Functional Programming Patterns in Scala and Clojure

Pattern 18

Lazy Sequence

Intent

To create a sequence whose members are computed only when needed—this allows us to easily stream results from a computation and to work with infinitely long sequences

Overview

We often deal with elements of a sequence one at a time. Since this is so, we generally don’t need to have the entire sequence realized before we start processing it. For instance, we may wish to stream lines of a file off of disk and process them without ever holding the entire file in memory. We could use Pattern 12, ​Tail Recursion​, to seek through the file, but Lazy Sequence provides a much cleaner abstraction for this sort of streaming computation.

Lazy Sequence does so by only creating an element in a sequence when it’s asked for. ...