Stream#withFilter should be override.

[scabug]

Now the method withFilter of scala.collection.immutable.Stream is defined in scala.collection.TraversableLike. This lead to some problems：

scala> val nums = Stream.from(1)
nums: scala.collection.immutable.Stream[Int] = Stream(1, ?)

scala> val even = for(n <- nums if n%2 == 0) yield n
java.lang.OutOfMemoryError: Java heap space
        at scala.collection.TraversableOnce$$class.toList(TraversableOnce.scala:393)
        at scala.collection.mutable.WrappedArray.toList(WrappedArray.scala:25)
        at scala.collection.immutable.List$$.apply(List.scala:448)
        at scala.collection.mutable.LazyBuilder.$$plus$$eq(LazyBuilder.scala:24)
        at scala.collection.mutable.LazyBuilder.$$plus$$eq(LazyBuilder.scala:21)
        at scala.collection.TraversableLike$$WithFilter$$$$anonfun$$map$$2.apply(TraversableLike.scala:775)
        at scala.collection.immutable.Stream.foreach(Stream.scala:195)
        at scala.collection.TraversableLike$$WithFilter.map(TraversableLike.scala:774)
        at .<init>(<console>:6)
        at .<clinit>(<console>)
        at RequestResult$$.<init>(<console>:9)
        at RequestResult$$.<clinit>(<console>)
        at RequestResult$$scala_repl_resul...
scala>

I think we should override withFilter in Stream like following(not test):

abstract class Stream[+A] extends LinearSeq[A] 
                             with GenericTraversableTemplate[A, Stream]
                             with LinearSeqOptimized[A, Stream[A]] {
self =>
  ...
  
  final override def withFilter(p: A => Boolean): WithFilter = new StreamFilter(p)
  
  class StreamFilter(p: A => Boolean) extends WithFilter(p) {
  
    def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Repr, B, That]): That = 
      (if(isEmpty) Stream.Empty
	   else if(p(head)) new Stream.Cons(f(head), tail.withFilter(p).map(f).asInstanceOf[Stream[B]])
       else tail.withFilter(p).map(f).asInstanceOf[Stream[B]]
	  ).asInstanceOf[That]
	
    def flatMap[B, That](f: A => Traversable[B])(implicit bf: CanBuildFrom[Repr, B, That]): That = 
	  (if(isEmpty) Stream.Empty
       else if(p(head)) f(head).toStream append tail.withFilter(p).flatMap(f).asInstanceOf[Stream[B]]
	   else tail.withFilter(p).flatMap(f).asInstanceOf[Stream[B]]
	  ).asInstanceOf[That]
 
    def foreach[U](f: A => U): Unit = 
      for (x <- self) 
        if (p(x)) f(x)

    def withFilter(q: A => Boolean): WithFilter = 
      new StreamFilter(x => p(x) && q(x))
  }
}

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-3265?orig=1
Reporter: Eastsun (eastsun)

[scabug]

@dubochet said:
Paul, would you like to comment on this proposal?

[scabug]

Eastsun (eastsun) said:
BTW, I think we could fix Ticket #2634 in a very simple way after this ticket be fixed.

[scabug]

@paulp said:
Replying to [comment:2 dubochet]:

Paul, would you like to comment on this proposal?

Sure! He's right in principle, and I don't know if I'm going to run into some nutty issue in practice. Shoot, I already added a whole withFilter implementation to poor little Option. It doesn't look like any collection with nontrivial differences can avoid it without suffering consequences. Anyway, I'll take that as a hint that I should check that patch out and commit something if I can.

[scabug]

@paulp said:
(In r21375) Gave Stream a lazy withFilter implementation. Now you too
can have a collection containing all the even numbers in the
universe and still be home in time for tea. Threw in some
Stream cleanups for free. Closes #3265, review by community.

[scabug]

@paulp said:
Replying to [comment:3 Eastsun]:

BTW, I think we could fix Ticket #2634 in a very simple way after this ticket be fixed.

You are now equipped to demonstrate! Because I tried, and it doesn't look like a simple task to me.

[scabug]

Eastsun (eastsun) said:
Replying to [comment:6 extempore]:

Replying to [comment:3 Eastsun]:

BTW, I think we could fix Ticket #2634 in a very simple way after this ticket be fixed.

You are now equipped to demonstrate! Because I tried, and it doesn't look like a simple task to me.

Take Tuple2 for example, I think #2634 would be fixed if we chang the implementation of class Tuple2#Zipped like this:

class Zipped[+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)
    }
	...
}

And Tuple2#zip should be changed in the same way.

Since I don't have chance to test it, I apologize if I have missed something.

[scabug]

@paulp said:
Replying to [comment:7 Eastsun]:

Since I don't have chance to test it, I apologize if I have missed something.

I encourage you to try it out. I'm hopeful that it's me who is missing something, because I'd like it to work. I agree that the for comprehensions should be rewritten like that now that withFilter exists, but it doesn't make any difference when it comes to termination.

[scabug]

Eastsun (eastsun) said:
Replying to [comment:8 extempore]:

Replying to [comment:7 Eastsun]:

Since I don't have chance to test it, I apologize if I have missed something.

I encourage you to try it out. I'm hopeful that it's me who is missing something, because I'd like it to work. I agree that the for comprehensions should be rewritten like that now that withFilter exists, but it doesn't make any difference when it comes to termination.

It confused me very much. I try to find the reason why it doesn't work as my expected. I find another Ticket #3273.

[scabug]

Eastsun (eastsun) said:
Replying to [comment:8 extempore]:

I encourage you to try it out. I'm hopeful that it's me who is missing something, because I'd like it to work. I agree that the for comprehensions should be rewritten like that now that withFilter exists, but it doesn't make any difference when it comes to termination.

I have some ideas about #2634 and I post it here: [http://old.nabble.com/Some-ideas-of-lazy-version-takeWhile-method-in-TraversableLike.-td28189940.html]