I am writing Fortran code in emacs but the tab key behaviour changes in F90 mode. Can I change it back?

Question

On loading a f90 file, emacs changes to F90 mode, which changes some emacs behaviours.

When I press <tab> emacs inserts two spaces at the left margin. Now when I use the backspace key it deletes those two spaces one at a time.

Is there a modification I can make which will allow me to delete tabs with a single keystroke in F90 mode?

Answer 1

Yes, what you want is a "hungry" or "greedy" delete/backspace. f90-mode doesn't come with such a thing, but cc-mode does. It includes two inter-related families of functions c-electric-delete* and c-hungry-delete-*. Specifically take a look at c-hungry-delete-backwards via Emacs's built in function help (C-h f c-hungry-delete-backwards). From there you can view its implementation; it's relatively small, and will give you a good start at implementing your own "hungry delete".

Edit: Someone's done the work for you, see https://github.com/nflath/hungry-delete

Edit: a naive function to delete two spaces could look something like this. In your ~/.emacs.d/init.el you would add:

;; define a toy function to delete two spaces at once 
;; if the preceding two characters are indeed spaces
(defun nega/two-space-delete ()
  (interactive)
  (if (and (eq ?\  (char-before))
           (eq ?\  (char-before (- (point) 1))))
      (delete-backward-char 2)
    (delete-backward-char 1)))

;; after f90-mode is loaded, remap instances of (delete-backward-char) to 
;; instead use our previously defined toy function
(eval-after-load 'f90-mode
  '(define-key f90-mode-map [remap delete-backward-char] #'nega/two-space-delete))

In your comment you stated

An elisp function which knows the tab-size in spaces and deletes the same number of spaces would solve the problem

This would be the idea solution, but that's not how f90-mode works. Unlike many other modes, it has more than one indent level. In fact it has seven.

• f90-do-indent -- Extra indentation within do blocks (default 3).
• f90-if-indent -- Extra indentation within if/select/where/forall blocks (default 3).
• f90-type-indent -- Extra indentation within type/enum/interface/block-data blocks (default 3).
• f90-program-indent -- Extra indentation within program/module/subroutine/function blocks (default 2).
• f90-associate-indent -- Extra indentation within associate blocks (default 2).
• f90-critical-indent -- Extra indentation within critical/block blocks (default 2).
• f90-continuation-indent -- Extra indentation applied to continuation lines (default 5).

This complicates writing a comprehensive function that "deletes backwards one indentation level". A "greedy backwards delete" that deletes whitespace back to the beginning of the line, followed by a stroke of the TAB key to set you at the correct indent level would serve you well.