Re: Predicate for true lists

[Paul Eggert]

On 06/04/2018 05:12 AM, Basil L. Contovounesios wrote:
> +(defun list-true-p (object)
> +  "Return t if OBJECT is a true list.
> +A true list is neither circular nor dotted (i.e., its last `cdr'
> +is nil)."
> +  (null (nthcdr (safe-length object) object)))

This traverses the list twice. Wouldn't it be better to traverse it just 
once? Also, why not return the length of the list when it is proper? 
That would not cost anything to compute, and would yield more 
information than just returning t. Like others, I prefer "proper" to "true".

Something like the following, perhaps:

(defun proper-list-length (obj)
  "Return OBJ's length if OBJ is a proper list, nil otherwise."
  (and (listp obj) (ignore-errors (length obj))))