Scala流评估

Question

Suppose I have this code

val stream = Stream.continually(1)
val s = stream.take(10).map(i => {
  throw new RuntimeException
  i
})

As far as I understand, the function passed to map is evaluated when the corresponding element in stream s is accessed. If s is never iterated and no element is ever accessed, why does the exception get thrown?

Answer 1

The first element of the Stream is always evaluated.

This will always throw.

val stream = Stream.iterate(0)(_ + 1)
val s = stream.take(10).map(i => {
  if (i == 0) throw new RuntimeException
  i
})

But this will not until s(1) is referenced.

val stream = Stream.iterate(0)(_ + 1)
val s = stream.take(10).map(i => {
  if (i > 0) throw new RuntimeException
  i
})

Answer 2

It is evaluated the first time,

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

scala> val newStream = stream.take(10).zipWithIndex.map{case (value, index) => { println(s"processing ${index}"); value * 2 } }
processing 0
newStream: scala.collection.immutable.Stream[Int] = Stream(2, ?)

So, throws exception in your case, because its evaluating first time

scala> val newStream = stream.take(10).zipWithIndex.map{case (value, index) => { println(s"processing ${index}"); throw new Exception; value * 2 } }
processing 0
java.lang.Exception
  at .$anonfun$newStream$1(<console>:12)
  at .$anonfun$newStream$1$adapted(<console>:12)
  at scala.collection.immutable.Stream.map(Stream.scala:415)
  ... 29 elided

You can verify by adding a check only if not the first time as below

scala> val newStream = stream.take(10).zipWithIndex.map{case (value, index) => { println(s"processing ${index}"); if(index > 0 ) throw new Exception; value * 2 } }
processing 0
newStream: scala.collection.immutable.Stream[Int] = Stream(2, ?)