Functional Programming in Scala

Chapter 13. External effects and I/O

In this chapter, we’ll take what we’ve learned so far about monads and algebraic data types and extend it to handle external effects like reading from databases and writing to files. We’ll develop a monad for I/O, aptly called IO, that will allow us to handle such external effects in a purely functional way.

We’ll make an important distinction in this chapter between effects and side effects. The IO monad provides a straightforward way of embedding imperative programming with I/O effects in a pure program while preserving referential transparency. It clearly separates effectful code—code that needs to have some effect on the outside world—from the rest of our program.

This will also illustrate a key technique ...