Path: utzoo!utgpu!water!watmath!clyde!att!rutgers!cmcl2!yale!Krulwich-Bruce
From: Krulwich-Bruce@cs.yale.edu (Bruce Krulwich)
Newsgroups: comp.lang.lisp
Subject: Re: list-length vs. listp in Common Lisp?
Message-ID: <37215@yale-celray.yale.UUCP>
Date: 6 Sep 88 19:47:06 GMT
References: <6317@orstcs.CS.ORST.EDU>
Sender: root@yale.UUCP
Reply-To: Krulwich-Bruce@cs.yale.edu (Bruce Krulwich)
Organization: Computer Science, Yale University, New Haven, CT 06520-2158
Lines: 21
In-reply-to: ruffwork@orstcs.CS.ORST.EDU (Ritchey Ruff)

In article <6317@orstcs.CS.ORST.EDU>, ruffwork@orstcs (Ritchey Ruff) writes:
>Anyone notice how hard it is to make sure you pass a "true"
>list to list-length (or length)?   The only way I've found to make sure an 
>arg is a true list is with -
>	(defun true-listp (l)
>	       (and (listp sexp)
>		    (null (cdr (last l)))))
...
>it would be nice if you could do something like
>	(list-length sexp :doted-ending-allowed)
>Isn't there an easier way?  (now watch, even though I just spent over
>an hour pouring through Steele somebody will say, "Oh, that's
>the function called true-listp on page xyz of CLtL...;-).

Well, regardless of whether TRUE-LISTP is built in or user-written, it will
still be slower to do this because the list will end up traversed twice.  The
only way to avoid this inefficiency is to write your own LIST-LENGTH that has
options to deal with a dotted ending (or to try to get such options added to
the CL standard).

Bruce