Making a circular list

How would you go about writing a function that converts a given list into a circular one? Obviously the last cons cell of the list must be made to point back to the first cons (conveniently ignoring some special cases like empty and improper lists).

(DEFUN CIRCULARIZE (LIST)
  (RPLACD (LAST LIST) (FIRST LIST))
  LIST)

That doesn’t quite cut it as this test expression shows:

;; Expecting #1=(#\a #\e #\i #\o #\u . #1#)
(LET ((*PRINT-CIRCLE* T))
  (WRITE (CIRCULARIZE (COERCE "aeiou" 'LIST))))

=> (#\a #\e #\i #\o #\u . #\a)
Filed under  //   conses  

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A growing compendium of erroneous Common Lisp code snippets by Ismail Ikram.

Lisp novices might try to correct these as a learning exercise. Need some help or just feel like nitpicking? Do post a comment and let me know.

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