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21 Implicit Conversions and Parameters

Sample run of chapter's interpreter examples

• 21.1 Implicit conversions
• 21.2 Rules for implicits
• 21.3 Implicit conversion to an expected type
• 21.4 Converting the receiver
• 21.5 Implicit parameters
• 21.6 View bounds
• 21.7 When multiple conversions apply
• 21.8 Debugging implicits
• 21.9 Conclusion

21.1 Implicit conversions

// In file implicits/Swing1.scala

  val button = new JButton
  button.addActionListener(
    new ActionListener {
      def actionPerformed(event: ActionEvent) {
        println("pressed!")
      }
    }
  )


// In file implicits/Swing2.scala

  button.addActionListener( // Type mismatch!
    (_: ActionEvent) => println("pressed!")
  )


// In file implicits/Swing2.scala

  implicit def function2ActionListener(f: ActionEvent => Unit) =
    new ActionListener {
      def actionPerformed(event: ActionEvent) = f(event)
    }


  button.addActionListener(
    function2ActionListener(
      (_: ActionEvent) => println("pressed!")
    )
  )


  // Now this works
  button.addActionListener(
    (_: ActionEvent) => println("pressed!")
  )


  

21.2 Rules for implicits

// In file implicits/Misc.scala

  implicit def intToString(x: Int) = x.toString


  object Dollar {
    implicit def dollarToEuro(x: Dollar): Euro = ...
  }
  class Dollar { ... }


  object MyConversions {
    implicit def stringWrapper(s: String):
        IndexedSeq[Char] = ...
    implicit def intToString(x: Int): String = ...
  }


  import MyConversions.stringWrapper
  ... // code making use of stringWrapper


  

21.3 Implicit conversion to an expected type

  scala> val i: Int = 3.5
  <console>:4: error: type mismatch;
   found   : Double(3.5)
   required: Int
         val i: Int = 3.5
                      ^


  scala> implicit def doubleToInt(x: Double) = x.toInt
  doubleToInt: (x: Double)Int

  scala> val i: Int = 3.5
  i: Int = 3


  val i: Int = doubleToInt(3.5)


// In file implicits/Misc.scala

  implicit def int2double(x: Int): Double = x.toDouble


  

21.4 Converting the receiver

  class Rational(n: Int, d: Int) {
    ...
    def + (that: Rational): Rational = ...
    def + (that: Int): Rational = ...
  }


  scala> val oneHalf = new Rational(1, 2)
  oneHalf: Rational = 1/2

  scala> oneHalf + oneHalf
  res0: Rational = 1/1

  scala> oneHalf + 1
  res1: Rational = 3/2


  scala> 1 + oneHalf
  <console>:6: error: overloaded method value + with
  alternatives (Double)Double <and> ... cannot be applied
  to (Rational)
       1 + oneHalf
         ^


  scala> implicit def intToRational(x: Int) = 
       |   new Rational(x, 1)
  intToRational: (x: Int)Rational


  scala> 1 + oneHalf
  res2: Rational = 3/2


  intToRational(1) + oneHalf


// In file implicits/Misc.scala

  Map(1 -> "one", 2 -> "two", 3 -> "three")


  package scala
  object Predef {
    class ArrowAssoc[A](x: A) {
      def -> [B](y: B): Tuple2[A, B] = Tuple2(x, y)
    }
    implicit def any2ArrowAssoc[A](x: A): ArrowAssoc[A] = 
      new ArrowAssoc(x)
    ...
  }


  

21.5 Implicit parameters

  class PreferredPrompt(val preference: String)


  object Greeter {
    def greet(name: String)(implicit prompt: PreferredPrompt) {
      println("Welcome, "+ name +". The system is ready.")
      println(prompt.preference)
    }
  }


  scala> val bobsPrompt = new PreferredPrompt("relax> ")
  bobsPrompt: PreferredPrompt = PreferredPrompt@74a138

  scala> Greeter.greet("Bob")(bobsPrompt)                    
  Welcome, Bob. The system is ready.
  relax> 


  object JoesPrefs {
    implicit val prompt = new PreferredPrompt("Yes, master> ")
  }


  scala> Greeter.greet("Joe")
  <console>:10: error: could not find implicit value for 
  parameter prompt: PreferredPrompt
         Greeter.greet("Joe")
                      ^


  scala> import JoesPrefs._         
  import JoesPrefs._

  scala> Greeter.greet("Joe")
  Welcome, Joe. The system is ready.
  Yes, master> 


  class PreferredPrompt(val preference: String)
  class PreferredDrink(val preference: String)

  object Greeter {
    def greet(name: String)(implicit prompt: PreferredPrompt,
        drink: PreferredDrink) {

      println("Welcome, "+ name +". The system is ready.")
      print("But while you work, ")
      println("why not enjoy a cup of "+ drink.preference +"?")
      println(prompt.preference)
    }
  }

  object JoesPrefs {
    implicit val prompt = new PreferredPrompt("Yes, master> ")
    implicit val drink = new PreferredDrink("tea")
  }


  scala> Greeter.greet("Joe") 
  <console>:14: error: could not find implicit value for 
  parameter prompt: PreferredPrompt
         Greeter.greet("Joe") 
                      ^


  scala> import JoesPrefs._
  import JoesPrefs._


  scala> Greeter.greet("Joe")(prompt, drink)
  Welcome, Joe. The system is ready.
  But while you work, why not enjoy a cup of tea?
  Yes, master> 


  scala> Greeter.greet("Joe")
  Welcome, Joe. The system is ready.
  But while you work, why not enjoy a cup of tea?
  Yes, master> 


