println in scala for-comprehension

This is how you need to write it:

scala> def prod(m: Int) = {
     |   for {
     |     a <- 2 to m / (2 * 3)
     |     _ = print(a + " ")
     |     b <- (a + 1) to (m / a)
     |     c = a * b
     |     if c < m
     |   } yield c
     | }
prod: (m: Int)scala.collection.immutable.IndexedSeq[Int]

scala> prod(20)
2 3 res159: scala.collection.immutable.IndexedSeq[Int] = Vector(6, 8, 10, 12, 14
, 16, 18, 12, 15, 18)

Starting Scala 2.13, the chaining operation tap, has been included in the standard library, and can be used with minimum intrusiveness wherever we need to print some intermediate state of a pipeline:

import util.chaining._

def prod(m: Int) =
  for {
    a <- 2 to m / (2 * 3)
    b <- (a + 1) to (m / a.tap(println)) // <- a.tap(println)
    c =  a * b
    if c < m
 } yield c

prod(20)
// 2
// 3
// res0: IndexedSeq[Int] = Vector(6, 8, 10, 12, 14, 16, 18, 12, 15, 18)

---

The tap chaining operation applies a side effect (in this case println) on a value (in this case a) while returning the value (a) untouched:

def tap[U](f: (A) => U): A

---

It's very convenient when debugging as you can use a bunch of taps without having to modify the code:

def prod(m: Int) =
  for {
    a <- (2 to m.tap(println) / (2 * 3)).tap(println)
    b <- (a + 1) to (m / a.tap(println))
    c = (a * b).tap(println)
    if c < m
 } yield c