In the eyes of many, the new collections framework is the most significant change in Scala 2.8. 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 are currently being 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)
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 are in the current development builds and will be released as part of Scala 2.9.)
These pages describe in depth the APIs of the Scala 2.8 collection 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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