Code Examples for

Programming in Scala, Second Edition

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19 Type Parameterization

Sample run of chapter's interpreter examples

19.1 Functional queues


scala> val q = Queue(1, 2, 3) q: Queue[Int] = Queue(1, 2, 3) scala> val q1 = q enqueue 4 q1: Queue[Int] = Queue(1, 2, 3, 4) scala> q res0: Queue[Int] = Queue(1, 2, 3)
// In file type-parameterization/Queues1.scala class SlowAppendQueue[T](elems: List[T]) { // Not efficient def head = elems.head def tail = new SlowAppendQueue(elems.tail) def enqueue(x: T) = new SlowAppendQueue(elems ::: List(x)) }
// In file type-parameterization/Queues2.scala class SlowHeadQueue[T](smele: List[T]) { // Not efficient // smele is elems reversed def head = smele.last def tail = new SlowHeadQueue(smele.init) def enqueue(x: T) = new SlowHeadQueue(x :: smele) }
// In file type-parameterization/Queues3.scala class Queue[T]( private val leading: List[T], private val trailing: List[T] ) { private def mirror = if (leading.isEmpty) new Queue(trailing.reverse, Nil) else this def head = mirror.leading.head def tail = { val q = mirror new Queue(q.leading.tail, q.trailing) } def enqueue(x: T) = new Queue(leading, x :: trailing) }

19.2 Information hiding


// In file type-parameterization/Queues4.scala class Queue[T] private ( private val leading: List[T], private val trailing: List[T] )
scala> new Queue(List(1, 2), List(3)) <console>:6: error: constructor Queue cannot be accessed in object $iw new Queue(List(1, 2), List(3)) ^
def this() = this(Nil, Nil)
def this(elems: T*) = this(elems.toList, Nil)
// In file type-parameterization/Queues4.scala object Queue { // constructs a queue with initial elements `xs' def apply[T](xs: T*) = new Queue[T](xs.toList, Nil) }
// In file type-parameterization/Queues5.scala trait Queue[T] { def head: T def tail: Queue[T] def enqueue(x: T): Queue[T] } object Queue { def apply[T](xs: T*): Queue[T] = new QueueImpl[T](xs.toList, Nil) private class QueueImpl[T]( private val leading: List[T], private val trailing: List[T] ) extends Queue[T] { def mirror = if (leading.isEmpty) new QueueImpl(trailing.reverse, Nil) else this def head: T = mirror.leading.head def tail: QueueImpl[T] = { val q = mirror new QueueImpl(q.leading.tail, q.trailing) } def enqueue(x: T) = new QueueImpl(leading, x :: trailing) } }

19.3 Variance annotations


scala> def doesNotCompile(q: Queue) {} <console>:5: error: trait Queue takes type parameters def doesNotCompile(q: Queue) {} ^
scala> def doesCompile(q: Queue[AnyRef]) {} doesCompile: (Queue[AnyRef])Unit
trait Queue[+T] { ... }
trait Queue[-T] { ... }
// In file type-parameterization/Misc.scala class Cell[T](init: T) { private[this] var current = init def get = current def set(x: T) { current = x } }
val c1 = new Cell[String]("abc") val c2: Cell[Any] = c1 c2.set(1) val s: String = c1.get
Cell.scala:7: error: covariant type T occurs in contravariant position in type T of value x def set(x: T) = current = x ^
// In file type-parameterization/JavaArrays.java // this is Java String[] a1 = { "abc" }; Object[] a2 = a1; a2[0] = new Integer(17); String s = a1[0];
Exception in thread "main" java.lang.ArrayStoreException: java.lang.Integer at JavaArrays.main(JavaArrays.java:8)
void sort(Object[] a, Comparator cmp) { ... }
scala> val a1 = Array("abc") a1: Array[java.lang.String] = Array(abc) scala> val a2: Array[Any] = a1 <console>:5: error: type mismatch; found : Array[java.lang.String] required: Array[Any] val a2: Array[Any] = a1 ^
scala> val a2: Array[Object] = | a1.asInstanceOf[Array[Object]] a2: Array[java.lang.Object] = Array(abc)

