RE: [External] : Determine whether a list is an alist

[uzibalqa]

------- Original Message -------
On Friday, July 14th, 2023 at 3:40 AM, Drew Adams <drew.adams@oracle.com> wrote:


> > > Every list is an alist.
> > 
> > Can you elaborate? '(1 2 3) is a list, but I don't think it's an
> > association list (at least according to Emacs manual - alist is supposed
> > to be a list of cons cells, which '(1 2 3) definitely isn't).
> 
> 
> Well, yes; and no.
> 
> "In Emacs Lisp, it is not an error if an element of an association list is 
> not a cons cell. The alist search functions simply ignore such elements. Many 
> other versions of Lisp signal errors in such cases." (elisp) `Association 
> Lists'
> 
> (let ((x 5))
> (assoc-default x '(1 2 3 4 5) nil x)) ; => 5
> 
> 
> None of those list elements is a cons.
> 
> (assq 5 '(nil (5 . foo))) ; => (5 . foo)
> 
> 
> Element nil is not a cons.
> 
> ___
> 
> The point is: specify what you really want when
> asking to test whether a list is an alist.
> 
> Even asking whether a Lisp object is a list is
> imprecise - proper/true list or just a nil or
> cons?
> 
> Knowing what you're asking and what you really
> want to ask is the first step (sometimes it's
> even sufficient) toward getting the answer you
> need.
> ___
> 
> And be aware that if you test every element of a
> list then that can be costly. It might in some
> cases be better to just test as you go, while
> you're trying to do something with the alist -
> e.g., while you're looking for an alist element
> match.

Agreed
 
> In many (most?) cases you don't really care
> whether each element of the list is a cons - you
> just want to retrieve the first match, and you
> only want to traverse then entire list if you
> have to (i.e., when there's no match or the last
> element matches), and you don't want to traverse
> it more than once.

If I want to print the alist in (Key . Value) way, how would one skip
extraneous (non-cons) list entries ?