Re: Making alistp matching plistp requirements

On Monday, April 13th, 2026 at 9:59 AM, tpeplt <[email protected]> wrote:

> Heime <[email protected]> writes:
> 
> > plistp is defined as
> >
> > (defun plistp (object)
> >   "Non-nil if and only if OBJECT is a valid plist."
> >   (declare (pure t) (side-effect-free error-free))
> >   (let ((len (proper-list-p object)))
> >     (and len (zerop (% len 2)))))
> >
> > For an equivalent alistp, checking cons cells with key and value,
> > how would the function be?
> >
> 
> The answer is going to depend on how strict you want your
> definition of an association list to be.
> 
> Here is a simple definition that does not check the contents
> of the items in the alist:
> 
>   (defun alistp (obj-list)
>     "Determine whether OBJ-LIST is an association list.
>      This function definition says that an empty list is not
>      an association list."
>     (when (and obj-list (proper-list-p obj-list))
>       (cl-every #'consp obj-list)))
> 
> You might decide to have additional, more strict conditions,
> in which case you could replace ‘#'consp’ with
> ‘#'your-alist-item-p’.
> 
>    (defun your-alist-item-p (pair)
>       (and (consp pair)
>            ... ;; your additional conditions))
> 
> For example, (a . nil) and (a) are the same, so they are a
> valid key/value pair, but you might want to restrict your
> alist pairs not to include nil as a possible value; likewise
> with not allowing nil as a key: (nil . some-value)

Emacs Lisp plistp allows nil as both keys and values.  
Have been thinking of keeping with same idea to accept
nil.
 
> --
> The lyf so short, the craft so long to lerne.
> - Geoffrey Chaucer, The Parliament of Birds.
>