19.4 Checking variance annotations


class StrangeIntQueue extends Queue[Int] { override def enqueue(x: Int) = { println(math.sqrt(x)) super.enqueue(x) } }
val x: Queue[Any] = new StrangeIntQueue x.enqueue("abc")
class Queue[+T] { def enqueue(x: T) = ... }
Queues.scala:11: error: covariant type T occurs in contravariant position in type T of value x def enqueue(x: T) = ^
abstract class Cat[-T, +U] { def meow[W-](volume: T-, listener: Cat[U+, T-]-) : Cat[Cat[U+, T-]-, U+]+ }

19.5 Lower bounds


class Queue[+T] (private val leading: List[T], private val trailing: List[T] ) { def enqueue[U >: T](x: U) = new Queue[U](leading, x :: trailing) // ... }

19.6 Contravariance


// In file type-parameterization/Misc.scala trait OutputChannel[-T] { def write(x: T) }
// In file type-parameterization/Misc.scala trait Function1[-S, +T] { def apply(x: S): T }
// In file type-parameterization/Customer.scala class Publication(val title: String) class Book(title: String) extends Publication(title) object Library { val books: Set[Book] = Set( new Book("Programming in Scala"), new Book("Walden") ) def printBookList(info: Book => AnyRef) { for (book <- books) println(info(book)) } } object Customer extends Application { def getTitle(p: Publication): String = p.title Library.printBookList(getTitle) }
Library.printBookList(getTitle)
class Queue[+T] private ( private[this] var leading: List[T], private[this] var trailing: List[T] ) { private def mirror() = if (leading.isEmpty) { while (!trailing.isEmpty) { leading = trailing.head :: leading trailing = trailing.tail } } def head: T = { mirror() leading.head } def tail: Queue[T] = { mirror() new Queue(leading.tail, trailing) } def enqueue[U >: T](x: U) = new Queue[U](leading, x :: trailing) }

19.7 Object private data


Queues.scala:1: error: covariant type T occurs in contravariant position in type List[T] of parameter of setter leading_= class Queue[+T] private (private var leading: List[T], ^ Queues.scala:1: error: covariant type T occurs in contravariant position in type List[T] of parameter of setter trailing_= private var trailing: List[T]) { ^

19.8 Upper bounds


scala> val robert = new Person("Robert", "Jones") robert: Person = Robert Jones scala> val sally = new Person("Sally", "Smith") sally: Person = Sally Smith scala> robert < sally res0: Boolean = true
class Person(val firstName: String, val lastName: String) extends Ordered[Person] { def compare(that: Person) = { val lastNameComparison = lastName.compareToIgnoreCase(that.lastName) if (lastNameComparison != 0) lastNameComparison else firstName.compareToIgnoreCase(that.firstName) } override def toString = firstName +" "+ lastName }
def orderedMergeSort[T <: Ordered[T]](xs: List[T]): List[T] = { def merge(xs: List[T], ys: List[T]): List[T] = (xs, ys) match { case (Nil, _) => ys case (_, Nil) => xs case (x :: xs1, y :: ys1) => if (x < y) x :: merge(xs1, ys) else y :: merge(xs, ys1) } val n = xs.length / 2 if (n == 0) xs else { val (ys, zs) = xs splitAt n merge(orderedMergeSort(ys), orderedMergeSort(zs)) } }
scala> val people = List( | new Person("Larry", "Wall"), | new Person("Anders", "Hejlsberg"), | new Person("Guido", "van Rossum"), | new Person("Alan", "Kay"), | new Person("Yukihiro", "Matsumoto") | ) people: List[Person] = List(Larry Wall, Anders Hejlsberg, Guido van Rossum, Alan Kay, Yukihiro Matsumoto)
scala> val sortedPeople = orderedMergeSort(people) sortedPeople: List[Person] = List(Anders Hejlsberg, Alan Kay, Yukihiro Matsumoto, Guido van Rossum, Larry Wall)
scala> val wontCompile = orderedMergeSort(List(3, 2, 1)) <console>:5: error: inferred type arguments [Int] do not conform to method orderedMergeSort's type parameter bounds [T <: Ordered[T]] val wontCompile = orderedMergeSort(List(3, 2, 1)) ^

19.9 Conclusion

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