We said in the introduction to part 4 that functional programming is a complete paradigm. Every imaginable program can be expressed functionally, including programs that interact with the external world. But it would be disappointing if the IO type were the only way to construct such programs. IO and ST work by simply embedding an imperative programming language into the purely functional subset of Scala. While programming within the IO monad, we have to reason about our programs much like we would in ordinary imperative programming.