Re: Two problems with directory-local variables

[Juri Linkov <juri@linkov.net>]

>>> On this note, while I don't use the commands myself, I've often wished
>>> that add-dir-local-variable would generate the dotted-pair syntax,
>>> because I've often seen confusion about the syntax of dir-locals arising
>>> from dotted vs non-dotted possibilities, and I feel strongly that the
>>> dotted syntax is the most readable for these files.
>
> Agreed.  We should probably have a `print-alist` kind of function for
> that purpose and then try and make use of it from add-dir-local-variable.
>
>> I don't know if something exists that could allow printing dotted syntax for
>>
>>   (pp-to-string '(eval message "hello"))
>>
>> In its implementation I see the references to print-escape-newlines
>> and print-quoted, but there is no print-dotted.
>
> I don't think we could add a print-dotted because as you say we don't
> want (eval . (message . ("hello" . nil))) and only the human coder can
> know which cons cell should be printed dotted and which shouldn't.

Examples in (info "(emacs) Directory Variables") recommend dotted syntax
for alists on the first level:

     ((nil . ((indent-tabs-mode . t)
              (fill-column . 80)))
      (c-mode . ((c-file-style . "BSD")
                 (subdirs . nil))))

as well as for the second level:

      (("src/imported"
        . ((nil . ((change-log-default-name
                    . "ChangeLog.local"))))))

Taking into account the original problematic case with `eval',
dotted alist should be also on the third level:

      (("src/imported"
        . ((nil . ((eval
                    . (message "hello")))))))

Given that, I don't see what would be a good choice for generalization:

1. Since requirements for this feature are too vague, it doesn't seem
   to fit well into low-level printing functions.

2. Reformatting the printed output in pp-buffer to add dots where necessary
   also doesn't look like an elegant solution.

3. Implementing print-alist poses a question how to define at which levels
   to print alists with dotted syntax.

4. Since only add-dir-local-variable knows the semantics of used data
   structures, the simplest way would be to implement printing of each level
   explicitly in add-dir-local-variable.