In Example 11.59 we showed how to implement interactive I/O in terms of the lazy evaluation of streams. Unfortunately, our code would not work as written, because Scheme uses applicative-order evaluation. We can make it work, however, with calls to delay and force.

Suppose we define input to be a function that returns an “istream”—a promise that when forced will yield a pair, the cdr of which is an istream:

(define input (lambda () (delay (cons (read) (input)))))

Now we can define the driver to expect an “ostream”—an empty list or a pair, the cdr of which is an ostream:

(define driver
    (lambda (s)
        (if (null? s) '()
            (display (car s))
            (driver (force (cdr s))))))

Note the use of force.

Show how to write the function squares so that it takes an istream as argument and returns an ostream. You should then be able to type (driver (squares (input))) and see appropriate behavior.