Adapting tab mode to file content not working

[fleutot]

The documentation made me expect dtrt-indent to adapt indent-tabs-mode to "the right thing":

;; `indent-tabs-mode' Setting
;;
;; For determining hard vs. soft tabs, dtrt-indent counts the number of
;; lines out of the eligible lines in the fixed segment that are
;; indented using hard tabs, and the number of lines indented using
;; spaces.  If either count is significantly higher than the other count,
;; `indent-tabs-mode' will be modified.

However, opening this file /tmp/asdf.sh with basic init file ~/tmp/basic-init.el does not seem to guess and modify to the right value of indent-tabs-mode.

---

Test material

/tmp/asdf.sh:

#! /usr/bin/env bash

function asdf()
{
    printf "asASDFASDF"
    echo "1234"
}

printf "qwerqwerqwer\n"
result=$(asdf)

if [ df ] ; then
    if [ er ] ; then
        wefwefew
    fi
fi

basic-init.el:

(require 'package)
(setq package-enable-at-startup nil)
(add-to-list 'package-archives '("melpa" . "http://melpa.org/packages/"))
(add-to-list 'package-archives '("gnu" . "http://elpa.gnu.org/packages/"))
(add-to-list 'package-archives '("melpamilk" . "http://melpa.milkbox.net/packages/"))
(package-initialize)

(unless package-archive-contents
  (package-refresh-contents))
(unless (package-installed-p 'use-package)
  (package-install 'use-package))
(require 'use-package)

(defun my-prog-mode-hook ()
  (dtrt-indent-mode))

(use-package dtrt-indent
  :ensure t
  :config (dtrt-indent-mode t))

(require 'whitespace) ;; builtin
(global-set-key (kbd "C-c b") 'whitespace-mode)
(delete 'indentation whitespace-style) ; no warning about indent with spaces

---

Test result

Starting emacs with emacs -q -l ~/tmp/basic-init.el /tmp/asdf.sh, then activating whitespace mode with C-c b gives me this:

After going to the row with 8 space indent (wefwefew), deleting one space, pressing TAB, I get this:

Expected result: dtrt-indent-mode indents with spaces, since all other indentations are with spaces.

---

Debug data

Here is what I have found so far.

Result of dtrt-indent-diagnosis:

Guessing offset for /tmp/asdf.sh

Elapsed time for analysis: 0.001 seconds

Total relevant lines: 5 out of 17 (limit: 5000)

Histogram:

     4x   4 spaces
     1x   8 spaces

Analysis:

  offset 2 works for 100.00% of relevant lines, matching 2 distinct offsets - merged with offset 4 (0.00% deviation, limit 20.00%)
  offset 4 works for 100.00% of relevant lines, matching 2 distinct offsets - CONSIDERED
  offset 8 works for  20.00% of relevant lines, matching 1 distinct offsets - CONSIDERED
  offset 3 works for   0.00% of relevant lines, matching 0 distinct offsets - rejected: too few distinct matching offsets (1 required)
  offset 5 works for   0.00% of relevant lines, matching 0 distinct offsets - rejected: too few distinct matching offsets (1 required)
  offset 6 works for   0.00% of relevant lines, matching 0 distinct offsets - rejected: too few distinct matching offsets (1 required)
  offset 7 works for   0.00% of relevant lines, matching 0 distinct offsets - rejected: too few distinct matching offsets (1 required)

Summary:

  Best guess is offset 4 with 100.00% matching lines (80.00% required)
  Hard tab percentage: 20.00% (1 lines), -75.00% superior to soft tabs (threshold 300.00%)
  Soft tab percentage: 80.00% (4 lines), 300.00% superior to hard tabs (threshold 300.00%)

Conclusion:

  Guessed offset 4 with 100% confidence.
  Change indent-tab-setting: yes, to nil

It seems dtrt-indent-mode could correctly guess that indent-tab-setting (although there is no such variable? Is there? Wrong name? Shouldn't it be indent-tabs-mode?) should be nil.
But it could only hint the user that they probably want to set it to nil (the message seems to be a recommendation, although it is not obvious. At first glance, it looks like dtrt-indent actually made the change).

