bug#59314: 29.0.50; EUDC and message-mode header completion

[Alexander Adolf <alexander.adolf@condition-alpha.com>]

Thomas Fitzsimmons <fitzsim@fitzsim.org> writes:

> [...]
>> For one, you are no longer adding `eudc-capf-complete` to
>> `completion-at-point-functions` in message mode. This seems fine given
>> that we are not there yet to make `eudc-capf-complete` the default thing
>> to happen in 29.
>
> To be clear, I wasn't going to push a patch to remove that, at least not
> yet.  It's not making anything worse, so might as well leave it as-is.

I see. Please kindly ignore yesterday's path then, please.

>> [...]
> I'm trying to get message--name-table's EUDC support working.  I only
> wanted to remove eudc-capf-complete from the debugging equation, because
> message--name-table existed way before eudc-capf-complete was
> introduced.
>
> I want to understand why it doesn't work (why it results in the " <..."
> expansion).  I haven't delved deep enough into the completion-at-point
> to figure that out.  I was hoping with your experience writing
> eudc-capf-complete that you'd know what was wrong with
> message--name-table's EUDC support.

I'll look into `message--name-table` and will find out what breaks with
the " <" prefix. Please allow until Friday USA time (assuming you're
there?); it's late already over here (Europe).

> I don't think completion-styles should have any bearing on this.
> message--name-table's EUDC support has to be made to work with the
> default completion-styles setting.  Do you know how to do that?

The thing is, they do have a bearing, and there is no way to avoid that.
`completion-at-point` filters the candidates returned by the
`completion-at-point-functions`, and it uses the completion style (set
by either `completion-styles`, or via a completion category signalled in
a completion table) for this.

When the completion table does not signal a completion style,
`completion-at-point` uses the value of `completion-styles` to filter
the candidates. Only matching candidates will be presented in any UI.
The default value of `completion-styles` is '(basic partial-completion
emacs22). Which - according to the manual [1] - effects the following:

---------------------------- Begin Quote -----------------------------
basic

   A matching completion alternative must have the same beginning as the
   text in the minibuffer before point. Furthermore, if there is any
   text in the minibuffer after point, the rest of the completion
   alternative must contain that text as a substring.

partial-completion

   This aggressive completion style divides the minibuffer text into
   words separated by hyphens or spaces, and completes each word
   separately. (For example, when completing command names, ‘em-l-m’
   completes to ‘emacs-lisp-mode’.)

   Furthermore, a ‘*’ in the minibuffer text is treated as a wildcard—it
   matches any string of characters at the corresponding position in the
   completion alternative.

emacs22

   This completion style is similar to basic, except that it ignores the
   text in the minibuffer after point. It is so-named because it
   corresponds to the completion behavior in Emacs 22.
----------------------------- End Quote ------------------------------
[1] https://www.gnu.org/software/emacs/manual/html_node/emacs/Completion-Styles.html

I.e. the default setting of `completion-styles` will match for
candidates that have the search string at their beginning only. Example:
when the text before point is "foo", the candidates "foo", "foox", and
"foobar" will be shown, but not the candidate "barfoo".

Clearly, this is fairly useless for email address completion. Thus, the
function `message--name-table` in message.el begins like this:

---------------------------- Begin Quote -----------------------------
(defun message--name-table (orig-string)
  (let ((orig-words (split-string orig-string "[ \t]+"))
        eudc-responses
        bbdb-responses)
    (lambda (string pred action)
      (pcase action
        ('metadata '(metadata (category . email)))
[...]
----------------------------- End Quote ------------------------------

In the last quoted line, it return the list '(metadata (category .
email)) in response to the 'metadata action.

When message.el is loaded, the init code there does this:

---------------------------- Begin Quote -----------------------------
(add-to-list 'completion-category-defaults '(email (styles substring
                                                           partial-completion)))
----------------------------- End Quote ------------------------------

This defines the 'email completion category to imply the completion
styles '(substring partial-completion). Thus, whenever
`message--name-table` comes into play, these completion styles will be
in effect.

Long story, short conclusion: you can't do meaningful email address
completion with the default value of `completion styles`.


Cheers,

  --alexander