[scala]
package examples
/** Illustrate the use of pattern matching in Scala.
Like patterns.scala, but uses extractors for representation independence
*/
object extractorPatterns {/** We need an abstract base class for trees. Subclasses with
- the ‘case’ modifier can be used in pattern matching expressions
- to deconstruct trees.
* - Here, we replaced case classes of patterns.scala with objects
- that hide the actual implementation of Branch and Leaf. Note
- that the remaining code does not change. In this way, we
- can change the implementation later without affecting clients,
- which is called representation independence.
*/
abstract class Tree
object Branch {
/ method to contruct branches @see extractorPatterns.tree1 /
def apply(left: Tree, right: Tree): Tree = new BranchImpl(left, right)
/ extractor method referenced in match expressions @see extractorPatterns.sumLeaves /
def unapply(x:Tree): Option[(Tree,Tree)] = x match {
case y:BranchImpl => Some(y.left, y.right)
case => None
}
private class BranchImpl(val left:Tree, val right:Tree) extends Tree
}
object Leaf {
/ method to contruct leaves @see tree1 /
def apply(x:Int): Tree = new LeafImpl(x);
/ extractor method referenced in match expressions @see extractorPatterns.sumLeaves /
def unapply(x:Tree): Option[Int] = x match {
case y:LeafImpl => Some(y.x)
case => None
}
private class LeafImpl(val x: Int) extends Tree
}/** Case classes have an implicit constructor methods which allows
- to create objects withouth the ‘new’ keyword. It saves some typing
- and makes code clearer.
* - Here, the task of the case class constructor is performed by the
- method Branch.apply - the singleton Branch is treated as if it
- were a function value. This trick works with any value that has
- an apply method.
*/
val tree1 = Branch(Branch(Leaf(1), Leaf(2)), Branch(Leaf(3), Leaf(4)))
/** Return the sum of numbers found in leaves.
- ‘match’ is a generalization of ‘switch’ in C-like languages
* - Patterns consist of case class constructors (which can
- be nested), and lower case variables which are
- bound to the values with which the class has been constructed.
* - For extractors, it is not the name of a case class, but the name of
- the singleton object Branch which is used to refer to its extractor method
- Branch.unapply - the pattern is the ‘reverse’ of a method
- call, with the result being matched in the subpatterns. This works
- for any value that has an appropriate extractor method.
*/
def sumLeaves(t: Tree): Int = t match {
case Branch(l, r) => sumLeaves(l) + sumLeaves(r)
case Leaf(x) => x
}
/** This illustrates the use of Option types. Since the
- method is not known in advance to find ‘x’, the
- return type is an Option. Options have two possible
- values, either ‘Some’ or ‘None’. It is a type-safe
- way around ‘null’ values.
*/
def findA, B: Option[B] = {
var result: Option[B] = None
while (it.hasNext && result == None) {
val Pair(x1, y) = it.next;
if (x == x1) result = Some(y)
}
result
}
def printFindsA =
find(xs.elements, x) match {
case Some(y) => println(y)
case None => println(“no match”)
}def main(args: Array[String]) {
println(“sum of leafs=” + sumLeaves(tree1));
printFinds(List(Pair(3, “three”), Pair(4, “four”)), 4)
}
}
package examples
/** Illustrate the use of pattern matching in Scala.
Like patterns.scala, but uses extractors for representation independence
*/
object extractorPatterns {/** We need an abstract base class for trees. Subclasses with
- the ‘case’ modifier can be used in pattern matching expressions
- to deconstruct trees.
* - Here, we replaced case classes of patterns.scala with objects
- that hide the actual implementation of Branch and Leaf. Note
- that the remaining code does not change. In this way, we
- can change the implementation later without affecting clients,
- which is called representation independence.
*/
abstract class Tree
object Branch {
/ method to contruct branches @see extractorPatterns.tree1 /
def apply(left: Tree, right: Tree): Tree = new BranchImpl(left, right)
/ extractor method referenced in match expressions @see extractorPatterns.sumLeaves /
def unapply(x:Tree): Option[(Tree,Tree)] = x match {
case y:BranchImpl => Some(y.left, y.right)
case => None
}
private class BranchImpl(val left:Tree, val right:Tree) extends Tree
}
object Leaf {
/ method to contruct leaves @see tree1 /
def apply(x:Int): Tree = new LeafImpl(x);
/ extractor method referenced in match expressions @see extractorPatterns.sumLeaves /
def unapply(x:Tree): Option[Int] = x match {
case y:LeafImpl => Some(y.x)
case => None
}
private class LeafImpl(val x: Int) extends Tree
}/** Case classes have an implicit constructor methods which allows
- to create objects withouth the ‘new’ keyword. It saves some typing
- and makes code clearer.
* - Here, the task of the case class constructor is performed by the
- method Branch.apply - the singleton Branch is treated as if it
- were a function value. This trick works with any value that has
- an apply method.
*/
val tree1 = Branch(Branch(Leaf(1), Leaf(2)), Branch(Leaf(3), Leaf(4)))
/** Return the sum of numbers found in leaves.
- ‘match’ is a generalization of ‘switch’ in C-like languages
* - Patterns consist of case class constructors (which can
- be nested), and lower case variables which are
- bound to the values with which the class has been constructed.
* - For extractors, it is not the name of a case class, but the name of
- the singleton object Branch which is used to refer to its extractor method
- Branch.unapply - the pattern is the ‘reverse’ of a method
- call, with the result being matched in the subpatterns. This works
- for any value that has an appropriate extractor method.
*/
def sumLeaves(t: Tree): Int = t match {
case Branch(l, r) => sumLeaves(l) + sumLeaves(r)
case Leaf(x) => x
}
/** This illustrates the use of Option types. Since the
- method is not known in advance to find ‘x’, the
- return type is an Option. Options have two possible
- values, either ‘Some’ or ‘None’. It is a type-safe
- way around ‘null’ values.
*/
def findA, B: Option[B] = {
var result: Option[B] = None
while (it.hasNext && result == None) {
val Pair(x1, y) = it.next;
if (x == x1) result = Some(y)
}
result
}
def printFindsA =
find(xs.elements, x) match {
case Some(y) => println(y)
case None => println(“no match”)
}def main(args: Array[String]) {
println(“sum of leafs=” + sumLeaves(tree1));
printFinds(List(Pair(3, “three”), Pair(4, “four”)), 4)
}
}
[/scala]