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CHAPTER 5 PATTERN MATCHING
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If you re building a web application, you might have particular URLs that need special handling while others get handled in the default manner. The URL can be expressed as a List[String]. We can do the following:
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def handleRequest(req: List[String])( exceptions: PartialFunction[List[String], String]): String = if (exceptions.isDefinedAt(req)) exceptions(req) else "Handling URL "+req+" in the normal way"
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So, if the partial function exceptions (the pattern) matches the request req according to the isDefinedAt method, then we allow the request to be handled by the exceptions function. Otherwise, we do default handling. We can call handleRequest and handle any api requests by a separate handler:
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handleRequest("foo" :: Nil) { case "api" :: call :: params => doApi(call, params) } def doApi(call: String, params: List[String]): String = "Doing API call "+call
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Partial functions can be composed into a single function using the orElse method.5 So, we can define a couple of partial functions:
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val f1: PartialFunction[List[String], String] = { case "stuff" :: Nil => "Got some stuff" } val f2: PartialFunction[List[String], String] = { case "other" :: params => "Other: "+params }
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And we can compose them:
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val f3 = f1 orElse f2
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And we can pass them into the handleRequest method:
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handleRequest("a" :: "b" :: Nil)(f3)
5. Technically, combining multiple partial functions using orElse is not functional composition. A roomful of wicked smart functional developers at LShift (http://lshift.com) were unable to come up with a good name other than functional smooshing.
CHAPTER 5 PATTERN MATCHING
In this way, Scala gives you a very nice, declarative way of handling complex filtering tasks. Partial functions can match on data and can be passed around like any other instances in Scala. Partial functions replace a lot of the XML configuration files in Java because pattern matching gives you the same declarative facilities as a configuration file, but they are type-safe, high-performance, and they can have guards and generally take advantage of any method in your code. Here s an example of using pattern matching to dispatch REST request in the ESME6 code:7
def dispatch: LiftRules.DispatchPF = { case Req("api" :: "status" :: Nil, "", GetRequest) => status case Req("api" :: "messages" :: Nil, "", GetRequest) => getMsgs case Req("api" :: "messages" :: "long_poll" :: Nil, "", GetRequest) => waitForMsgs case Req("api" :: "messages" :: Nil, "", PostRequest) => () => sendMsg(User.currentUser.map(_.id.is), S) case Req("api" :: "follow" :: Nil, _, GetRequest) => following(calcUser) case Req("api" :: "followers" :: Nil, _, GetRequest) => followers(calcUser) case Req("api" :: "follow" :: Nil, _, PostRequest) => performFollow(S.param("user")) }
Object-Oriented and Functional Tensions
At this point, the hard-core object-oriented designer folks may be somewhat unhappy about Scala case class s exposure of lots of internal information. Data hiding is an important part of OOP s abstraction. But in fact, most of the Java classes we define have getters and setters, so there is data exposed in OOP. But there is a tension between the amount of internal state that s exposed in our program and the amount of state that s hidden. In this section, we ll explore OOP and functional programming (FP) patterns for data hiding and exposure. Another tension in OOP is how to define methods on class and interface hierarchies. Where does a method definition belong What happens when a library is deployed but it s necessary to add new functionality to subclasses How do we retrofit the defined-in-stone
6. ESME is the Enterprise Social Messaging Experiment (http://blog.esme.us). 7. This code will not compile without the rest of the ESME code, but it serves as an illustration of using pattern matching as an alternative to XML configuration files or annotations.
CHAPTER 5 PATTERN MATCHING
library classes to add this functionality Put more concretely, if we have a library of shapes circle, square, rectangle that each have an area method but hide all their other data, how do we add a perimeter method to the shapes Let s explore the tension and the tools Scala and FP give us to address the tension.
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