Exercise 4.70

Read Exercise 4.70 ~ Solution

When we investigated streams we saw the constructor, cons-stream only evaluates the first item, not the second. That led to the surprising (at the time) infinite list of 1s (define ones (cons-stream 1 ones). To see why this is infinite look at the box and pointer diagrams.

infinite stream of ones
The dotted lines show how the value of ones will be evaluated once forced.
The exact same pattern and result is in the implementation of add-assertion! in this question.

(define (add-assertion! assertion)
  (store-assertion-in-index assertion)
        (cons-stream assertion THE-ASSERTIONS))

THE-ASSERTIONS becomes an infinite, self-referential stream.
Using let in add-assertion! and add-rules! forces their delayed values and avoids the infinite stream.
I’m glad we weren’t asked to implement these primitives. I’m not sure I would have spotted that problem and it could be a headache to find and fix.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s