For Expressions

for expressions can nest, with later generators varying more rapidly than earlier ones: 

val xValues = 1 to 4
val yValues = 1 to 2
val coordinates = for {
  x <- xValues
  y <- yValues
} yield (x, y)
coordinates(4) should be((res0, res1))

Using for we can make more readable code: 

val nums = List(List(1), List(2), List(3), List(4), List(5))

val result = for {
  numList <- nums
  num <- numList
  if (num % 2 == 0)
} yield (num)

result should be(res0)

// Which is the same as
nums.flatMap(numList => numList).filter(_ % 2 == 0) should be(result)

// or the same as
nums.flatten.filter(_ % 2 == 0) should be(result)