Chapter 4. Handling errors without exceptions


We noted briefly in chapter 1 that throwing exceptions is a side effect. If exceptions aren’t used in functional code, what is used instead? In this chapter, we’ll learn the basic principles for raising and handling errors functionally. The big idea is that we can represent failures and exceptions with ordinary values, and we can write higher-order functions that abstract out common patterns of error handling and recovery. The functional solution, of returning errors as values, is safer and retains referential transparency, and through the use of higher-order functions, we can preserve the primary benefit of exceptions—consolidation of error-handling logic. We’ll see how this works over the course of this chapter, after we take a closer look at exceptions and discuss some of their problems.

For the same reason that we created our own List data type in the previous chapter, we’ll re-create in this chapter two Scala standard library types: Option and Either. The purpose is to enhance your understanding of how these types can be used for handling errors. After completing this chapter, you should feel free to use the Scala standard library version of Option and Either (though you’ll notice that the standard library versions of both types are missing some of the useful functions we define in this chapter).

4.1. The good and bad aspects of exceptions

4.2. Possible alternatives to exceptions

4.3. The Option data type

4.4. The Either data type

4.5. Summary