How to add tracing within a 'for' comprehension?

Question

For log tracing inside a for comprehension, I\'ve used dummy assignment like this:

val ll = List(List(1,2),List(1))            

for {
  outer <         


        

Answer 1

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._

// val lists = List(List(1, 2), List(1))
for {
  outer <- lists
  inner <- outer.tap(println)
} yield inner
// List(2, 4, 6)
// List(4, 8, 12)
// ls: List[Int] = List(4, 8, 12)

---

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

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