Scala Cookbook

11.6. Using Stream, a Lazy Version of a List

Problem

You want to use a collection that works like a List but invokes its transformer methods (map, filter, etc.) lazily.

Solution

A Stream is like a List, except that its elements are computed lazily, in a manner similar to how a view creates a lazy version of a collection. Because Stream elements are computed lazily, a Stream can be long ... infinitely long. Like a view, only the elements that are accessed are computed. Other than this behavior, a Stream behaves similar to a List.

Just like a List can be constructed with ::, a Stream can be constructed with the #:: method, using Stream.empty at the end of the expression instead of Nil:

scala> val stream = 1 #:: 2 #:: 3 #:: Stream.empty
stream: scala.collection.immutable.Stream[Int] = Stream(1, ?)

The REPL output shows that the stream begins with the number 1 but uses a ? to denote the end of the stream. This is because the end of the stream hasn’t been evaluated yet.

For example, given a Stream:

scala> val stream = (1 to 100000000).toStream
stream: scala.collection.immutable.Stream[Int] = Stream(1, ?)

you can attempt to access the head and tail of the stream. The head is returned immediately:

scala> stream.head
res0: Int = 1

but the tail isn’t evaluated yet:

scala> stream.tail
res1: scala.collection.immutable.Stream[Int] = Stream(2, ?)

The ? symbol is the way a lazy collection shows that the end of the collection hasn’t been evaluated yet.

As discussed in Recipe 10.24, transformer methods are computed ...