gistfile1.txt

val a = Some(1)
val b = Some(2)
val c = Some(3)

for {
  x <- a
  y <- b
  z <- c
} yield x + y + z // Some(6)

a.flatMap(x => // A => Option[B]
  b.flatMap(y => // B => Option[C]
    c.map(z => // C => D
      x + y + z
    )
  )
) // Option[B] Some(6)
a.map(x => // A => B A => Option[_]
  b.map(y => 
    c.map(z => 
    x + y + z)
  )
) // Option[Option[Option[B]]] Some(Some(Some(6)))