Exploring Emacs

Here’s a fun idea in case you want to practice reading code and using Emacs: read Emacs in Emacs!

Emacs makes it easy to see the code behind its commands. For instance, if you want to see how a particular command-key sequence works, type C-h k and then the key sequence into the modeline prompt.

If you’re curious how C-h k itself works, type C-h k C-h k. According to the *Help* buffer that appears, C-h k corresponds to the Emacs Lisp function describe-key defined in help.el. Move your cursor onto the help.el text in the Help buffer and press enter. You’re now looking at the Emacs Lisp source for describe-key.

If you’d like to see how a particular Emacs Lisp function or special form works, type C-h f and then the name of the function or special form.

Perhaps you’re curious how lambda works: type C-h f lambda and then hit enter. Another *Help* buffer will appear, saying that “lambda is a Lisp macro in subr.el”. As before, place your cursor inside subr.el and then press enter; Emacs will transport you right to the definition of lambda.

Emacs isn’t Emacs Lisp all the way down, though. For instance, addition is presumably defined in C. C-f + confirms this, noting that “+ is a built-in function defined in data.c”. If you move your cursor over data.c and press enter, Emacs will probably prompt you at the modeline for the location of your Emacs C source directory; it’s likely that when you installed Emacs, you only downloaded pre-compiled binaries. You can find source corresponding to your version of Emacs here. Once you tell Emacs where to look, you can hop into Emacs’s C source code, too.

It’s fun to guess whether a particular function or special form is defined in Emacs Lisp or C. The answer can also change between versions of Emacs. For example, until quite recently, the special form for defining a new Emacs Lisp function, defun, was defined in C. If you’re unfamiliar with Lisp, you might wonder how it could be otherwise! But according to my *Help* buffer in Emacs 24.3.50.1, “defun is a Lisp macro defined in byte-run.el”.

The exuberant comment above defun’s definition in byte-run.el is worth reinstalling Emacs from trunk: “Now that we defined defmacro we can use it!”. Bootstrapping defun is now the very first use of defmacro in all of Emacs. If you’re curious where defmacro comes from, you know what to do :)