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strF: (String) => java.lang.String = <function>
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CHAPTER 4 FUN WITH FUNCTIONS, AND NEVER HAVING TO CLOSE THAT JDBC CONNECTION
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Let s call strF a couple of times:
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scala> strF("a")
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res0: java.lang.String = a Registered
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scala> strF("b")
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res1: java.lang.String = b Registered
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Let s inspect strs:
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scala> strs
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res2: List[String] = List(b, a)
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Cool. The side effect of calling strF is to update strs. Is this a local magical phenomenon or does it always work Let s see:
scala> List("p", "q", "r").map(strF)
res3: List[java.lang.String] = List(p Registered, q Registered, r Registered)
Let s test strs:
scala> strs
res4: List[String] = List(r, q, p, b, a)
Yes, the strF function is still updating strs.
CHAPTER 4 FUN WITH FUNCTIONS, AND NEVER HAVING TO CLOSE THAT JDBC CONNECTION
Putting Functions in Containers
Functions are instances, which means that whatever you can do with an instance, you can do with a function. Let s create a function, bf, which takes an Int and returns a function:
scala> def bf: Int => Int => Int = i => v => i + v
bf: (Int) => (Int) => Int
Next, let s create a sequence of 1 to 100 and map bf over the sequence:
scala> val fs = (1 to 100).map(bf).toArray
fs: Array[(Int) => Int] = Array(<function>, ...
We ve got an Array[Int => Int], otherwise known as an array of functions that will convert an Int to an Int. Let s get the first element in the array and apply it to 1:
scala> fs(0)(1)
res34: Int = 2
The first element of the array was the 1 applied to the bf function. We applied 1 to this new function, and discover that 1 + 1 does equal 2. Does it work with other members of the array
scala> fs(44)(3)
res35: Int = 48
It s theoretically cool that functions are instances that can be manipulated like any other instance. There are practical uses of putting functions in Maps and Lists. Functions represent blocks of code instructions on how to do something that is within a particular
CHAPTER 4 FUN WITH FUNCTIONS, AND NEVER HAVING TO CLOSE THAT JDBC CONNECTION
context and that is bound to variables in a particular scope. The ability to bind functions to events, such as the user clicking a button on a screen, which may occur in the future, provides a powerful way to build interactive, event-based applications.
Functions and Interactive Applications
Callbacks are very common in interactive applications. For example, if a button is clicked, perform a particular action. Creating a callback in web applications is a particularly difficult task unless you ve got powerful tools like the ones Scala gives you. Let s create a method that generates a random String that will serve as a globally unique identifier (GUID):
scala> def randomName = "I"+Math.abs((new java.util.Random).nextLong)
randomName: java.lang.String
Next, let s define a generic JavaScript trait. We need not flesh it out, but we can assume that it contains JavaScript commands that can be run in the browser:
scala> trait JavaScript
defined trait JavaScript
Next, let s create a Map to associate the GUID with a function that will generate some JavaScript:
scala> var callbacks: Map[String, () => JavaScript] = Map()
callbacks: Map[String,() => JavaScript] = Map()
Finally, we can create a method that registers the function and generates an HTML
<button/>. When the button is clicked in the browser, an Ajax call will be made to the
server, the function will be invoked, and the resulting JavaScript will be returned to the browser.
CHAPTER 4 FUN WITH FUNCTIONS, AND NEVER HAVING TO CLOSE THAT JDBC CONNECTION
scala> def register(f: () => JavaScript) = { val name = randomName callbacks += name -> f <button onclick={"invokeSeverCall('"+name+"')"}>ClickMe</button> }
register: (() => JavaScript)scala.xml.Elem
When the user clicks the button, an Ajax HTTP request is generated with the GUID. The servlet looks up the GUID in the Map, and if the GUID is found, the function is invoked, and the resulting JavaScript is returned to the browser. The code looks like the following:
def handleAjax(guid: String): HttpResponse = functionMap.get(guid).map(f => f()) match { case Some(javaScript) => JavaScriptResponse(javaScript) case _ => Http404Response() }
This code is a simplified version of what is done in the Lift Web Framework (http:// liftweb.net). This code demonstrates a practical way that Scala and Lift abstract away the HTTP request/response cycle by associating a function with a client-side event. The developer writing code using this style gets to spend more brain cycles on the business logic of what to do when the user clicks the button and far fewer cycles worrying about the plumbing of servicing an HTTP request.
Building New Functions
So far, we ve created simple functions and manipulated the function instances. However, we can also build functions from other functions. Functional composition provides the basis for a lot of cool things in Scala including the parser combinator, which we will explore in 8. But for now, let s see the difference between interpreting a series of commands and compiling a function that interprets them. First, let s define a grammar. In our grammar, we have expressions, which can be constant values or named variables. Expressions can also be addition or multiplication of other expressions. Here s a collection of case classes that describes our grammar (recall that we covered case classes in 2):
sealed trait Expr case class Add(left: Expr, right: Expr) extends Expr case class Mul(left: Expr, right: Expr) extends Expr case class Val(value: Int) extends Expr case class Var(name: String) extends Expr
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