Common Lisp setting print-circle T causes list compression?

Question

(setq *print-circle* T)

;; then create some list with a large number of items, of which a bunch
;; of them repeat

If you then try to print the list in the repl, you will see a lot of unfamiliar text. Each of your items (in the beginning) will show a #1 or #2, etc, depending on the items location in your list. Where the list repeats itself, instead of the item printing out, you will have a #1# #2#, etc, depending on which item it the list it is a repeat of. This threw me, because I wasn't expecting it. I'm not sure if there is a way to mark a list so it won't do this behaviour when you print the object in the repl.

JEPrice · Accepted Answer · 2024-11-20 01:03:42Z

0

Nothing has been modified in the list, just the way it was represented was changed.

The way I worked around it is to defun a function to concatenate the list items in a single string, this forced an expansion of the list items.

(defun join-list-items( list )
  "joins all items in a list as one big string"
  (format nil "~{~a~}" list))

This isn't really a question, more of a warning that "here there be dragons" if you're unfamiliar with how SBCL handles the repl printing of a list that has duplicate items, if print-circle is set.

answered Nov 20, 2024 at 1:03

JEPrice

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1 Comment

kd4ttc Mar 8 at 5:43

No reason to downvote this guy. He's new to SBCL. It is all a mite overwhelming. He's just picking up on sharp notation and he's picking up on the repetion of items. JE: I might add that you'll find that crashes if you have a circular list. Have fun!

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