通过实例学习Either树和模式匹配

Either<Exception, Integer> result = RomanNumeralParser.parseNumber("XLII");

val VALID_GRADES = Set("A", "B", "C", "D", "F")
def letterGrade(value: Any) : String = value match {
case x:Int if (90 to 100).contains(x) => "A"
case x:Int if (80 to 90).contains(x) => "B"
case x:Int if (70 to 80).contains(x) => "C"
case x:Int if (60 to 70).contains(x) => "D"
case x:Int if (0 to 60).contains(x) => "F"
case x:String if VALID_GRADES(x.toUpperCase) => x.toUpperCase
}
printf("Amy scores %d and receives %s\n", 91, letterGrade(91))
printf("Bob scores %d and receives %s\n", 72, letterGrade(72))
printf("Sam never showed for class, scored %d, and received %s\n", 
44, letterGrade(44))
printf("Roy transfered and already had %s, which translated as %s\n",
"B", letterGrade("B"))

class Color(val red:Int, val green:Int, val blue:Int)
case class Red(r:Int) extends Color(r, 0, 0)
case class Green(g:Int) extends Color(0, g, 0)
case class Blue(b:Int) extends Color(0, 0, b)
def printColor(c:Color) = c match {
case Red(v) => println("Red: " + v)
case Green(v) => println("Green: " + v)
case Blue(v) => println("Blue: " + v)
case col:Color => {
print("R: " + col.red + ", ")
print("G: " + col.green + ", ")
println("B: " + col.blue)
}
case null => println("invalid color")
}

import fj.data.Either;
import static fj.data.Either.left;
import static fj.data.Either.right;
public abstract class Tree {
private Tree() {}
public abstract Either<Empty, Either<Leaf, Node>> toEither();
public static final class Empty extends Tree {
public Either<Empty, Either<Leaf, Node>> toEither() {
return left(this);
}
public Empty() {}
}
public static final class Leaf extends Tree {
public final int n;
public Either<Empty, Either<Leaf, Node>> toEither() {
return right(Either.<Leaf, Node>left(this));
}
public Leaf(int n) { this.n = n; }
}
public static final class Node extends Tree {
public final Tree left;
public final Tree right;
public Either<Empty, Either<Leaf, Node>> toEither() {
return right(Either.<Leaf, Node>right(this));
}
public Node(Tree left, Tree right) {
this.left = left;
this.right = right;
}
}
}

static public int depth(Tree t) {
for (Empty e : t.toEither().left())
return 0;
for (Either<Leaf, Node> ln: t.toEither().right()) {
for (Leaf leaf : ln.left())
return 1;
for (Node node : ln.right())
return 1 + max(depth(node.left), depth(node.right));
}
throw new RuntimeException("Inexhaustible pattern match on tree");
}

static public boolean inTree(Tree t, int value) {
for (Empty e : t.toEither().left())
return false;
for (Either<Leaf, Node> ln: t.toEither().right()) {
for (Leaf leaf : ln.left())
return value == leaf.n;
for (Node node : ln.right())
return inTree(node.left, value) | inTree(node.right, value);
}
return false;
}

@Test
public void more_elaborate_searchp_test() {
Tree t = new Node(new Node(new Node(new Node(
new Node(new Leaf(4),new Empty()), 
new Leaf(12)), new Leaf(55)), 
new Empty()), new Leaf(4));
assertTrue(inTree(t, 55));
assertTrue(inTree(t, 4));
assertTrue(inTree(t, 12));
assertFalse(inTree(t, 42));
}

static public int occurrencesIn(Tree t, int value) {
for (Empty e: t.toEither().left())
return 0;
for (Either<Leaf, Node> ln: t.toEither().right()) {
for (Leaf leaf : ln.left())
if (value == leaf.n) return 1;
for (Node node : ln.right())
return occurrencesIn(node.left, value) + occurrencesIn(node.right, value);
}
return 0;
}

@Test
public void multi_branch_tree_test() {
Tree t = new Node(new Node(new Node(new Leaf(4),
new Node(new Leaf(1), new Node(
new Node(new Node(new Node(
new Node(new Node(new Leaf(10), new Leaf(0)),
new Leaf(22)), new Node(new Node(
new Node(new Leaf(4), new Empty()),
new Leaf(101)), new Leaf(555))),
new Leaf(201)), new Leaf(1000)),
new Leaf(4)))),
new Leaf(12)), new Leaf(27));
assertEquals(12, depth(t));
assertTrue(inTree(t, 555));
assertEquals(3, occurrencesIn(t, 4));
}