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CHAPTER 4 FUN WITH FUNCTIONS, AND NEVER HAVING TO CLOSE THAT JDBC CONNECTION
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And you would call this code:
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log(INFO, "The value is "+value)
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The Scala version passes "The value is "+value as a function that is evaluated each time it is accessed in the log method. The log method will access it only if the log message is going to be printed. Your code is cleaner because you don t have to repeatedly test the log level, but it performs as well as the previous Java code that has the inline test. In order to make something call-by-name, just put => before the type. So, foo(s: String) is call-by-reference, and foo(s: => String) is call-by-name. You may be wondering how the code could possibly perform as well if a function object is being created and handed off to the log method. In the JVM, the cost of creating an object that never escapes the current thread and is very short-lived is zero or very near zero. The JVM may also inline the log method such that the test is performed without an actual method call. The result is that your code will run as quickly with the Scala code as it will with the Java code that has the repeated test for log level. The first use of call-by-name is passing an expression that takes a long time to evaluate that may not be evaluated. The second use for call-by-name is the situation where we want to evaluate the expression many times in the target method, for example, if we want to evaluate an expression until some condition is met. That condition could be until the expression returns false or until the expression returns null. For example, we could collect all the Strings returned from an expression until we encounter a null:
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def allStrings(expr: => String): List[String] = expr match { case null => Nil case s => s :: allStrings(expr) }
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We can test this method:
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scala> import java.io._
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import java.io._
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scala> val br = new BufferedReader(new FileReader("foo.txt"))
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br: java.io.BufferedReader = java.io.BufferedReader@2bfa91
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CHAPTER 4 FUN WITH FUNCTIONS, AND NEVER HAVING TO CLOSE THAT JDBC CONNECTION
scala> allStrings(br.readLine)
res0: List[String] = List(import scala.xml._, , object Morg {, )
Each time the call-by-name parameter, expr, is accessed, it is applied. If it is passed as a parameter that is also call-by-name, it will be passed without evaluation. In the previous code, we pattern match against the application of expr. If it s null, we return an empty List, a Nil. If it s not null, we return a List that is the current String and the result of allStrings(expr). Call-by-name is a very useful construct. In the next section, we ll use it to build complex control structures.
Build Your Own Control Structures
In this section, we ll use call-by-name variables and functions to create our own control structures. Scala has very limited control structures: try/catch/finally, if/else, and while. Most languages have a plethora of control structures including for, foreach, and so on. C# even has the using control structure. using provides an auto-close feature where the parameter passed to using will be closed when using s code block is exited. For example:
using (TextReader textReader = new StreamReader(filename)) { return textReader.ReadLine(); }
This provides a convenient mechanism to make sure that files, database connections, TCP/IP connections, and so on are closed without having to write a try/finally block for each thing you want to close. Scala does not have a using statement, but we can write one. I ll show you all the code and then step through the pieces:
object Control { def using[A <: {def close(): Unit}, B](param: A)(f: A => B): B = try { f(param) } finally { param.close() } import scala.collection.mutable.ListBuffer
CHAPTER 4 FUN WITH FUNCTIONS, AND NEVER HAVING TO CLOSE THAT JDBC CONNECTION
def bmap[T](test: => Boolean)(block: => T): List[T] = { val ret = new ListBuffer[T] while(test) ret += block ret.toList } }
Let s step through the code. First, we define the Control singleton object:
object Control {
Next, we define the using method. It takes two type parameters: A and B. The B type parameter is much like what we ve seen in the past: it can be any type. We have put a structural type bound on A. A can be an instance of any class as long as that class has a close method on it. This is called structural typing. Scala allows you to define types based on their structure rather than their class. This is familiar to ECMAScript developers. It s also closer to the duck typing of Ruby and Python, but in the case of Scala, you have to declare the methods of the duck as part of the parameter. The param parameter is one of these A ducks that has a close method. The f parameter is something that takes the A and transforms it to a B.
def using[A <: {def close(): Unit}, B](param: A)(f: A => B): B = try { f(param) } finally { param.close() }
The code is pretty simple. It wraps the function application in a try/finally block and makes sure that param is closed before the method returns. We could call the code like
using(new BufferedReader(otherReader)) { reader => reader.readLine() }
The next control statement we ll build is something that loops as long as a test is true. In each iteration, the method will collect the output of a pass-by-name value and append it to the list accumulator. First, let s import ListBuffer so we can accumulate the results:
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