How can I set Emacs tab settings by file type?

Question

I need to be able to set the tab settings for the following file types:

• .rb: 2 soft spaces
• .css, .html.erb: 4 space tabs

I have tried the following, but none of it seems to alter my default tab settings for each of the file types.

;; js-mode-hook has also been tried
(add-hook 'javascript-mode-hook 
      '(lambda() 
        (setq tab-width 4)))

(add-hook 'css-mode-hook 
      '(lambda() 
        (setq tab-width 4)))

(add-hook 'html-mode-hook 
      '(lambda() 
        (setq tab-width 8)))

I am pretty new to emacs so my knowledge of configuration is pretty low.

Answer 1

Theres a number of aspects to how Emacs does indentation. Setting the tab-width only specifics how big a tab is if a literal tab is inserted. If you don't wish to use literal tabs for indentation, then you should first disable their insertion (from the manual ):

Emacs normally uses both tabs and spaces to indent lines. If you prefer, all indentation can be made from spaces only. To request this, set indent-tabs-mode to nil. This is a per-buffer variable, so altering the variable affects only the current buffer, but there is a default value which you can change as well.which you can change as well.

However, to specify the indentation levels, you'll also need to set the c-basic-offset value variable as well:

(add-hook 'html-mode-hook 
      '(lambda() 
        (setq c-basic-offset 4)
        (setq indent-tabs-mode nil))

In your case, you may only need the c-basic-offset but try a few combinations and see what works best.

Answer 2

In emacs each mode has it's own indentation style. The main command to indent (bound to TAB) is indent-for-tab-command. This command calls mode specific indentation function found in the variable indent-line-function. So each mode has it's own way of doing it.

For Ruby (for my emacs 2 is a default):

 (setq ruby-indent-level 2)

For CSS (again, default is 4 for me):

 (setq css-indent-offset 4)

Unfortunately SGML mode (on which HTML mode is based) has a very simple indentation mechanism and apparently the level is not configurable. See the source code of sgml-calculate-indent function.

I personally find it weird. I am not writing HTML, but you can try to modify the sgml-calculate-indent function yourself :). Learn some lisp.

I am using js2 mode, and it indents perfectly by default. For js you have to search for js-indent-level or something similar.

Cheers.