Lazy stream does not work

Question

I'm using Stream to create a lazy sequence. My sequence is combined from others sequences by ++. But my code does not work. Why?

def select[T1, T2](s: Stream[T1], map: T1=>T2): Stream[T2] = {
    for (sv <- s) yield map(sv)
}
var innerCounter = 0
var proxy = (x: Int) => { innerCounter += 1; x }
var list: Stream[Int] = select[Int,Int](Stream.empty, k => k)

for (nc <- 0 until 10) {
     list = select[Int,Int](Stream.empty, k => k)
     var i: Int = 0
      while (i < CountConcat) {
          i += 1
          list = list ++ select[Int,Int](Stream(0), k => proxy(i + 100))
       }
}
assert(innerCounter == 0)

yes, why? please post the error message. and what's your code trying to achieve? — Erik Kaplun, Jul 31 '14 at 14:09
I tried to implement lazy access to a file. By my scenario the proxy function would to read from an opened file at random positions. But! I have big gap between the Stream init and a time of opening a file. And strictly evaluating the head of a Stream unacceptably for me — Chronos, Aug 01 '14 at 05:53

score 1 · Answer 1 · answered Jul 31 '14 at 15:27

your failure can be much simplified:

var innerCounter = 0
val proxy = (x: Int) => { innerCounter += 1; x }
Stream(0).map(k => proxy(100))
assert(innerCounter == 0)

I'm taking into account that your select function is nothing more than map.

map/flatMap for a Stream will strictly evaluate the head of the stream but not the tail of the stream when the tail is lazy. so here, we are mapping on a single element stream and or proxy function is evaluated the one time.

So, since you are mapping on the single element streams BEFORE you append them to the stream you are accumulating, you will see your proxy function being called each time.

try this one:

var innerCounter = 0
val proxy = (x: Int) => { innerCounter += 1; x }
(Stream.empty ++ Stream(0) ++ Stream(1)).map(k => proxy(100))
 assert(innerCounter == 1)

and you'll find that the assertion holds. The head of our stream is strictly mapped, but the tail of the stream hasn't been evaluated.

Lazy stream does not work

1 Answers1