collections

Introduction

Martin Odersky, and Lex Spoon

In the eyes of many, the new collections framework is the most significant change in the Scala 2.8 release. Scala had collections before (and in fact the new framework is largely compatible with them). But it’s only 2.8 that provides a common, uniform, and all-encompassing framework for collection types.

Even though the additions to collections are subtle at first glance, the changes they can provoke in your programming style can be profound. In fact, quite often it’s as if you work on a higher-level with the basic building blocks of a program being whole collections instead of their elements. This new style of programming requires some adaptation. Fortunately, the adaptation is helped by several nice properties of the new Scala collections. They are easy to use, concise, safe, fast, universal.

Easy to use: A small vocabulary of 20-50 methods is enough to solve most collection problems in a couple of operations. No need to wrap your head around complicated looping structures or recursions. Persistent collections and side-effect-free operations mean that you need not worry about accidentally corrupting existing collections with new data. Interference between iterators and collection updates is eliminated.

Concise: You can achieve with a single word what used to take one or several loops. You can express functional operations with lightweight syntax and combine operations effortlessly, so that the result feels like a custom algebra.

Safe: This one has to be experienced to sink in. The statically typed and functional nature of Scala’s collections means that the overwhelming majority of errors you might make are caught at compile-time. The reason is that (1) the collection operations themselves are heavily used and therefore well tested. (2) the usages of the collection operation make inputs and output explicit as function parameters and results. (3) These explicit inputs and outputs are subject to static type checking. The bottom line is that the large majority of misuses will manifest themselves as type errors. It’s not at all uncommon to have programs of several hundred lines run at first try.

Fast: Collection operations are tuned and optimized in the libraries. As a result, using collections is typically quite efficient. You might be able to do a little bit better with carefully hand-tuned data structures and operations, but you might also do a lot worse by making some suboptimal implementation decisions along the way. What’s more, collections have been recently adapted to parallel execution on multi-cores. Parallel collections support the same operations as sequential ones, so no new operations need to be learned and no code needs to be rewritten. You can turn a sequential collection into a parallel one simply by invoking the par method.

Universal: Collections provide the same operations on any type where it makes sense to do so. So you can achieve a lot with a fairly small vocabulary of operations. For instance, a string is conceptually a sequence of characters. Consequently, in Scala collections, strings support all sequence operations. The same holds for arrays.

Example: Here’s one line of code that demonstrates many of the advantages of Scala’s collections.

val (minors, adults) = people partition (_.age < 18)

It’s immediately clear what this operation does: It partitions a collection of people into minors and adults depending on their age. Because the partition method is defined in the root collection type TraversableLike, this code works for any kind of collection, including arrays. The resulting minors and adults collections will be of the same type as the people collection.

This code is much more concise than the one to three loops required for traditional collection processing (three loops for an array, because the intermediate results need to be buffered somewhere else). Once you have learned the basic collection vocabulary you will also find writing this code is much easier and safer than writing explicit loops. Furthermore, the partition operation is quite fast, and will get even faster on parallel collections on multi-cores. (Parallel collections have been released as part of Scala 2.9.)

This document provides an in depth discussion of the APIs of the Scala collections classes from a user perspective. They take you on a tour of all the fundamental classes and the methods they define.

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