C-h v indent-tabs-mode is t, so it was not changed. According to the code comment I posted at the top, it should?

The embedded help I open from that emacs session says:

When dtrt-indent mode is enabled, the proper indentation offset
and ‘indent-tabs-mode’ will be guessed for newly opened files and
adjusted transparently.

package-list-packages tells me that what I have installed is this:

     Status: Installed in ‘dtrt-indent-20210423.745/’ (unsigned). Delete
    Version: 20210423.745
     Commit: 9714f2c5f1c9b7c21e732df8c15a870a88caba84
    Summary: Adapt to foreign indentation offsets
   Keywords: convenience files languages c 
Other versions: 20210423.745 (melpa), 20200430.1023 (melpamilk).

[jscheid]

@fleutot set dtrt-indent-run-after-smie and it should work.

@rrthomas when dtrt-indent doesn't run because SMIE is active (and dtrt-indent-run-after-smie is nil) there is no indication to users: dtrt-indent-diagnosis doesn't mention it, the mode-line lighter is displayed, and the minor mode is listed as active. This is clearly confusing.

The diagnosis should say something like "dtrt-indent doesn't run in this buffer because...".

As for the minor mode, I haven't been involved with its refactoring: do you think it would be better to check inside the globalized mode to decide whether or not it should be enabled for a given buffer?

[jscheid]

I might see if I can make time to improve this.

[rrthomas]

Thanks very much, @jscheid, for looking into this and other issues; also thanks @fleutot for a beautifully detailed report for this issue.

[fleutot]

And thank you @jscheid and @rrthomas for looking into this :) If any of you wants to earn free internet pointz, here is the question I wrote on StackExchange about this.

Incidentally, and this is a little embarrassing, I didn't load the package correctly in my minimal example above either:

(use-package dtrt-indent
  :ensure t
  :config (dtrt-indent-mode t))  ; This line is NOT going to load the module by itself

I did create a custom prog mode hook, but never actually loaded it. How silly.

This works:

(defun my-prog-mode-hook ()
  (dtrt-indent-mode))

(add-hook 'prog-mode-hook 'my-prog-mode-hook)

(use-package dtrt-indent
  :ensure t
  :config
  (setq dtrt-indent-run-after-smie t)
  )

I kind of expected dtrt to replace smie altoghether. But now it looks like it rather requires it?

I suppose there is a use for dtrt-indent-mode without dtrt-indent-run-after-smie, but I don't see what it could be? If there isn't, why not make it default?

[rrthomas]

As far as the interaction with SMIE goes (since I introduced that) the idea is that for modes that use SMIE, dtrt-indent should defer to SMIE. If you don't use dtrt-indent-run-after-smie, then dtrt-indent will still be used in non-SMIE modes.

[fleutot]

I don't understand "defer to SMIE". Do you mean that dtrt-mode does nothing by default if SMIE is in use? Even if dtrt-mode is active? And dtrt-indent-run-after-smie overrides that and makes it run anyway?

If that is the case, I understand @jscheid's comment better, it really is confusing having the mode active but no effect. Why not have dtrt-indent-run-after-smie on by default? Whoever activates the mode (at least locally) certainly wants it active?

[rrthomas]

I don't understand "defer to SMIE". Do you mean that dtrt-mode does nothing by default if SMIE is in use? Even if dtrt-mode is active? And dtrt-indent-run-after-smie overrides that and makes it run anyway?

That's right.

Why not have dtrt-indent-run-after-smie on by default? Whoever activates the mode (at least locally) certainly wants it active?

Indeed, if you're activating dtrt-indent-mode locally, then it may make sense to turn dtrt-indent-run-after-smie on. The default setting of off makes sense when you're just using dtrt-indent-global-mode. The idea is that SMIE is supposed to solve the same problem, and it's the "default" solution, so we don't want to interfere with it by default.