elisp - How to achieve code folding effects in Emacs?

Question

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.

Answer 1

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.