interesting LazyList evaluation semantics

lazylist.scala

% mscala -v 2.13.0-M4
Welcome to Scala 2.13.0-M4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_181).
Type in expressions for evaluation. Or try :help.

scala> val counter = {
     |   var count = 0
     |   () => {
     |     val res = try "x" + count finally count = count + 1
     |     try res finally println(res)
     |   }
     | }
counter: () => String = $$Lambda$831/992524208@22048bd6

scala> def f = counter()
f: String

scala> def mk1(n: Int): LazyList[String] = if (n <= 0) LazyList() else f #:: mk1(n - 1)
mk1: (n: Int)LazyList[String]

scala> def mk2(n: Int): LazyList[String] = LazyList.fill(n)(f)
mk2: (n: Int)LazyList[String]

scala> def xs = mk1(5)
xs: LazyList[String]

scala> def ys = mk2(5)
ys: LazyList[String]

scala> xs(4)
x0
res0: String = x0

scala> ys(4)
x1
x2
x3
x4
x5
res1: String = x5

scala> xs.zipWithIndex.apply(4)
x6
res2: (String, Int) = (x6,4)

scala> xs.zipWithIndex.filter(_ => true).apply(4)
x7
x8
x9
x10
x11
res3: (String, Int) = (x11,4)

scala> xs.zipWithIndex.map(_ => 0).apply(4)
x12
res4: Int = 0

:pasteable

val counter = {
  var count = 0
  () => {
    val res = try "x" + count finally count = count + 1
    try res finally println(res)
  }
}

def f = counter()

def mk1(n: Int): LazyList[String] = if (n <= 0) LazyList() else f #:: mk1(n - 1)

def mk2(n: Int): LazyList[String] = LazyList.fill(n)(f)

def xs = mk1(5)

def ys = mk2(5)

xs(4)

ys(4)

xs.zipWithIndex.apply(4)

xs.zipWithIndex.filter(_ => true).apply(4)

xs.zipWithIndex.map(_ => 0).apply(4)