Scala: Stream concatenation. Thanks to Pavel Lepin.

stream_concat.scala

val a = Stream(1)
//a: scala.collection.immutable.Stream[Int] = Stream(1, ?)

def b: Stream[Int] = Stream(b.head)
//b: Stream[Int]

a #::: b
//res0: scala.collection.immutable.Stream[Int] = Stream(1, ?)

a append b
//res1: scala.collection.immutable.Stream[Int] = Stream(1, ?)

a ++ b
//java.lang.StackOverflowError
//        at scala.package$.Stream(package.scala:74)
//        at .b(<console>:7)
//        (etc.)

this has nothing to do with concatenation. Just doing b.take(2) results in a StackOverFlowError. It's a recursive method without a base case so no surprise there.