Streams Are Delayed Lists

Stream Constructor and Selectors

On the surface, streams are just lists with different names for the procedures that manipulate them. They have a constructor cons-stream , and two selectors, stream-car and stream-cdr, which satisfies these constraints:

  • (stream-car (cons-stream x y)) returns x
  • (stream-cdr (cons-stream x y)) returns y

In order to construct the stream as we use it, we will arrange for the cdr of a stream to be evaluated when it is accessed by the stream-cdr procedure rather than when the stream is constructed.

As a data abstraction, streams are the same as lists. The difference is the time at which the elements are evaluated. With ordinary lists, both the car and the cdr are evaluated at construction time. With streams, the cdr isn't evaluated until selection time.

Our implementation of streams will be based on a special form called delay. Evaluating (delay [exp]) does not evaluate the expression [exp], but rather returns a so-called delayed object, which we can think of as a "promise" to evaluate [exp] at some future time. As a companion to delay, there is a procedure called force that takes a delayed object as argument and performs the evaluation -- in effect, forcing the delay to fulfill its promise. We will see below how delay and force can be implemented, but first let us use these to construct streams.

cons-stream is a special form such that (cons-stream [a] [b]) is equivalent to (cons [a] (delay [b])).

This means that we will construct using pairs of cars and delayed cdrs. These will be our stream-car and stream-cdr procedures:

(define (stream-car stream) (car stream))

(define (stream-cdr stream) (force (cdr stream)))

Note that cons-stream is a special form. If it weren't, calling (cons-stream a b) would evaluate b, meaning b wouldn't be delayed.


There is a distinguishable object, the-empty-stream, which cannot be the result of any cons-stream operation, and which can be identified with the predicate stream-null?.

Stream Analogs of List Procedures

We can make and use streams, in just the same way as we can make and use lists, to represent aggregate data arranged in a sequence. In particular, we can build stream analogs of list-ref, map, for-each, and so on.


(define (stream-ref s n)
  (if (= n 0)
      (stream-car s)
      (stream-ref (stream-cdr s) (- n 1))))

If we define x as

(define x (cons-stream 0 (cons-stream 1 (cons-stream 2 the-empty-stream))))

then (stream-ref x 0) returns 0 and (stream-ref x 2) returns 2. (Note that n starts counting from 0)


(define (stream-map proc s)
  (if (stream-null? s)
      (cons-stream (proc (stream-car s))
                   (stream-map proc (stream-cdr s)))))

If x is the same as above, then (stream-map square x) returns a stream with (0 1 4)


(define (stream-for-each proc s)
  (if (stream-null? s)
      (begin (proc (stream-car s))
             (stream-for-each proc (stream-cdr s)))))

stream-for-each is useful for viewing streams. The following may be helpful for checking what's going on:

(define (display-stream s)
  (stream-for-each display-line s))

(define (display-line x)
  (display x))

Computation Using Streams

Let's take another look at the second prime computation we saw earlier, reformulated in terms of streams:

  (stream-filter prime?
                 (stream-enumerate-interval 10000 1000000))))

So we begin by calling stream-enumerate-interval with the arguments 10,000 and 1,000,000. This creates a stream of the numbers from 10,000 to 1,000,000.

(define (stream-enumerate-interval low high)
  (if (> low high)
       (stream-enumerate-interval (+ low 1) high))))

The result returned is (cons 10000 (delay (stream-enumerate-interval 10001 1000000))) This is a stream represented as a pair whose car is 10,000 and whose cdr is a promise to enumerate more of the interval if it becomes necessary. Now we filter it using stream-filter

(define (stream-filter pred stream)
  (cond ((stream-null? stream) the-empty-stream)
        ((pred (stream-car stream))
         (cons-stream (stream-car stream)
                      (stream-filter pred
                                     (stream-cdr stream))))
        (else (stream-filter pred (stream-cdr stream)))))

stream-filter tests the stream-car of the stream (the car of the pair, which is 10,000). Since this is not prime, stream-filter examines the stream-cdr of its input stream. The call to stream-cdr forces evaluation of the delayed stream-enumerate-interval, which now returns

(cons 10001
      (delay (stream-enumerate-interval 10002 1000000)))

stream-filter now looks at the stream-car of this stream, 10,001, sees that this is not prime either, forces another stream-cdr, and so on, until stream-enumerate-interval yields the prime 10,007, whereupon stream- filter, according to its definition, returns

(cons-stream (stream-car stream)
             (stream-filter pred (stream-cdr stream)))

which is

(cons 10007
         (cons 10008
                 (stream-enumerate-interval 10009

This result is now passed to stream-cdr in our original expression. This forces the delayed stream-filter, which in turn keeps forcing the delayed stream-enumerate-interval until it finds the next prime, which is 10,009. Finally, the result passed to stream-car in our original expression is

(cons 10009
         (cons 10010
                 (stream-enumerate-interval 10011

Stream-car returns 10,009, and the computation is complete. Only as many integers were tested for primality as were necessary to find the second prime, and the interval was enumerated only as far as was necessary to feed the prime filter.

Implementing delay and force

Although delay and force may seem like mysterious operations, their implementation is really quite straightforward. delay must package an expression so that it can be evaluated later on demand, and we can accomplish this simply by treating the expression as the body of a procedure. delay can be a special form such that

 (delay [exp])

is syntactic sugar for

 (lambda () [exp])

force simply calls the procedure (of no arguments) produced by delay, so we can implement force as a procedure:

 (define (force delayed-object)

Again, note the importance of delay being a special form. If it is not, then when we call (delay b), b will be evaluated before we evaluate the body.

This implementation suffices for delay and force to work as advertised, but there is an important optimization that we can include. In many applications, we end up forcing the same delayed object many times. This can lead to serious inefficiency in recursive programs involving streams. The solution is to build delayed objects so that the first time they are forced, they store the value that is computed. Subsequent forcings will simply return the stored value without repeating the computation. In other words, we implement delay as a special-purpose memoized procedure. One way to accomplish this is to use the following procedure, which takes as argument a procedure (of no arguments) and returns a memoized version of the procedure. The first time the memoized procedure is run, it saves the computed result. On subsequent evaluations, it simply returns the result.

(define (memo-proc proc)
  (let ((already-run? false) (result false))
    (lambda ()
      (if (not already-run?)
          (begin (set! result (proc))
                 (set! already-run? true)

Delay is then defined so that (delay [exp]) is equivalent to

(memo-proc (lambda () [exp]))

and force is unchanged


In this section, you learned:

  1. What a stream is
  2. Some useful applications of streams
  3. How to implement delay and force

What's Next?

Let's go to the next subsection and learn about infinite lists!