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CHAPTER 5 PATTERN MATCHING
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Each element derives from a trait that creates a contract, which requires that the class implement the accept method:
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trait OCarElement { def accept(visitor: OCarVisitor): Unit }
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We implement each subclass and implement the accept method:
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class OWheel(val name: String) extends OCarElement { def accept(visitor: OCarVisitor) = visitor.visit(this) } class OEngine extends OCarElement { def accept(visitor: OCarVisitor) = visitor.visit(this) } class OBody extends OCarElement { def accept(visitor: OCarVisitor) = visitor.visit(this) } class OCar extends OCarElement { val elements = List(new OEngine, new OBody, new OWheel("FR"), new OWheel("FL"), new OWheel("RR"), new OWheel("RL")) def accept(visitor: OCarVisitor) = (this :: elements).foreach(_.accept(visitor)) }
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That s a lot of boilerplate.8 Additionally, it violates the data-hiding principles of OOP because the visitor has to access some of the data in each element that it visits. Let s compare the pattern-matching version:
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trait CarElement case class Wheel(name: String) extends CarElement case class Engine() extends CarElement case class Body() extends CarElement case class Car(elements: List[CarElement]) extends CarElement
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8. Here is where a unityped language such as Ruby or Python has a material advantage over a static language such as Java. In Ruby, you don t need all the boilerplate, and the class hierarchy is not fixed at the time the OCarVisitor interface is defined.
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CHAPTER 5 PATTERN MATCHING
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The code is cleaner because there s no boilerplate accept method. Let s see what we do when we want to traverse the object hierarchy:
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def doSomething(in: CarElement): Unit = in match { case Wheel(name) => case Engine() => case Body() => case Car(e) => e.foreach(doSomething) }
More generally, Burak Emir, one of Scala s authors, wrote an excellent paper on the intersection of pattern matching and object-oriented design. See http://library.epfl.ch/ theses/ nr=3899.
Summary
In this chapter, we explored pattern matching and saw how pattern matching provides powerful declarative syntax for expressing complex logic. Pattern matching provides an excellent and type-safe alternative to Java s test/cast paradigm. Pattern matching used with case classes and extraction provides a powerful way to traverse object hierarchies and is an excellent alternative to the visitor pattern. And because patterns are functions and objects, they can be passed as parameters and used wherever functions are used. In the next chapter, we ll explore Actors. Actors provide a great paradigm for concurrency without locks. Actors in Scala are entirely library-based. They take advantage of Scala s flexible syntax, interoperability with Java libraries, and pattern matching to provide awesome power and flexibility for building multicore-friendly applications.
Actors and Concurrency
ava introduced the synchronized keyword, which provided language-level concurrency management. Coming from C++, built-in language-level concurrency had the benefits of a unified model, so each project or module had the same concurrency mechanism and there was no need to roll your own. Java s synchronization semantics are very simple. You lock an object for exclusive use on a given thread, and the JVM assures you that the object will not be locked by another thread. Furthermore, because the JVM assures you that you can enter the lock multiple times on the same thread and at the bytecode level, you know that the lock will be released no matter how your application unwinds the stack.1 In practical use, Java s synchronized mechanism is fraught with peril. The granularity at which you lock objects is a very tough call. If you lock too coarsely, then you wind up with a single-threaded application, because in practical terms the global lock will be asserted by the first thread that needs the given high-level object. If your granularity is too fine, there s a high likelihood of deadlocks, as locks are asserted by different threads on mutually interdependent objects. As a practical matter, when you code Java, you never know when something is going to be synchronized. Even if your team defines a set of concurrency and synchronization patterns that work, enforcing the model is non-trivial, and often the only time the defects will be detected is during high-load production situations. There has to be a better way, and in fact there is. The Actor model of concurrency offers a different and generally superior mechanism for doing multithreaded and multicore coding.
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