递归流抛出StackOverflowError

Question

我正在定义一个流本身（递归定义）。当试图访问流的第二个元素时，StackOverflowError被抛出。从斯卡拉控制台代码：

scala> val s1 = Stream.iterate(1)(identity _) 
s1: scala.collection.immutable.Stream[Int] = Stream(1, ?) 

scala> lazy val s2 : Stream[Int]= Stream.cons(1, (s2, s1).zipped.map { _ + _ }) 
s2: Stream[Int] = <lazy> 

scala> s1 take 5 print 
1, 1, 1, 1, 1, empty 
scala> s2(0) 
res4: Int = 1 

scala> s2(1) 
java.lang.StackOverflowError 
     at $anonfun$s2$1$$anonfun$apply$1.apply(<console>:9) 
     at $anonfun$s2$1$$anonfun$apply$1.apply(<console>:9) 
     at scala.Tuple2$Zipped$$anonfun$map$1.apply(Tuple2.scala:62) 
     at scala.collection.immutable.Stream.foreach(Stream.scala:255) 
     at scala.Tuple2$Zipped.map(Tuple2.scala:60) 
     at $anonfun$s2$1.apply(<console>:9) 
     at $anonfun$s2$1.apply(<console>:9) 
     at scala.collection.immutable.Stream$Cons.tail(Stream.scala:556) 
     at scala.collection.immutable.Stream$Cons.tail(Stream.scala:550) 
     at scala.collection.immutable.Stream.foreach(Stream.scala:256) 
     at scala.Tuple2$Zipped.map(Tuple2.scala:60) 
     at $anonfun$s2$1.apply(<console>:9) 
     at $anonfun$s2$1.apply(<console>:9) 
     at scala.collection.immutable.Stream$Cons.tail(Stream.scala:556) 
     at scala.collection.immutable.Str... 

我无法理解堆栈溢出的原因。由于流本身就是懒惰的，所以递归映射应该可以工作。

这种情况有什么问题？

我正在使用Scala版本2.8.0.RC2。

Answer 1

问题是zipped不是懒惰 - 它实际上试图在那里评估那张地图。你可以用zip做你想做的事。

scala> val s1 = Stream.iterate(1)(identity _) 
s1: scala.collection.immutable.Stream[Int] = Stream(1, ?) 

scala> lazy val s2: Stream[Int] = Stream.cons(1, (s2 zip s1).map {s=>s._1+s._2}) 
s2: Stream[Int] = <lazy> 

scala> s2 take 5 print 
1, 2, 3, 4, 5, empty