Common Lisp: Why does this create circular list: (setf (car ) (append ))?

Question

I am trying to understand why this produces what seems to be circular list.

 * (progn
 (setf (car *x*) (append '(3) *x*))
 2)

2  ;; No "apparent issue setting the value. Hence it is related to printing `*x*`
*x* ;; infinite loop, perhaps due to the structure of *x*??

Why is it a circular list? I would expect that it should not be a circular list

What is different between this question and the "duplicate" question:

In this question, I believe *x* should not be a circular list. In the duplicate answer chain, it is shown how to create a circular list, and neither of the example uses the result of append in the setf.

Alright, I found the answer: My confusion arises from misunderstanding the spec where they say that append returns a new list.

Evidently a new list does not mean that each and every member of it is new (does not mean a copy is returned). The last argument of append is actually shared...

in the marked as duplicate response, the `setf` function does not set the variable to a result from `append` — Makketronix, Apr 05 '18 at 00:22
Why shouldn't it be a circular list? Take a piece of paper and draw the graph of cons cells. — Rainer Joswig, Apr 05 '18 at 06:47
@coredump my question is, why does this create a cycle, not how to print it. — Makketronix, Apr 05 '18 at 13:41

score 2 · Answer 1 · edited Apr 04 '18 at 23:30

2

It's not the reader, but the printer that is in an infinite loop.
Most implementations have a variable to limit the top level printer, see *PRINT-LEVEL*, *PRINT-LENGTH*

edited Apr 04 '18 at 23:30

Barmar

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answered Apr 04 '18 at 23:29

ddyer

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Thanks for the correction. I will edit the question – Makketronix Apr 05 '18 at 00:15

Common Lisp: Why does this create circular list: (setf (car ) (append ))?

1 Answers1