This post is part of an ongoing series to try to make the Emacs Lisp learning curve less steep, which I introduced here.

So far, you should be able to read Elisp and understand how to get around Emacs' documentation. Now, its time to get into writing real Emacs Lisp.

Saying "Hello, World" in Emacs

Lets get right to it: our very first, very minimal function. Run M-x elm to open up an elisp interpreter, and evaluate the following code:

(defun say-hello ()
    "hello")

Next, run it via (say-hello). Your *ielm* buffer should look something like this:

ELISP> (defun say-hello ()
         "Heya, World!")
say-hello
ELISP> (say-hello)
"Heya, World!"

This function definition is very simple. It declares a function (via defun) named say-hello This function takes no arguments (signified by the empty parameter list ()). It returns the string "Heya, World!". Easy. Lets do the same thing, but slightly differently this time:

(defun say-hello ()
   "Says hello."
   (interactive)
   (message "Hello, World!"))

Lets slow down a bit and explain these new features. They are still easy, but do not necessarily directly translate from experience programming in other language

The first piece we come to is the documentation string, "Says Hello." Documentation strings are used to document what a function does. The function describe-function displays the documentation string of the function that is being described.

Other things also get documentation strings, such as variables with the defvar declaration. We will revisit defvars later, though.

The code (interactive) signifies that this function should be considered a command; that is, it should be user-invocable. This command can be invoked via "M-x say-hello", and it can also be bound to a keyboard key.

We begin to see the power and flexibility of Emacs with interactive functions. A user can easily create new commands; they need not be constrained to what the authors thought he would need, or use. More on this later.

Finally, we use message to say "Hello, World!" The function message is used to send a message to the user. The string passed to message will be displayed in the echo area and output to the *Messages* buffer for reference. Its a really handy little function for telling the user about all sorts of things.

We're almost done with the basic introduction to Emacs and Emacs lisp. At this point, you can get really far by understanding how to build your own functions, look at the way other functions are built, and experimenting. Next time, we will look at variables, and the various ways to use and define them.