Exercise 1.4.17

Circular Lists

The destructive equivalent of append is nconc. Using such side-effects, it's possible to create circular lists.

* (defparameter *cycle* (list 'a 'b))
*CYCLE*

* (first *cycle*)
A

* (second *cycle*)
B

* (third *cycle*)
NIL

* (fourth *cycle*)
NIL

Before we create an actual cycle, we need to tell the interpreter to print them (otherwise the request to print a circular list would never return; unlike Haskell, Common Lisp is not a lazy language by default).

* (setf *print-circle* t)
T

* (nconc *cycle* *cycle*)
#1=(A B . #1#)

* (third *cycle*)
A

* (fourth *cycle*)
B

* (loop repeat 15 for elem in *cycle* collect elem)
(A B A B A B A B A B A B A B A)