module fors {

  trait Person {
    val name: String;
    val age: Int;
  }

  def printOlderThan20(xs: Seq[Person]): Iterator[String] =
    printOlderThan20(xs.elements);

  def printOlderThan20(xs: Iterator[Person]): Iterator[String] =
    for (val p <- xs; p.age > 20) yield p.name;

  def divisors(n: Int): List[Int] =
    for (val i <- List.range(1, n); n % i == 0) yield i;

  def isPrime(n: Int) = divisors(n).length == 2;

  def findNums(n: Int): Iterator[Pair[Int, Int]] =
    for (val i <- Iterator.range(1, n);
         val j <- Iterator.range(1, i-1);
         isPrime(i+j)) yield Pair(i, j);

  def sum(xs: List[Double]): Double =
    (0.0 foldl_: xs) { (x, y) => x + y }

  def scalProd(xs: List[Double], ys: List[Double]) =
    sum(for(val Pair(x, y) <- xs zip ys) yield x * y);

  type Lst = List[Any];

  val books = List(
    'book('title("Structure and Interpretation of Computer Programs"),
          'author("Abelson, Harald"),
          'author("Sussman, Gerald J.")),
    'book('title("Principles of Compiler Design"),
          'author("Aho, Alfred"),
          'author("Ullman, Jeffrey")),
    'book('title("Programming in Modula-2"),
	  'author("Wirth, Niklaus")));

  def findAuthor(books: Lst) =
    for (val 'book(book: Lst) <- books;
         val 'title(title: String) <- book;
         (title indexOf "Program") >= 0;
         val 'author(author: String) <- book) yield author;

  for (val 'book(b: Lst) <- books;
       val 'author(author: String) <- b;
       author startsWith "Ullman";
       val 'title(title: String) <- b) yield title;

  removeDuplicates(
    for (val 'book(b1: Lst) <- books;
       val 'book(b2: Lst) <- books;
	 b1 != b2;
	 val 'author(a1: String) <- b1;
	 val 'author(a2: String) <- b2;
	 a1 == a2) yield Pair(a1, a2));

  def removeDuplicates[a](xs: List[a]): List[a] =
    xs.head :: removeDuplicates(for (val x <- xs.tail; x != xs.head) yield x)
}