`dtrt-indent-mode` erroneously uses Tab characters

Question

Here is an example file (/tmp/asdf.sh), for which I want dtrt-indent-mode to do the right thing.

#! /usr/bin/env bash

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

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

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

I have created a minimal Emacs init file (~/tmp/basic-init.el) to test this:

(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))

(use-package blank-mode
  :ensure t)
(global-set-key (kbd "C-c b") 'blank-mode)

Starting emacs with that init file, opening that shell file:

emacs -q -l ~/tmp/basic-init.el /tmp/asdf.sh

then showing blank characters with C-c b, gives this:

(Don't mind the yellow highlight, blank-mode should be setup to leave that alone.)

Going to that yellow line, deleting a space char then pressing TAB gives this:

According to me, the right thing to do is not to put a Tab character there.

---

What I have found trying to understand what is going on:

Luckily, dtrt-indent has a diagnosis tool. This is the output of dtrt-indent-diagnosis, on the original file (no Tab character):

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: 0.00% (0 lines), -100.00% superior to soft tabs (threshold 300.00%)
  Soft tab percentage: 100.00% (5 lines), inf% superior to hard tabs (threshold 300.00%)

Conclusion:

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

I did not manage to find help on indent-tab-setting.

However, C-h k on TAB key press gives this:

TAB (translated from <tab>) runs the command indent-for-tab-command
(found in global-map), which is an interactive compiled Lisp function
in ‘indent.el’.

It is bound to TAB.

(indent-for-tab-command &optional ARG)

Indent the current line or region, or insert a tab, as appropriate.
This function either inserts a tab, or indents the current line,
or performs symbol completion, depending on ‘tab-always-indent’.
The function called to actually indent the line or insert a tab
is given by the variable ‘indent-line-function’.

and C-h v on indent-line-function:

indent-line-function is a variable defined in ‘indent.el’.
Its value is ‘smie-indent-line’
Local in buffer asdf.sh; global value is indent-relative

Apparently, smie- is a default naive indentation function.

I was expecting to find a dtrt- function here.

How can I make dtrt-indent-mode do the right thing?

GNU Emacs 26.1.

Answer 1

dtrt-indent is a minor mode that will guess the indentation offset of a file and adjusts those local settings accordingly. It hooks in to several major modes, and works reasonably well.

@Gauthier is wondering why dtrt-indent is inserting a TAB character to perform indentation after guessing, with 100% confidence, that the indentation offset is 4 SPACE characters. I have always taken the line in the dtrt-indent-diagnosis report to be a recommendation to the user to change the "indent settings" (indent-tabs-mode). How the user chooses to do that is up to the user. (File local variables, directory local variables, global configuration, etc). In this case Change indent-tab-setting: yes, to nil is saying "Hey, maybe you want to disable indent-tabs-mode here. I dunno. You do you."

I believe the reason why @Guathier was able to delete 2 indentation levels of 4 spaces, and insert a single tab character to match those 2 indentation levels is because indent-tabs-mode was enabled (t). This short example file didn't meet the heuristics required for dtrt-indent to change indent-tabs-mode. The code in fact states that it may have trouble with small files.

Note that in a comment I stated:

Oh, also, dtrt-indent only adjusts indent level, it won't change what the indentation is (tabs/spaces). Hence the recommendation on whether or not to change the "indent-tab-settings". – nega 5 hours ago

That comment is incorrect, and I've deleted it. I confirmed I was wrong when I globally set my indent-tabs-mode to nil and opened a rather large C++ file that was indented with tabs, and dtrt-indent locally changed indent-tabs-mode to t. By default dtrt-indent will tell you what it changes in the mini-buffer and *Messages* buffer. You can change the verbosity of these messages by customizing dtrt-indent-verbosity. Said C++ example reported:

Note: c-basic-offset adjusted to 8
Note: indent-tabs-mode adjusted to t

I haven't studied the code in depth, but I suspect that this test case, and maybe other examples that @Gauthier has seen just didn't have enough data for dtrt-indent to make a decision. Personally, I think that conservatism is a good thing, as I think it would be more jarring to have my indent "things" changed, than just my indent "sizes".

Answer 2

I received help from the devs at github.

Basically, at its default, dtrt-indent-mode does nothing if SMIE also is active. Even if dtrt-indent-mode is active. Activating it (even buffer-locally) always defers to SMIE if active, which it is in many programming language modes.

This deferring can be overridden with dtrt-indent-run-after-smie.

Here is the relevant section of the basic init file in the question, modified to activate dtrt always:

(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) ;; Run even if SMIE is active
  )