Stream#iterator should be override. #3273
Comments
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Imported From: https://issues.scala-lang.org/browse/SI-3273?orig=1 |
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Eastsun (eastsun) said: val fib: Stream[Int] = 1 #:: 1 #:: (fib, fib.tail).zippedx.map(_ + _)While I expected to get fib(3) without any exception. I must have something missed. Is there any one to tell me why. I believe it is the key to fix #3273 import scala.collection.{TraversableLike, IterableLike}
import scala.collection.generic.CanBuildFrom
class LazyItrSt[A](stream: Stream[A]) {
class LazyCell[T](st: => Stream[T]){ lazy val v = st }
def iterator: Iterator[A] = new Iterator[A] {
var these = new LazyCell(stream)
def hasNext: Boolean = !these.v.isEmpty
def next: A =
if (hasNext) {
val result = these.v.head;
these =new LazyCell(these.v.tail);
result
} else Iterator.empty.next
override def toList: List[A] = stream.toList
}
}
class Zippedx[+Repr1, +El1, +Repr2, +El2](coll1: TraversableLike[El1, Repr1], coll2: IterableLike[El2, Repr2]) {
def map[B, To](f: (El1, El2) => B)(implicit cbf: CanBuildFrom[Repr1, B, To]): To = {
val elems2 = (new LazyItrSt(coll2.asInstanceOf[Stream[El2]])).iterator
for(el1 <- coll1 if elems2.hasNext) yield f(el1, elems2.next)
}
}
class Zippedx2[+Repr1, +El1, +Repr2, +El2](coll1: TraversableLike[El1, Repr1], coll2: IterableLike[El2, Repr2]) {
def map[B, To](f: (El1, El2) => B)(implicit cbf: CanBuildFrom[Repr1, B, To]): To = {
val elems2 = coll2.iterator
for(el1 <- coll1 if elems2.hasNext) yield f(el1, elems2.next)
}
}
object Debug extends Application {
implicit def t2t[T1,T2](t: (T1,T2)) = new {
def zippedx[Repr1, El1, Repr2, El2](implicit w1: T1 => TraversableLike[El1, Repr1],
w2: T2 => IterableLike[El2, Repr2]): Zippedx[Repr1, El1, Repr2, El2]
= new Zippedx[Repr1, El1, Repr2, El2](t._1, t._2)
}
implicit def t2t2[T1,T2](t: (T1,T2)) = new {
def zippedx2[Repr1, El1, Repr2, El2](implicit w1: T1 => TraversableLike[El1, Repr1],
w2: T2 => IterableLike[El2, Repr2]): Zippedx2[Repr1, El1, Repr2, El2]
= new Zippedx2[Repr1, El1, Repr2, El2](t._1, t._2)
}
val fib: Stream[Int] = 1 #:: 1 #:: (fib, fib.tail).zippedx2.map(_ + _)
//val fib: Stream[Int] = 1 #:: 1 #:: (fib, fib.tail).zippedx.map(_ + _)
println(fib(2))
println(fib(3))
}
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Eastsun (eastsun) said: |
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Eastsun (eastsun) said: class Stream[A] {
...
class LazyCell[T](st: => Stream[T]){ lazy val v = st }
def iterator: Iterator[A] = new Iterator[A] {
var these = new LazyCell(Stream.this)
def hasNext: Boolean = !these.v.isEmpty
def next: A =
if (hasNext) {
val result = these.v.head
val tmp = these //Here...
these =new LazyCell(tmp.v.tail);
result
} else Iterator.empty.next
override def toList: List[A] = Stream.this.toList
}
} |
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@paulp said:
I applaud your progress, but so far in my limited fiddling with your implementation I haven't gotten zipped to behave fully lazily. I can't spend any more time on it, but it would be great if you could take this all the way to a patch against trunk. (I can't guarantee it'll go into 2.8 though.) |
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Eastsun (eastsun) said:
It works fine in my tests. Anyway, this ticket(SI-3273) should be fixed override /*LinearSeqLike*/
def iterator: Iterator[A] = new Iterator[A] {
class LazyCell(st: => Stream[A]){ lazy val v = st }
var these = new LazyCell(self)
def hasNext: Boolean = !these.v.isEmpty
def next: A =
if (hasNext) {
val cur = these.v
val result = cur.head
these = new LazyCell(cur.tail)
result
} else Iterator.empty.next
override def toStream: Stream[A] = these.v
override def toList: List[A] = these.v.toList
} |
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@dubochet said: |
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@SethTisue said: is there something short I can type into the REPL to see for myself that Stream.iterator needs to be "more lazy"? |
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Eastsun (eastsun) said:
scala> val num1: Stream[Int] = 1 #:: num1.map{ _+1 }
num1: Stream[Int] = Stream(1, ?)
scala> num1.take(10).toList
res1: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
scala> val num2: Stream[Int] = 1 #:: num2.iterator.map{ _+1 }.toStream
num2: Stream[Int] = Stream(1, ?)
scala> num2.take(10).toList
java.lang.StackOverflowError
at scala.collection.LinearSeqLike$$$$anon$$1.hasNext(LinearSeqLike.scala:54)
at scala.collection.Iterator$$$$anon$$19.hasNext(Iterator.scala:368)
at scala.collection.Iterator$$class.toStream(Iterator.scala:1011)
at scala.collection.Iterator$$$$anon$$19.toStream(Iterator.scala:367)
at $$anonfun$$1.apply(<console>:5)
at $$anonfun$$1.apply(<console>:5)
at scala.collection.immutable.Stream$$Cons.tail(Stream.scala:571)
at scala.collection.immutable.Stream$$Cons.tail(Stream.scala:565)
at scala.collection.LinearSeqLike$$$$anon$$1.next(LinearSeqLike.scala:57)
at scala.collection.Iterator$$$$anon$$19.next(Iterator.scala:369)
at scala.collection.Iterator$$class.toStream(Iterator.scala:1011)
at scala.collection.Iterator$$$$anon$$19.toStream(Iterator.scala:367)
at $$anonfun$$1.apply(<...And with lazy iterator: scala> def lazyItr[A](xs: Stream[A]): Iterator[A] = new Iterator[A] {
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| class LazyCell(st: => Stream[A]){ lazy val v = st }
|
| var these = new LazyCell(xs)
| def hasNext: Boolean = !these.v.isEmpty
| def next: A =
| if (hasNext) {
| val cur = these.v
| val result = cur.head
| these = new LazyCell(cur.tail)
| result
| } else Iterator.empty.next
| }
lazyItr: [A](xs: Stream[A])Iterator[A]
scala> val num3: Stream[Int] = 1 #:: lazyItr(num3).map{ _+1 }.toStream
num1: Stream[Int] = Stream(1, ?)
scala> num3.take(10).toList
res0: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10) |
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Eastsun (eastsun) said: |
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@paulp said:
In case you're not on the list, see my comments at http://permalink.gmane.org/gmane.comp.lang.scala.internals/3890 . (Further discussion at #3526 please.) |
/*My English is not so good, forgive my expression */
The iterator method of Stream is defined in LinearSeqLike now. I think it should be override in class Stream to make it more lazy.
In my opinion, it should be something like following:
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