Append a stream A to a stream B doesn't imply evaluation of all items of the first Stream A.

gistfile1.txt

#!/bin/sh
exec scala -deprecation -nocompdaemon "$0" "$@"
!#

def trace(x:Int) = {println(x); x}

val x=(1 to 5).toStream.map(trace _)
// 1 is printed

val y=(5 to 1 by -1).toStream.map(trace _)
// 5 is printed

val z=x++y
// nothing is printed

println("NOW let's materialize the stream")

z.toList
// 2 3 4 5 4 3 2 1 is printed