scala> val l = List(1, 2, 3)
l: List[Int] = List(1, 2, 3)

scala> val l2 = List(1, 2, 3, 4)
l2: List[Int] = List(1, 2, 3, 4)

scala> l.foldLeft
   override def foldLeft[B](z: B)(op: (B, Int) => B): B

scala> l.foldLeft(0)((acc, cur) => acc + cur)
res12: Int = 6

scala> l2.foldLeft(0)((acc, cur) => acc + cur)
res13: Int = 10

scala> l2.foldLeft(0)((acc, cur) => acc * cur)
res14: Int = 0

scala> l2.foldLeft(1)((acc, cur) => acc * cur)
res15: Int = 24

scala> l2.product
res16: Int = 24

scala> l2.foldLeft("")((acc, cur) => acc + ", " + cur.toString)
res17: String = , 1, 2, 3, 4

scala> l2.foldLeft("")((acc, cur) => if(acc == "") cur.toString else acc + ", " + cur.toString)
res19: String = 1, 2, 3, 4

scala> l2.foldLeft("List(")((acc, cur) => if(acc == "List(") cur.toString else acc + ", " + cur.toString)
res20: String = 1, 2, 3, 4

scala> l2.foldLeft("List(")((acc, cur) => if(acc == "List(") "List(" + cur.toString else acc + ", " + cur.toString)
res21: String = List(1, 2, 3, 4

scala> l2.foldLeft("List(")((acc, cur) => if(acc == "List(") "List(" + cur.toString else acc + ", " + cur.toString) + ")"
res22: String = List(1, 2, 3, 4)

scala> l2.foldLeft("0")((acc, cur) => acc + " + " + cur.toString)
res23: String = 0 + 1 + 2 + 3 + 4

scala> l2.foldLeft("0")((acc, cur) => "(" + acc + " + " + cur.toString + ")")
res24: String = ((((0 + 1) + 2) + 3) + 4)

scala> l2.foldLeft("1")((acc, cur) => "(" + acc + " * " + cur.toString + ")")
res25: String = ((((1 * 1) * 2) * 3) * 4)

scala> l2.foldRight
   override def foldRight[B](z: B)(op: (Int, B) => B): B

scala> l2.foldRight(0)((acc, cur) => acc + cur)
res26: Int = 10

scala> l2.foldRight(0)(_ + _)
res27: Int = 10

scala> l2.foldRight("0")((cur, acc) => cur.toString + " + " + acc)
res28: String = 1 + 2 + 3 + 4 + 0

scala> l2.foldRight("0")((cur, acc) => "(" + cur.toString + " + " + acc + ")")
res29: String = (1 + (2 + (3 + (4 + 0))))

scala> l2.foldLeft("0")((acc, cur) => "(" + acc + " + " + cur.toString + ")")
res30: String = ((((0 + 1) + 2) + 3) + 4)

scala> l2.foldLeft(List[Int]())((acc, cur) => acc ++ List(cur))
res31: List[Int] = List(1, 2, 3, 4)

scala> l2.foldLeft(List[Int]())((acc, cur) => acc ++ List(cur * 2))
res32: List[Int] = List(2, 4, 6, 8)

scala> l2.foldLeft(List[Int]())((acc, cur) => if(cur % 2 == 1) acc ++ List(cur) else acc)
res33: List[Int] = List(1, 3)

scala> l2.filter(x => x % 2 == 1)
res34: List[Int] = List(1, 3)

scala> l2.foldLeft(List[Int]())((acc, cur) => if(cur % 2 == 1) acc ++ List(cur))
<console>:13: error: type mismatch;
 found   : Unit
 required: List[Int]
       l2.foldLeft(List[Int]())((acc, cur) => if(cur % 2 == 1) acc ++ List(cur))
                                              ^

scala> l2.map(x => List(x, x))
res36: List[List[Int]] = List(List(1, 1), List(2, 2), List(3, 3), List(4, 4))

scala> l2.map(x => x)
res37: List[Int] = List(1, 2, 3, 4)

scala> l2.map(x => List(x, x)).flatten
res38: List[Int] = List(1, 1, 2, 2, 3, 3, 4, 4)

scala> l2.flatMap(x => List(x, x))
res39: List[Int] = List(1, 1, 2, 2, 3, 3, 4, 4)

scala> l2.flatMap(x => List(x * 2))
res40: List[Int] = List(2, 4, 6, 8)

scala> l2.flatten
<console>:13: error: No implicit view available from Int => scala.collection.GenTraversableOnce[B].
       l2.flatten
          ^

scala> List(List(List(1, 1), List(2, 2)))
res42: List[List[List[Int]]] = List(List(List(1, 1), List(2, 2)))

scala> List(List(List(1, 1), List(2, 2))).flatten
res43: List[List[Int]] = List(List(1, 1), List(2, 2))

scala> def mp[A, B](l: List[A])(f: A => B): List[B] = l.flatMap(x => List(f(x)))
mp: [A, B](l: List[A])(f: A => B)List[B]

scala> mp(l2)(x => x * 2)
res44: List[Int] = List(2, 4, 6, 8)

scala> mp(l2)(x => x * 3)
res45: List[Int] = List(3, 6, 9, 12)

scala> l2.flatMap(x => List(x % 2 == 1))
res46: List[Boolean] = List(true, false, true, false)

scala> l2.flatMap(x => if(x % 2 == 1) List(x) else List())
res47: List[Int] = List(1, 3)