inverse of 'indent-tabs-mode'

Question

According to indent-tabs-mode,

Indentation can insert tabs if this is non-nil.

nil means indentation cannot use tabs. I need the opposite of this option, prevent indentation from using spaces as tabs.

I need this to match a formatting convention (eclipse) where tabs are mandatory.

What is the simplest way to achieve this? I'm using plain java-mode, whose tab is bound to (c-indent-line-or-region &optional ARG REGION), which seems to use (symbol-function 'indent-according-to-mode), which eventually uses c-shift-line-indentation, but I don't see any options to configure using tabs only. Am I looking in the wrong place?

Answer 1

Ultimately it's up to the individual mode to decide how it indents things, but indent-tabs-mode is generally respected, and all you need do is set it to a non-nil value.

The canonical non-nil value is t when no other value is more appropriate.

indent-tabs-mode is automatically buffer-local, and you can use the mode hook for java-mode to set it in any buffer using that mode.

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

(defun my-java-mode-hook ()
  "Custom behaviours for `java-mode-hook'."
  (setq indent-tabs-mode t))

Provided java-mode only ever indents to multiples of tab-width, this ought to give you tab-only indentation.

If not, you would need to determine how to control the level of indentation in java-mode so that you can set that (and/or tab-width -- another buffer-local value) to coincide, so that only tabs will be required.

Naturally if there are circumstances when java-mode needs to indent to a column which is not an exact tab-width multiple, it won't be possible for it to use only tabs.

Answer 2

TLDR: (setf tab-width 4)

I was seeing the unwanted behavior of 4 spaces being inserted despite indent-tabs-mode being t. Thankfully, emacs is free software, so I was able to follow my TAB key press all the way down to the C source:

`TAB->c-indent-line-or-region->(symbol-function 'indent-according-to-mode)->c-shift-line-indentation->indent-to`

indent-to is defined in C source. the relevant portion is:

if (indent_tabs_mode)
{
  Lisp_Object n;
  XSETFASTINT (n, mincol / tab_width - fromcol / tab_width);
  if (XFASTINT (n) != 0)
{
  Finsert_char (make_number ('\t'), n, Qt);

  fromcol = (mincol / tab_width) * tab_width;
}
}

clearly n was being 0. This is because mincol / tab_width was 4 / 8, because tab-width was too big. Setting (setf tab-width 4) allows the function to indent using a tab, and no spaces are needed.

Finally:

(add-hook 'java-mode-hook (lambda () (setf tab-width 4)))