// In file implicits/Misc.scala

  def maxListUpBound[T <: Ordered[T]](elements: List[T]): T = 
    elements match {
      case List() =>
        throw new IllegalArgumentException("empty list!")
      case List(x) => x
      case x :: rest =>
        val maxRest = maxListUpBound(rest)
        if (x > maxRest) x
        else maxRest
    }


// In file implicits/MaxList1.scala

  def maxListImpParm[T](elements: List[T])
        (implicit orderer: T => Ordered[T]): T =

    elements match {
      case List() => 
        throw new IllegalArgumentException("empty list!")
      case List(x) => x
      case x :: rest =>
        val maxRest = maxListImpParm(rest)(orderer)
        if (orderer(x) > maxRest) x
        else maxRest
    }


  scala> maxListImpParm(List(1,5,10,3))
  res9: Int = 10

  scala> maxListImpParm(List(1.5, 5.2, 10.7, 3.14159))
  res10: Double = 10.7

  scala> maxListImpParm(List("one", "two", "three"))
  res11: java.lang.String = two


  def maxListPoorStyle[T](elements: List[T])
        (implicit orderer: (T, T) => Boolean): T


  

21.6 View bounds

// In file implicits/Misc.scala

  def maxList[T](elements: List[T])
        (implicit orderer: T => Ordered[T]): T =

    elements match {
      case List() => 
        throw new IllegalArgumentException("empty list!")
      case List(x) => x
      case x :: rest =>
        val maxRest = maxList(rest)  // (orderer) is implicit
        if (x > maxRest) x           // orderer(x) is implicit
        else maxRest
    }


// In file implicits/Misc.scala

  def maxList[T](elements: List[T])
        (implicit converter: T => Ordered[T]): T =
    // same body...


// In file implicits/Misc.scala

  def maxList[T](elements: List[T])
        (implicit iceCream: T => Ordered[T]): T =
    // same body...


// In file implicits/MaxList2.scala

  def maxList[T <% Ordered[T]](elements: List[T]): T =
    elements match {
      case List() => 
        throw new IllegalArgumentException("empty list!")
      case List(x) => x
      case x :: rest =>
        val maxRest = maxList(rest)  // (orderer) is implicit
        if (x > maxRest) x           // orderer(x) is implicit
        else maxRest
    }


  implicit def identity[A](x: A): A = x


  

21.7 When multiple conversions apply

  scala> def printLength(seq: Seq[Int]) = println(seq.length)
  printLength: (seq: Seq[Int])Unit

  scala> implicit def intToRange(i: Int) = 1 to i
  intToRange: (i: Int)scala.collection.immutable.Range.Inclusive
  with scala.collection.immutable.Range.ByOne

  scala> implicit def intToDigits(i: Int) =
       |   i.toString.toList.map(_.toInt)
  intToDigits: (i: Int)List[Int]

  scala> printLength(12)
  <console>:21: error: type mismatch;
   found   : Int(12)
   required: Seq[Int]
  Note that implicit conversions are not applicable because 
  they are ambiguous:
  ...


  val cba = "abc".reverse


// In file implicits/mocha.scala

  object Mocha extends Application {

    class PreferredDrink(val preference: String)

    implicit val pref = new PreferredDrink("mocha")

    def enjoy(name: String)(implicit drink: PreferredDrink) {
      print("Welcome, "+ name)
      print(". Enjoy a ")
      print(drink.preference)
      println("!")
    }

    enjoy("reader")
  }


  $ scalac -Xprint:typer mocha.scala
  [[syntax trees at end of typer]]// Scala source: mocha.scala
  package <empty> {
    final object Mocha extends java.lang.Object with Application
        with ScalaObject {

      // ...

      private[this] val pref: Mocha.PreferredDrink =
        new Mocha.this.PreferredDrink("mocha");
      implicit <stable> <accessor>
        def pref: Mocha.PreferredDrink = Mocha.this.pref;
      def enjoy(name: String)
          (implicit drink: Mocha.PreferredDrink): Unit = {
        scala.this.Predef.print("Welcome, ".+(name));
        scala.this.Predef.print(". Enjoy a ");
        scala.this.Predef.print(drink.preference);
        scala.this.Predef.println("!")
      };
      Mocha.this.enjoy("reader")(Mocha.this.pref)
    }
  }


  

21.8 Debugging implicits

  scala> val chars: List[Char] = "xyz"
  <console>:19: error: type mismatch;
   found   : java.lang.String("xyz")
   required: List[Char]
         val chars: List[Char] = "xyz"
                                 ^


  scala> val chars: List[Char] = wrapString("xyz")
  <console>:19: error: type mismatch;
   found   : scala.collection.immutable.WrappedString
   required: List[Char]
         val chars: List[Char] = wrapString("xyz")
                                           ^


  Mocha.this.enjoy("reader")(Mocha.this.pref)


  

21.9 Conclusion