// FOR is very unlike for in Java

val xss = List(List(1,2),List(3,4))

for (xs <- xss) { 
  for (x <- xs) print(x + "\t") 
  println() 
} 

// is really just a special case of the general
// FOR comprehension which returns a result for
// each iteration and collects it into a list:

for (p <- persons if p.age > 20) yield p.name 

// is equivalent to 

persons filter (p => p.age > 20) map (p => p.name)

/*
 generally:  for ( s ) yield e 
 s is a sequence of generators, definitions and filters.
 generators:   x <- e   where e is a list (or expression resulting in a list)
 definitions:   val x = e
 filters:      boolean expression
*/

// sometimes more readable than map/filter/...

{ i <- List.range(1, n) 
  j <- List.range(1, i) 
  if isPrime(i+j) } yield {i, j} 

// n-queens example ...

// similar to SQL select statements

case class Book(title: String, authors: List[String]) 


val books: List[Book] = List( 
  Book("Structure and Interpretation of Computer Programs", 
       List("Abelson, Harold", "Sussman, Gerald J.")), 
  Book("Principles of Compiler Design", 
       List("Aho, Alfred", "Ullman, Jeffrey")), 
  Book("Programming in Modula-2", 
       List("Wirth, Niklaus")), 
  Book("Introduction to Functional Programming", 
       List("Bird, Richard")), 
  Book("The Java Language Specification", 
       List("Gosling, James", "Joy, Bill", "Steele, Guy", "Bracha, Gilad")))


/* try
 * 
for (b <- books; a <- b.authors if a startsWith "Ullman") 
  yield b.title

for (b <- books if (b.title indexOf "Program") >= 0) 
  yield b.title

  and similar
*/

// translation of FOR only depends on  map, flatMap, and filter (see 10.4)
// therefore if your container implements these, you can use FOR


def map[B](f: A => B): C[B] 
def flatMap[B](f: A => C[B]): C[B] 
def filter(p: A => Boolean): C[A]