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Basic Concepts

There are quite a few concepts in Emacs which relate to white space at the beginning of a line. Enough, in fact, to be potentially quite confusing. I'll try to clear up the confusion in this little document. The following concepts come into play:

The TAB key.

Contrary to popular belief, the TAB key does not insert a tab character in most modes. Instead, it does syntax-driven indentation in most programming language modes. In fundamental mode, it advances to the next tab stop. In text mode, it does relative indentation (relative to the previous line, that is). You can type C-q TAB to insert a tab character.

The tab character.

It is possible to change the way a tab character is displayed. `vi' users do this. For example, they can set the width of a tab character to 4, and this enables them to hit TAB at the beginning of a line to indent the body of a loop, say, by one indentation step (of 4 characters). Normally, the width of tab is eight characters, so a file created by a `vi' user with a tab width of 4 will look different when viewed in a program which uses the default tab width setting.

In order to correctly view files with non-standard tab width settings, it is possible to set the variable tab-width, like this:

(setq-default tab-width 4)

If you only want this in a particular mode, add (setq tab-width 4) to that mode's hook. (See below for an explanation of mode hooks.)

Whether or not tab characters are used for indentation.

Whether by syntax-driven indentation, or relative indentation (text mode), or advancing to the next tab stop (fundamental mode), indentation needs to insert white space. The variable indent-tabs-mode controls whether or not tab characters are used for inserting the white space. If this variable is t, tab characters are used to make the file as short as possible. For example, if tab-width is equal to 4 and white space of 10 characters needs to be inserted at the beginning of the line, Emacs will insert 2 tab characters (2 times 4 equals 8 spaces), plus two space characters. With tab-width equal to the default value of 8, Emacs would insert 1 tab plus 2 spaces.

Use the following line to tell Emacs to never use tab characters for indentation:

(setq-default indent-tabs-mode nil)

Use t rather than nil to tell Emacs to use tab characters where appropriate.

If you only want this in a particular mode, add (setq indent-tabs-mode nil) to that mode's hook. (See below for an explanation of mode hooks.)

By the way, actual tab characters are really needed in Makefiles; thus, makefile mode overrides this setting.

The location of the tab stops.

The variable tab-stop-list contains a list of tab stop positions, and M-i moves to the next tab stop. (In fundamental mode, TAB also does this.) You can define your own list with a statement like the following:

(setq tab-stop-list '(4 8 12 16))

Whether or not actual tab characters are inserted is controlled by the variable indent-tabs-mode, and the number of tab characters inserted is controlled by tab-width (among others).

Obviously, if you set tab-stop-list to a list containing multiples of tab-width, then one M-i will insert one tab character at all times (if indent-tabs-mode isn't nil).

The variable tab-stop-list controls which spots M-i moves to, and indent-tabs-mode controls whether tab characters (plus maybe a few space characters) are used to move to that spot, and tab-width controls how many tab characters are needed.

Syntax driven indentation.

This means that the current line is indented in a way that is useful for the syntax of the programming language used. For example, in C-like languages, the lines between an opening and a closing brace are usually indented two to four spaces with respect to the braces themselves. In Pascal, the lines between begin and end would be indented two to four spaces.

Since the syntax driven indentation depends a lot on the syntax of the programming language at hand, the major mode provides the indentation. And customizing each major mode works a little differently, because the programming languages are different.

For C-like languages, there is `CC mode'. It provides major modes for C, C++, Objective C, IDL, and Java. The indentation engine is quite flexible, and the CC mode info file has the whole story on customizing indentation for these major modes. I'll just mention a few things.

CC mode provides for different coding styles. Type M-x c-set-style RET, then use tab completion to have a look at them, and try them out. If you find one you like, you can add something like (c-set-style "gnu") to your mode hook. These coding styles often refer to a basic indentation offset, determined by the variable c-basic-offset. Add stuff like (setq c-basic-offset 4) to the mode hook in question.

If this isn't sufficient, you can go to the line where you don't like the indentation, and type C-c C-o. Follow the prompts. Hit TAB to re-indent the current line. Repeat until you like the result. You can then type C-x ESC ESC and use M-n and M-p to locate the right `c-set-offset' statement to add to your hook. Please note that you can enter numbers for the indentation level, but you can also use + and - to stand for one basic offset to the right or left, respectively, or ++ and -- for twice the basic offset.

I use `CPerl' mode for editing Perl code. It also provides a style mechanism similar to the one provided by CC mode, but it doesn't provide an interactive command which allows one to customize indentation, like C-c C-o in CC mode. Type C-h f cperl-mode RET for a list of variables affecting indentation. Choose an indentation style which is closest to what you want, then change the remaining variables from your mode hook. You can choose an indentation style by adding something like (cperl-set-style "gnu") to your mode hook.

CCC Can people provide info for other modes?

Relative indentation.

This means indent to a spot indicated by the previous non-blank line. Below, the spots relative indentation moves to are indicated with a caret.

    This is a line to show relative indentation.
    ^    ^  ^ ^    ^  ^    ^        ^

I find this useful for text where it can be used to format tables or to indent itemized lists. When in text mode, the TAB key performs relative indentation by default. The command name is indent-relative.

Usage scenarios.

In the above, I explained the concepts behind the things Emacs is doing, but I suspect you'd like some advice on when to frob which options.

I want to turn off syntax driven indentation!

Since `vi' doesn't have syntax driven indentation, its users usually set the tab width to some value they like, and whenever they want to indent a line more than the previous one, they hit TAB. Of course, a file which was edited in this way will look strange when loaded into Emacs. The variable tab-width was made for this, just set it to the tab width your `vi' loving colleagues use, and you can continue to use syntax driven indentation and your cow-orkers will wonder why you can edit code so fast...

Perhaps you also want to set indent-tabs-mode to t (which is the default) such that Emacs actually uses tabs with the new width. But then, maybe you want to set it to nil such that Emacs uses only spaces and the looks of your code won't depend on the tab width setting somebody uses.

And you might need to customize your indentation to be similar to the style used by the others, lest you need to do a lot of manual reformatting. A consistent coding style is a good thing.

Background information.

This section provides background information referred to in the above.

Mode hooks.

A central concept in Emacs is the major mode. For each kind of text, a major mode provides functions useful in that mode. Most (if not all) major modes provide a so-called hook which allows you to specify Lisp code to be run when that major mode is entered. Most of the time, this feature is used just for setting a few variables.

A mode hook is a variable which contains a list of functions to be executed. Thus, if you want something to happen, you have to write a function first. Writing functions is rather simple, finding a name for the function might be the most difficult part, in fact!

So, let's say you wish to set the variable c-basic-offset to 3 when in C++ mode. Additionally, you want to use no tabs for indentation. Here's a function which does this:

(defun my-c++-indent-setup ()
  (setq c-basic-offset 3)
  (setq indent-tabs-mode nil))

Pay careful attention to the nesting of the parentheses. The empty pair of parentheses is always needed, and the parenthesis opened before the word defun is closed at the very end of the function.

So, all that's left is to add this function to the mode hook for C++ mode. Easy:

(add-hook 'c++-mode-hook 'my-c++-indent-setup)

You may wish to replace the `my' prefix with something else such that you can be sure not to reuse a function name already defined elsewhere. I use the prefix `kai', for example. This means I've got to be careful when snarfing code from Kai-Uwe Rommel :-)


  • file local variables for tab-width (Per Abrahamsen)

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