Re: Making alistp matching plistp requirements

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)

-- 
The lyf so short, the craft so long to lerne.
- Geoffrey Chaucer, The Parliament of Birds.