Why does iterating over multiple streams only iterate over the first element? - scala

Question

I've recently run into a bug in my code, in which iterating over multiple streams causes them to only iterate only through the first item. I converted my streams to buffers (I wasn't even aware that the function's implementation that I was calling returns a stream) and the problem was fixed. I found this hard to believe, so I created a minimum verifiable example:
def f(as: Seq[String], bs: Seq[String]): Unit =
for {
a <- as
b <- bs
} yield println((a, b))
val seq = Seq(1, 2, 3).map(_.toString)
f(seq, seq)
println()
val stream = Stream.iterate(1)(_ + 1).map(_.toString).take(3)
f(stream, stream)
A function that prints every combination of its inputs, and is invoked with the Seq [1, 2, 3] and the Stream [1, 2, 3].
The result with the seq is:
(1,1)
(1,2)
(1,3)
(2,1)
(2,2)
(2,3)
(3,1)
(3,2)
(3,3)
And the result with the stream is:
(1,1)
I've only been able to replicate this when iterating through multiple generators, iterating through a single stream seems to work fine.
So my questions are: why does this happen, and how can I avoid this kind of glitch? That is, short of using .toBuffer or .to[Vector] before every multi-generator iteration?
Thanks.

Answer 1

The manner in which you're using the for-comprehension (with the println in the yield) is a bit strange and probably not what you want to do. If you really just want to print out the entries, then just use foreach. This will force lazy sequences like Stream, i.e.
def f_strict(as: Seq[String], bs: Seq[String]): Unit = {
for {
a <- as
b <- bs
} println((a, b))
}
The reason you're getting the strange behavior with your f is that Streams are lazy, and elements are only computed (and then memoized) as needed. Since you never use the Stream created by f (necessarily because your f returns a Unit), only the head ever gets computed (which is why you're seeing the single (1, 1).) If you were instead to have it return the sequence it generated (which will have type Seq[Unit]), i.e.
def f_new(as: Seq[String], bs: Seq[String]): Seq[Unit] = {
for {
a <- as
b <- bs
} yield println((a, b))
}
Then you'll get the following behavior which should hopefully help to elucidate what's going on:
val xs = Stream(1, 2, 3)
val result = f_new(xs.map(_.toString), xs.map(_.toString))
//prints out (1, 1) as a result of evaluating the head of the resulting Stream
result.foreach(aUnit => {})
//prints out the other elements as the rest of the entries of Stream are computed, i.e.
//(1,2)
//(1,3)
//(2,1)
//...
result.foreach(aUnit => {})
//probably won't print out anything because elements of Stream have been computed,
//memoized and probably don't need to be computed again at this point.