Return to chapter index

9 Control Abstraction

Sample run of chapter's interpreter examples

• 9.1 Reducing code duplication
• 9.2 Simplifying client code
• 9.3 Currying
• 9.4 Writing new control structures
• 9.5 By-name parameters
• 9.6 Conclusion

9.1 Reducing code duplication

// In file control-abstraction/Files1.scala

  object FileMatcher:
    private def filesHere = (new java.io.File(".")).listFiles

    def filesEnding(query: String) =
      for file <- filesHere if file.getName.endsWith(query)
      yield file


// In file control-abstraction/Files1.scala

  def filesContaining(query: String) =
    for file <- filesHere if file.getName.contains(query)
    yield file


// In file control-abstraction/Files1.scala

  def filesRegex(query: String) =
    for file <- filesHere if file.getName.matches(query)
    yield file


  def filesMatching(query: String, method) =
    for file <- filesHere if file.getName.method(query)
    yield file


// In file control-abstraction/Files2.scala

  def filesMatching(query: String,
      matcher: (String, String) => Boolean) =

    for file <- filesHere if matcher(file.getName, query)
    yield file


// In file control-abstraction/Files2.scala

  def filesEnding(query: String) =
    filesMatching(query, _.endsWith(_))

  def filesContaining(query: String) =
    filesMatching(query, _.contains(_))

  def filesRegex(query: String) =
    filesMatching(query, _.matches(_))


  (fileName: String, query: String) => fileName.endsWith(query)


// In file control-abstraction/Files.scala

  object FileMatcher:
    private def filesHere = (new java.io.File(".")).listFiles

    private def filesMatching(matcher: String => Boolean) =
      for file <- filesHere if matcher(file.getName)
      yield file
  
    def filesEnding(query: String) =
      filesMatching(_.endsWith(query))
  
    def filesContaining(query: String) =
      filesMatching(_.contains(query))
  
    def filesRegex(query: String) =
      filesMatching(_.matches(query))


  

9.2 Simplifying client code

  def containsNeg(nums: List[Int]): Boolean =
    var exists = false
    for num <- nums do
      if num < 0 then
        exists = true
    exists


  containsNeg(List(1, 2, 3, 4))    // false
  containsNeg(List(1, 2, -3, 4))   // true


  def containsNeg(nums: List[Int]) = nums.exists(_ < 0)


  containsNeg(Nil)               // false
  containsNeg(List(0, -1, -2))   // true


  def containsOdd(nums: List[Int]): Boolean =
    var exists = false
    for num <- nums do
      if num % 2 == 1 then
        exists = true
    exists


  def containsOdd(nums: List[Int]) = nums.exists(_ % 2 == 1)


  

9.3 Currying

  def plainOldSum(x: Int, y: Int) = x + y
  plainOldSum(1, 2)   // 3


  def curriedSum(x: Int)(y: Int) = x + y
  curriedSum(1)(2)   // 3


  def first(x: Int) = (y: Int) => x + y


  val second = first(1) // second has type Int => Int


  second(2)   // 3


  val onePlus = curriedSum(1) // onePlus has type Int => Int


  onePlus(2)   // 3


  val twoPlus = curriedSum(2)
  twoPlus(2)   // 4


  

9.4 Writing new control structures

  def twice(op: Double => Double, x: Double) = op(op(x))
  twice(_ + 1, 5)   // 7.0


// In file control-abstraction/WithPrintWriter1.scala

  def withPrintWriter(file: File, op: PrintWriter => Unit) =
    val writer = new PrintWriter(file)
    try op(writer)
    finally writer.close()


// In file control-abstraction/WithPrintWriter1.scala

  withPrintWriter(
    new File("date.txt"),
    writer => writer.println(new java.util.Date)
  )


  val s = "Hello, world!"
  s.charAt(1)      // 'e'


  s.charAt { 1 }   // 'e'


  s.substring { 7, 9 }
  1 |s.substring { 7, 9 }
    |               ^
    |               end of statement expected but ',' found
  1 |s.substring { 7, 9 }
    |                 ^
    |                 ';' expected, but integer literal found


  s.substring(7, 9)   // "wo"


// In file control-abstraction/WithPrintWriter2.scala

  def withPrintWriter(file: File)(op: PrintWriter => Unit) =
    val writer = new PrintWriter(file)
    try op(writer)
    finally writer.close()


// In file control-abstraction/WithPrintWriter2.scala

  val file = new File("date.txt")

  withPrintWriter(file) { writer =>
    writer.println(new java.util.Date)
  }


  

9.5 By-name parameters

  withPrintWriter(file) { writer =>
    writer.println(new java.util.Date)
  }


// In file control-abstraction/Assert.scala

  var assertionsEnabled = true

  def myAssert(predicate: () => Boolean) =
    if assertionsEnabled && !predicate() then
      throw new AssertionError


  myAssert(() => 5 > 3)


  myAssert(5 > 3) // Won't work, because missing () => 


// In file control-abstraction/Assert.scala

  def byNameAssert(predicate: => Boolean) =
    if assertionsEnabled && !predicate then
      throw new AssertionError


  byNameAssert(5 > 3)


  def boolAssert(predicate: Boolean) =
    if assertionsEnabled && !predicate then
      throw new AssertionError


  boolAssert(5 > 3)


  val x = 5
  assertionsEnabled = false

  boolAssert(x / 0 == 0)
  java.lang.ArithmeticException: / by zero
    ... 27 elided


  byNameAssert(x / 0 == 0)   // Returns normally


  

9.6 Conclusion