Scala NullPointerException unless lazy

Question

  def fix[A, B](fn : Function2[Function1[A, B], A, B]) : Function1[A, B] =
    (x : A) => fn(fix(fn), x)

  lazy val fibs1 = fix[(Int, Int), Stream[Int]](
    (fn, a) => a._1 #:: fn((a._2, a._1 + a._2))
  )

  val fibs2 = fix[(Int, Int), Stream[Int]](
    (fn, a) => a._1 #:: fn((a._2, a._1 + a._2))
  )

While learning Scala I ran into a strange error. Why is it that fibs1((1,1)) produces no error, while fibs2((1, 1)) gives a null pointer exception?

EDIT:

This code was inside a App class. It seems like vals are not initialized until instance.main(...) is evaluated. I assume the implementation for lazy vals is different.

I don't have any NPE on fibs2 evaluation – Nyavro Oct 07 '16 at 02:15 — Nyavro, Oct 07 '16 at 02:15
same here, no NPE when I ran it on repl – Leo Bufi Barrameda Oct 07 '16 at 02:19 — Leo Bufi Barrameda, Oct 07 '16 at 02:19

score 0 · Answer 1 · answered Oct 07 '16 at 20:51

0

you are right. some val was not initialized, but since you make it a lazy val, it was only evaluated when needed, and at that time it was already initialized by some code you don't control

answered Oct 07 '16 at 20:51

pedrorijo91

7,635
9
44
82

Scala NullPointerException unless lazy

1 Answers1