Learning Functional Data Structures and Algorithms

Streams

Scala's streams are lazy lists, an infinite sequence of elements. For example, just like we zip two lists together, we could zip two streams too:

scala> def zip(p: Stream[Int], q: Stream[Int]): Stream[(Int, Int)] = 
     | (p.head, q.head) #:: zip(p.tail, q.tail) 
zip: (p: Stream[Int], q: Stream[Int])Stream[(Int, Int)] 
 
scala> val r = zip(Stream.from(9), Stream.from(10)) 
r: Stream[(Int, Int)] = Stream((9,10), ?) 

As you can see, the type of r variable is another stream. Also, the stream has a head element, a pair (9,10), and a function (also called a thunk). The function (in this case, zip) is called to produce successive elements.

As you can see, the tail is a question, meaning it is not yet computed; therefore, it is not yet known.

Why are streams ...