if monthIdFrom or monthIdUntil cannot be found in the returns Vector