How to make emacs properly indent if-then-else construct in elisp

Question

When I indent if-then-else construct in emacs lisp, the else block doesn't indent properly. What I get is:

(defun swank-clojure-decygwinify (path)
  "Convert path from CYGWIN UNIX style to Windows style"
  (if (swank-clojure-cygwin)
      (replace-regexp-in-string "\n" "" (shell-command-to-string (concat "cygpath -w " path)))
    (path)))

where else form is not indented at the same level as the then form. Is there an obvious way to fix this?

Answer 1

That is the proper indentation. A quote from the manual:

The word "else" is not written in the Lisp code; the else-part of an `if' expression comes after the then-part. In the written Lisp, the else-part is usually written to start on a line of its own and is indented less than the then-part:

 (if TRUE-OR-FALSE-TEST
     ACTION-TO-CARRY-OUT-IF-THE-TEST-RETURNS-TRUE
   ACTION-TO-CARRY-OUT-IF-THE-TEST-RETURNS-FALSE)

For example, the following if expression prints the message 4 is not greater than 5! when you evaluate it in the usual way:

 (if (> 4 5)                               ; if-part
     (message "4 falsely greater than 5!") ; then-part
   (message "4 is not greater than 5!"))   ; else-part

Note that the different levels of indentation make it easy to distinguish the then-part from the else-part. (GNU Emacs has several commands that automatically indent if expressions correctly. *Note GNU Emacs Helps You Type Lists: Typing Lists.)

This is a feature, not a bug :-)

Answer 2

I don't have the relevant documentation handy, but this seems to do what you want:

(put 'if 'lisp-indent-function nil)

Also, you misused the word "properly"; by definition, however emacs indents is "proper" :)

Answer 3

The default style is proper -- for Emacs Lisp. For Common Lisp and other flavors of Lisp, the "else" clause should be aligned underneath the "then" clause. To get Common Lisp indendation, you have to do something like this:

(set (make-local-variable lisp-indent-function)
     'common-lisp-indent-function)

To get this to happen automatically, you can do something like this:

(add-hook 'lisp-mode-hook
          '(lambda ()
             (set (make-local-variable lisp-indent-function)
                  'common-lisp-indent-function))))

Note, however, that Lisp-Emacs interaction packages, like Slime, may override the indentation behavior, and in that case, the above might do nothing. The above should work in a basic Emacs.