bug#21922: Indentation of Emacs Lisp list constants is surprising

bug#21922: Indentation of Emacs Lisp list constants is surprising

[Clément Pit--Claudel]

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Hi all,

I'm posting this following a suggestion from Stefan on a discussion on
Emacs' stackexchange site at https://emacs.stackexchange.com/questions/16942/

Emacs' indentation of Emacs Lisp code is really great, except for one thing:

(defconst one-to-ten '(one two three
                           four five six seven
                           eight nine ten))

Is this actually the preferred way to indent this block? As opposed to

(defconst one-to-ten '(one two three
                       four five six seven
                       eight nine ten))

I find it especially confusing when compared to the default for alists:

(defconst one-to-ten '((one . 1) (two . 2) (three . 3)
                       (four . 4) (five . 5) (six . 6) (seven . 7)
                       (eight . 8) (nine . 9) (ten . 10)))

Is there a reason for this behaviour? I could possibly understand it for 
back-quoted lists, as it would yield better indentation for macros, but 
what about lists? On stackexchange, Oleh suggested using  

(setq lisp-indent-function 'common-lisp-indent-function)

Cheers,
Clément.

In GNU Emacs 25.1.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.8)
 of 2015-11-14
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bug#21922: Indentation of Emacs Lisp list constants is surprising

[Eli Zaretskii]

> From: Clément Pit--Claudel <clement.pitclaudel@live.com>
> Date: Sat, 14 Nov 2015 13:29:54 -0500
> 
> I'm posting this following a suggestion from Stefan on a discussion on
> Emacs' stackexchange site at https://emacs.stackexchange.com/questions/16942/
> 
> Emacs' indentation of Emacs Lisp code is really great, except for one thing:
> 
> (defconst one-to-ten '(one two three
>                            four five six seven
>                            eight nine ten))
> 
> Is this actually the preferred way to indent this block? As opposed to
> 
> (defconst one-to-ten '(one two three
>                        four five six seven
>                        eight nine ten))
> 
> I find it especially confusing when compared to the default for alists:
> 
> (defconst one-to-ten '((one . 1) (two . 2) (three . 3)
>                        (four . 4) (five . 5) (six . 6) (seven . 7)
>                        (eight . 8) (nine . 9) (ten . 10)))
> 

FWIW, Stefan was wrong: Emacs behaved like that since at least
Emacs 22, so recent changes didn't change this in any way.

bug#21922: Indentation of Emacs Lisp list constants is surprising

[Dmitry Gutov]

Hi Clément,

On 11/14/2015 08:29 PM, Clément Pit--Claudel wrote:

> I'm posting this following a suggestion from Stefan on a discussion on
> Emacs' stackexchange site at https://emacs.stackexchange.com/questions/16942/

See Stefan's comment under your question. I also think it's a bug.

> Is there a reason for this behaviour? I could possibly understand it for
> back-quoted lists, as it would yield better indentation for macros, but
> what about lists?

We'll have to keep back-quoted lists an exception either way, for 
macros, and because of that we'll sometimes mis-indent "value" lists as 
well, because it can be handy to use backquotes for them, too.

That might be the main reason we haven't bothered to fix this yet. But 
someone should look into fixing the straight-quote case at least, and 
propose a patch.

> On stackexchange, Oleh suggested using
>
> (setq lisp-indent-function 'common-lisp-indent-function)

We recommend against this, e.g. because it would cause a lot of 
indentation changes in the Emacs codebase.

bug#21922: Patch for fixing the "straight-quote" case

[Luis Henriquez-Perez]

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Hi,

In regards to the issue on the indentation of quoted lists, I'd like to
propose a patch.

It would be changing the predicate of this conditional in the function
`calculate-lisp-indent`.

(if
    (= (point) calculate-lisp-indent-last-sexp)
    ;; Containing sexp has nothing before this line
    ;; except the first element.  Indent under that element.
    nil
  ;; Skip the first element, find start of second (the first
  ;; argument of the function call) and indent under.
  (progn (forward-sexp 1)
         (parse-partial-sexp (point)
                             calculate-lisp-indent-last-sexp
                             0 t)))

It would to this:

(or
 (= (point) calculate-lisp-indent-last-sexp)

 (when-let (point (char-before containing-sexp))
   (char-equal point ?'))

 (let ((quoted-p nil)
       (point nil)
       (positions (nreverse (butlast (elt state 9)))))
   (while (and positions (not quoted-p))
     (setq point (pop positions))
     (setq quoted-p
           (or
            (and (char-before point)
                 (char-equal (char-before point) ?'))
            (save-excursion
              (goto-char (1+ point))
              (looking-at-p "quote[\t\n\f\s]+(")))))
   quoted-p))

This code checks if the `containing-sexp` is quoted and if so indents it
normally (under the first element).

It works for forms quoted with the quote abbreviation ie:
'(a b c d
  e f g)

It also works for explicit quoting:
(quote (a b c
            d e))

Additionally it works for nested lists that are quoted.

'((a b c
   d e))

Please let me know what you think.

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bug#21922: Patch for fixing the "straight-quote" case

[Noam Postavsky]

Luis Henriquez-Perez <luishenriquezperez@gmail.com> writes:

>  (when-let (point (char-before containing-sexp))
>    (char-equal point ?'))

>             (and (char-before point)
>                  (char-equal (char-before point) ?'))

You can use eq instead of char-equal, and then the extra check for nil
isn't needed.

>             (save-excursion
>               (goto-char (1+ point))
>               (looking-at-p "quote[\t\n\f\s]+(")))))

I would suggest (looking-at-p "[[:whitespace:]\n]*quote\_>") instead,
and maybe move the save-excursion outside of the loop.

Overall, the approach looks reasonable to me.