Code folding in Emacs

Code folding is a feature I’ve taken for granted in Windows IDEs, but now I am using Emacs more and more, I wanted it there too, and it turns out getting it is straightforward enough.

1. Download Folding Mode and save folding.el in the Emacs site lisp directory (on my Debian machine that is /usr/share/emacs23/site-lisp/, probably the same on any Linux). Surprisingly there’s no .deb package for it or apt-get install emacs-goodies-el.
2. Add the following to your ~/.emacs:

   (load "folding" 'nomessage 'noerror)
   (folding-add-to-marks-list 'haskell-mode "--{{{"  "--}}}"  nil t)
   (folding-mode-add-find-file-hook)
   (add-hook 'haskell-mode-hook 'folding-mode)
   
   (folding-add-to-marks-list 'tuareg-mode "(*{{{"  "(*}}}*)"  nil t)
   (add-hook 'tuareg-mode-hook 'folding-mode)
   

   Lines 4 and 7 cause folding-mode to be automatically entered after haskell-mode or tuareg-mode respectively, which seems to be necessary, rather than just loading it on Emacs startup with auto-load. This is slightly different from the instructions on the page, perhaps it was written for an older Emacs.

3. Delimit the code with the appropriate markers. These can be anything you put in .emacs, {{{ and }}} in conjunction with the language’s own comment characters seems to be a convention. Unfortunately Emacs doesn’t seem to be able to do this automagically the way other language-sensitive editors would (then again, those editors don’t allow you to define folding positions yourself, they always do it on functions or other obvious blocks)

As an example you can then toggle between:

And:

With right-click.