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

[Platon Pronko <platon7pronko@gmail.com>]

On 2023-07-13 18:40, Drew Adams 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.
> 
> 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.

My apologies for not reading the manual thoroughly enough. Thanks for the explanation!

-- 
Best regards,
Platon Pronko
PGP 2A62D77A7A2CB94E