reevik

public class CatBar {
    private List<Cat> catFriends = new ArrayList<>();
    public void enter(Cat cat) {
        catFriends.add(cat);
    }
    public void singTogether() {
        for (Cat cat: catFriends) {
            cat.sing();
        }
    }

reevik

Lion lion = new Lion(AnimalSoundLib.ROAR);
lion.sing(); // roarrr

reevik

Cat cat = catService.getCat(); // a real cat created which meows
catbar.enter(cat); 
Cat lion = catService.getCat(); // this time the service gives you a lion instance back.
catbar.enter(lion);
catbar.singTogether(); // party begins: meoww moewww ROOARRR meow

reevik

trait M[T] {  def flatMap[U](f: T => M[U]): M[U] }

reevik

val list = List(1,2,3,4,5) 
list.flatMap(e => List(e - 1, e, e + 1))  

> res0: List[Int] = List(0, 1, 2, 1, 2, 3, 2, 3, 4, 3, 4, 5, 4, 5, 6)

reevik

val list = List(1,2,3,4,5)
list.map(e => List(e - 1, e, e + 1))

> res0: List[List[Int]] = List(List(0, 1, 2), List(1, 2, 3), List(2, 3, 4), List(3, 4, 5), List(4, 5, 6))

reevik

for (i  1 to n; if i > j)   yield (i, j)

reevik

(2,1), (3,1), (3,2), (4,1), (4,2), ...

reevik

$ scala -Xprint:parser -e "for (i <- 1 to n; if i % 2 == 0) yield i"

reevik

(1 to m).flatMap(i => 
  (1 to n).filter(j => i > j) 
     map(k => (i, k)))