At first glance you may think scala for comprehensions are a slightly more sophisticated version of the for loop. Under the hood, they are much more. What they really are is syntactic sugar over the higher order functions flatMap, map, withFilter, and foreach, and they are compiled down to a series of these higher order functions chained together.

The challenge for you, reader, is to write an equivalent expression for each of the following for comprehensions using only the higher order functions mentioned. Each comprehension is independent and you may assume the types for the named values (a, b, etc.) are correct. Some answers are given at the end of the post.

1.

for (x <- a) yield x + 1

2.

for {
  x <- a
  y <- b
  z <- c
} yield x + y + z

3.

for {
  x <- a
  y <- x
  z <- y if z % 69 == 0
} yield s"z: ${z + 1}"

4.

for {
  (x, y) <- a
  z = x * y
  w <- b if w - z > 0 && z != 0
  v = w / z
} println(v)

Answers

1.

a map (_ + 1)

2.

a flatMap (x => b flatMap (y => c map (z => x + y + z)))

3.

a flatMap { x =>
  x flatMap { y =>
    y withFilter (_ == 2) map { z =>
      z + 1
    }
  }
}

4.

a flatMap { case (x, y) =>
  val z = x * y
  b withFilter (_ - z > 0 && z != 0) map (_ / z)
} foreach println