How to achieve code folding effects in Emacs?

What's the best way to achieve something like code folding, or the type of cycling that org-mode uses. What would be the best solution in elisp to create this type of behavior?

EDIT: I'm sorry I was not clear. I want to program something in elisp that does things very similar to code folding, or actually most like org-mode with the hierarchy that can be expanded. I am wondering the best way to achieve this effect. I think I have heard emacs overlays are a good solution, but I don't know.

As far as folding I just use the builtin set-selective-display

EDIT NUMBER 2:

Thanks for the answers but I think I am asking the wrong question so let me try to be more clear on what I am trying to do. I would like to create the following

When you put your point on a function and call this elisp function it will put the function definition from wherever it is (I am thinking of just using find-tag for this) and unfold it in the current buffer. The idea is if you have to jump to a different buffer to read the function definition I feel like it's a context switch to another file. So I would like it to behave like code folding only it pulls in code external from the buffer. This presents some problems as it can not actually paste the code into the buffer or if someone saves it will save the pulled in code. So I am wondering if there is a way to create an area inside a buffer that is also not part of the buffer. I think that makes sense.

Folding is generally unnecessary with emacs, as it has tools that explicitly implement the actions people do manually when folding code.

Most people have good success with simple incremental searches. See "foo" mentioned somewhere? Type C-sfoo, find the definition, press enter, read it, and then press C-x C-x to go back to where you were. Simple and very useful.

Most modes support imenu. M-ximenu will let you jump to a function definition (etc.) by name. You can also bind it to a mouse click to get a menu of functions (or add it to the menubar; see the Info page for more detail). It provides data for which-function-mode, which will let you see which function you are currently inside in the modeline. (Why are your functions this long, though?)

There is also speedbar, which displays the imenu information (and other things) graphically.

If you want to get an overview of your file, try M-xoccur". Given a regex, it will create a new buffer with each match in the current buffer. You can search for "(defun" to get an overview of the functions the current file implements. Clicking on the result will move you to that position in the file.

So anyway, think about what you really want to achieve, and Emacs probably implements that. Don't fight with imperfect tools that make you fold and unfold things constantly.

You can use the hide-show mode (hs-minor-mode). To activate this mode on the fly for the current buffer only, type M-x hs-minor-mode RET

Assuming you have the default keybinding:

• C-c @ C-M-h To fold it all (hs-hide-all).
• C-c @ C-h Hides the current block (hs-hide-block). The point (cursor) has to be inside the block.
• C-c @ C-s Show current block (hs-show-block). The point has to be in the {...} marker that is hiding the code.

Another way to skin the cat:

As it happens, you don’t need any package or extra configuration for that. Just go to any source file, type

M-1 C-x $ and magic happens!

As usual, it’s white magic: C-x $ will bring your code back.

We can use Emacs’ help system to discover what’s going on: C-h k C-x $ tells us that the above key combination is calling set-selective-display, a function that takes one numerical argument (the M-1 prefix passes 1 as the value of that argument) and, unsurprisingly, sets the variable selective-display to the value of that argument.

from minor emacs wizardry blog

Just for completeness: M-3 C-x $ would show deeper nested code and so on.

FWIW I made a tiny helper today based on smth. found here so that F5 toggles code folding based on the current cursor position:

(global-set-key (kbd "<f5>") 'set-selective-display-dlw)

(defun set-selective-display-dlw (&optional level)
"Fold text indented same of more than the cursor.
If level is set, set the indent level to LEVEL.
If 'selective-display' is already set to LEVEL, clicking
F5 again will unset 'selective-display' by setting it to 0."
  (interactive "P")
  (if (eq selective-display (1+ (current-column)))
      (set-selective-display 0)
    (set-selective-display (or level (1+ (current-column))))))

I use the outline minor mode to fold my python code. Instead of needing to place {{{ and }}} as in folding mode, it uses where the block is defined.

http://www.gnu.org/software/emacs/manual/html_node/emacs/Outline-Mode.html http://www.emacswiki.org/emacs/OutlineMinorMode

I am pretty sure that it comes with emacs. I then add this to my .emacs

;;======= Code folding =======
(add-hook 'python-mode-hook 'my-python-outline-hook)
; this gets called by outline to deteremine the level. Just use the length of the whitespace
(defun py-outline-level ()
  (let (buffer-invisibility-spec)
    (save-excursion
      (skip-chars-forward "    ")
      (current-column))))
; this get called after python mode is enabled
(defun my-python-outline-hook ()
  ; outline uses this regexp to find headers. I match lines with no indent and indented "class"
  ; and "def" lines.
  (setq outline-regexp "[^ \t]\\|[ \t]*\\(def\\|class\\) ")
  ; enable our level computation
  (setq outline-level 'py-outline-level)
  ; do not use their \C-c@ prefix, too hard to type. Note this overides some bindings.
  (setq outline-minor-mode-prefix "\C-t")
  ; turn on outline mode
  (outline-minor-mode t)
  ; initially hide all but the headers
  ;(hide-body)
  ; make paren matches visible
  (show-paren-mode 1)
)