Recursive LazyList construction inside def gives error

Asked Apr 24 '21 at 22:41

Active Apr 24 '21 at 22:41

Viewed 53 times

If I write the following everything works as expected:

object Working extends App {
  val whl: LazyList[Int] = 1 #:: 2 #:: 3 #:: whl
  (1 to 10) foreach {n => println(whl(n)) }
}

However, if I wrap my LazyList in a def:

object NotWorking extends App {
  def f(): Unit = {
    val whl: LazyList[Int] = 1 #:: 2 #:: 3 #:: whl
    (1 to 10) foreach {n => println(whl(n)) }
  }

  f()
}

then I get an error message

forward reference extends over definition of value whl val whl: LazyList[Int] = 1 #:: 2 #:: 3 #:: whl

Does anyone know why this happens? I am using Scala 2.13.5 with IntelliJ Idea, if this makes any difference.

asked Apr 24 '21 at 22:41

Daniel Robert-Nicoud

It needs to be lazy inside a method, object vals are always lazy. – Luis Miguel Mejía Suárez Apr 24 '21 at 22:55
1

Local variables cannot refer to themselves normally, so they need to be declared lazy. Object fields aren't really lazy, but the code above should be safe to use because `#::` doesn't evaluate `whl` until necessary. – user Apr 24 '21 at 23:35
@LuisMiguelMejíaSuárez Thanks, changing `val` to `lazy val` indeed works – Daniel Robert-Nicoud Apr 25 '21 at 09:43
I retracted my close vote and cannot vote again, but does this answer your question? [Should I use val or def when defining a Stream?](https://stackoverflow.com/q/13217222) – user Apr 25 '21 at 15:04
@user No, that is another question altogether (`def` vs. `val`) – Daniel Robert-Nicoud Apr 26 '21 at 08:19

Recursive LazyList construction inside def gives error

0 Answers0