Introduction

At its simplest, a lazy sequence is a hybrid of two things:

• A list (not a vector!) of zero or more items
• A reference to possible future items of the list that can be computed if necessary

In other words, there's a real part and a virtual part. Most of the time, you don't need to think about this distinction. That's the whole point of lazy sequences: the virtual part becomes real when you need it, if you need it. When you can stay away from the edge cases, you don't need to worry about the virtual part because as soon as it's needed, it will become real.

Over the last few chapters, we've already used lazy sequences in many different ways. They are an important, distinctive feature of Clojure and they are something that you'll use every day as a Clojure programmer. You've seen by now that they are list-like structures with a twist: while you are using the first elements of the list, the rest of the list may not...