bug#66509: 29.1.50; let-alist should support numeric indexing

[Stefan Kangas <stefankangas@gmail.com>]

Artur, do you have any thoughts on the below patch?

Spencer Baugh <sbaugh@janestreet.com> writes:

> From 28e52a343f72dddd991cafd23fea910cc5f64ac5 Mon Sep 17 00:00:00 2001
> From: Spencer Baugh <sbaugh@janestreet.com>
> Date: Thu, 12 Oct 2023 18:01:46 -0400
> Subject: [PATCH] Support numeric indexing in let-alist
>
> let-alist is very useful.  But sometimes an alist contains a list in
> the middle, which contains yet more alists.  Previously, this was
> somewhat painful to deal with, and required something like:
>
> (let-alist alist
>   (let-alist (nth 0 .a)
>     (let-alist (nth 3 .b)
>        .c)))
>
> Now, the following works:
>
> (let-alist alist
>   .a.0.b.3.c)
>
> * lisp/emacs-lisp/let-alist.el (let-alist--access-sexp): Properly
> parse numbers.
> (let-alist--list-to-sexp): Use nth to handle numbers.
> (let-alist): Update docs.
> ---
>  lisp/emacs-lisp/let-alist.el | 25 +++++++++++++++----------
>  1 file changed, 15 insertions(+), 10 deletions(-)
>
> diff --git a/lisp/emacs-lisp/let-alist.el b/lisp/emacs-lisp/let-alist.el
> index d9ad46b2af7..de7c087bf2a 100644
> --- a/lisp/emacs-lisp/let-alist.el
> +++ b/lisp/emacs-lisp/let-alist.el
> @@ -36,22 +36,23 @@
>  ;; symbol inside body is let-bound to their cdrs in the alist.  Dotted
>  ;; symbol is any symbol starting with a `.'.  Only those present in
>  ;; the body are let-bound and this search is done at compile time.
> +;; A number will result in a list index.
>  ;;
>  ;; For instance, the following code
>  ;;
>  ;;   (let-alist alist
> -;;     (if (and .title .body)
> +;;     (if (and .title.0 .body)
>  ;;         .body
>  ;;       .site
>  ;;       .site.contents))
>  ;;
>  ;; essentially expands to
>  ;;
> -;;   (let ((.title (cdr (assq 'title alist)))
> +;;   (let ((.title.0 (nth 0 (cdr (assq 'title alist))))
>  ;;         (.body  (cdr (assq 'body alist)))
>  ;;         (.site  (cdr (assq 'site alist)))
>  ;;         (.site.contents (cdr (assq 'contents (cdr (assq 'site alist))))))
> -;;     (if (and .title .body)
> +;;     (if (and .title.0 .body)
>  ;;         .body
>  ;;       .site
>  ;;       .site.contents))
> @@ -93,14 +94,17 @@ let-alist--access-sexp
>      (if (string-match "\\`\\." name)
>          clean
>        (let-alist--list-to-sexp
> -       (mapcar #'intern (nreverse (split-string name "\\.")))
> +       (mapcar #'read (nreverse (split-string name "\\.")))
>         variable))))
>
>  (defun let-alist--list-to-sexp (list var)
>    "Turn symbols LIST into recursive calls to `cdr' `assq' on VAR."
> -  `(cdr (assq ',(car list)
> -              ,(if (cdr list) (let-alist--list-to-sexp (cdr list) var)
> -                 var))))
> +  (let ((sym (car list))
> +        (rest (if (cdr list) (let-alist--list-to-sexp (cdr list) var)
> +                 var)))
> +    (cond
> +     ((numberp sym) `(nth ,sym ,rest))
> +     (t `(cdr (assq ',sym ,rest))))))
>
>  (defun let-alist--remove-dot (symbol)
>    "Return SYMBOL, sans an initial dot."
> @@ -116,22 +120,23 @@ let-alist
>    "Let-bind dotted symbols to their cdrs in ALIST and execute BODY.
>  Dotted symbol is any symbol starting with a `.'.  Only those present
>  in BODY are let-bound and this search is done at compile time.
> +A number will result in a list index.
>
>  For instance, the following code
>
>    (let-alist alist
> -    (if (and .title .body)
> +    (if (and .title.0 .body)
>          .body
>        .site
>        .site.contents))
>
>  essentially expands to
>
> -  (let ((.title (cdr (assq \\='title alist)))
> +  (let ((.title (nth 0 (cdr (assq \\='title alist))))
>          (.body  (cdr (assq \\='body alist)))
>          (.site  (cdr (assq \\='site alist)))
>          (.site.contents (cdr (assq \\='contents (cdr (assq \\='site alist))))))
> -    (if (and .title .body)
> +    (if (and .title.0 .body)
>          .body
>        .site
>        .site.contents))