Scala Cookbook

10.24. Creating a Lazy View on a Collection

Problem

You’re working with a large collection and want to create a “lazy” version of it so it will only compute and return results as they are actually needed.

Solution

Except for the Stream class, whenever you create an instance of a Scala collection class, you’re creating a strict version of the collection. This means that if you create a collection that contains one million elements, memory is allocated for all of those elements immediately. This is the way things normally work in a language like Java.

In Scala you can optionally create a view on a collection. A view makes the result nonstrict, or lazy. This changes the resulting collection, so when it’s used with a transformer method, the elements will only be calculated as they are accessed, and not “eagerly,” as they normally would be. (A transformer method is a method that transforms an input collection into a new output collection, as described in the Discussion.)

You can see the effect of creating a view on a collection by creating one Range without a view, and a second one with a view:

scala> 1 to 100
res0: scala.collection.immutable.Range.Inclusive =
    Range(1, 2, 3, 4, ... 98, 99, 100)

scala> (1 to 100).view
res0: java.lang.Object with 
  scala.collection.SeqView[Int,scala.collection.immutable.IndexedSeq[Int]] =
  SeqView(...)

Creating the Range without a view shows what you expect, a Range with 100 elements. However, the Range with the view shows different output in the REPL, showing something ...