Emacs Lisp in 15 minutes

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This gives an introduction to Emacs Lisp in 15 minutes (v0.2d)

Author: Bastien / @bzg2 / http://bzg.fr (the Org maintainer for many years)

First make sure you read this text by Peter Norvig: http://norvig.com/21-days.html

Going through this tutorial won't damage your computer unless you get so angry that you throw it on the floor. In that case, I hereby decline any responsibility. Have fun!


Write and execute lisp

Fire up Emacs.

Hit the q key to dismiss the welcome message.

Now look at the gray line at the bottom of the window:

"*scratch*" is the name of the editing space you are now in. This editing space is called a "buffer".

The scratch buffer is the default buffer when opening Emacs. You are never editing files: you are editing buffers that you can save to a file.

"Lisp interaction" refers to a set of commands available here.

Emacs has a built-in set of commands available in every buffer, and several subsets of commands available when you activate a specific mode. Here we use the `lisp-interaction-mode', which comes with commands to evaluate and navigate within Elisp code.


Semi-colons start comments anywhere on a line.

Elisp programs are made of symbolic expressions ("sexps"): 

(+ 2 2)  ;; this is a comment

This symbolic expression reads as "Add 2 to 2".

Sexps are enclosed into parentheses, possibly nested: 

(+ 2 (+ 1 1))

A symbolic expression contains atoms or other symbolic expressions. In the above examples, 1 and 2 are atoms, (+ 2 (+ 1 1)) and (+ 1 1) are symbolic expressions.

From `lisp-interaction-mode' you can evaluate sexps. Put the cursor right after the closing parenthesis then hold down the control and hit the j keys ("C-j" for short). 


(+ 3 (+ 1 2))

[ C-j ] => 6

[ C-j ] inserts the result of the evaluation in the buffer.

[ C-xC-e ] displays the same result in Emacs bottom line, called the "minibuffer". We will generally use `C-xC-e', as we don't want to clutter the buffer with useless text.

Variables

setq stores a value into a variable: 

(setq my-name "Bastien")

[ C-xC-e ] => "Bastien" (displayed in the mini-buffer)

Insert text

insert will insert "Hello!" where the cursor is: 

(insert "Hello!")

[ C-xC-e ] => "Hello!"

We used insert with only one argument "Hello!", but we can pass more arguments -- here we use two: 


(insert "Hello" " world!")

[ C-xC-e ] => "Hello world!"

You can use variables instead of strings: 

(insert "Hello, I am " my-name)

[ C-xC-e ] => "Hello, I am Bastien"

Functions

You can combine sexps into functions: 

(defun hello () (insert "Hello, I am " my-name))

[ C-xC-e ] => hello

You can evaluate functions: 

(hello)

[ C-xC-e ] => Hello, I am Bastien

The empty parentheses in the function's definition means that it does not accept arguments. But always using `my-name' is boring, let's tell the function to accept one argument (here the argument is called "name"): 


(defun hello (name) (insert "Hello " name))

[ C-xC-e ] => hello

Now let's call the function with the string "you" as the value for its unique argument: 

(hello "you")

[ C-xC-e ] => "Hello you"

Yeah!

Take a breath.


Switch buffers

Now switch to a new buffer named "*test*" in another window: 


(switch-to-buffer-other-window "*test*")

[ C-xC-e ] => [screen has two windows and cursor is in the *test* buffer]

Mouse over the top window and left-click to go back. Or you can use `C-xo' (i.e. hold down control-x and hit o) to go to the other window interactively.

Expression blocks

You can combine several sexps with progn: 

(progn
  (switch-to-buffer-other-window "*test*")
  (hello "you"))

[ C-xC-e ] => [The screen has two windows and cursor is in the *test* buffer]

Now if you don't mind, I'll stop asking you to hit `C-xC-e': do it for every sexp that follows.

Always go back to the *scratch* buffer with the mouse or `C-xo'.

It's often useful to erase the buffer: 

(progn
  (switch-to-buffer-other-window "*test*")
  (erase-buffer)
  (hello "there"))


Or to go back to the other window: 

(progn
  (switch-to-buffer-other-window "*test*")
  (erase-buffer)
  (hello "you")
  (other-window 1))


Local variables

You can bind a value to a local variable with let: 

(let ((local-name "you"))
  (switch-to-buffer-other-window "*test*")
  (erase-buffer)
  (hello local-name)
  (other-window 1))


No need to use progn in that case, since let also combines several sexps.

Strings handling

Let's format a string: 

(format "Hello %s!\n" "visitor")

%s is a place-holder for a string, replaced by "visitor". \n is the newline character.

Let's refine our function by using format: 

(defun hello (name)
  (insert (format "Hello %s!\n" name)))

(hello "you")

Let's create another function which uses let: 

(defun greeting (name)
  (let ((your-name "Bastien"))
    (insert (format "Hello %s!\n\nI am %s."
                    name       ; the argument of the function
                    your-name  ; the let-bound variable "Bastien"
                    ))))


And evaluate it: 

(greeting "you")

Interactive functions

Some function are interactive: 

(read-from-minibuffer "Enter your name: ")

Evaluating this function returns what you entered at the prompt.

Let's make our greeting function prompt for your name: 

(defun greeting (from-name)
  (let ((your-name (read-from-minibuffer "Enter your name: ")))
    (insert (format "Hello!\n\nI am %s and you are %s."
                    from-name ; the argument of the function
                    your-name ; the let-bound var, entered at prompt
                    ))))

(greeting "Bastien")

Let's complete it by displaying the results in the other window: 

(defun greeting (from-name)
  (let ((your-name (read-from-minibuffer "Enter your name: ")))
    (switch-to-buffer-other-window "*test*")
    (erase-buffer)
    (insert (format "Hello %s!\n\nI am %s." your-name from-name))
    (other-window 1)))

Now test it: (greeting "Bastien")

Take a breath.


Lists

Let's store a list of names: 

(setq list-of-names '("Sarah" "Chloe" "Mathilde"))

Get the first element of this list with car: 

(car list-of-names)

Get a list of all but the first element with cdr: 

(cdr list-of-names)

Add an element to the beginning of a list with push: 

(push "Stephanie" list-of-names)

NOTE: car and cdr don't modify the list, but push does. This is an important difference: some functions don't have any side-effects (like car) while others have (like push).

Let's call hello for each element in `list-of-names': 

(mapcar 'hello list-of-names)

Refine greeting to say hello to everyone in `list-of-names': 

(defun greeting ()
    (switch-to-buffer-other-window "*test*")
    (erase-buffer)
    (mapcar 'hello list-of-names)
    (other-window 1))

(greeting)

Remember the hello function we defined above? It takes one argument, a name. mapcar calls hello, successively using each element of `list-of-names' as the argument for hello.

Now let's arrange a bit what we have in the displayed buffer: 


(defun replace-hello-by-bonjour ()
    (switch-to-buffer-other-window "*test*")
    (goto-char (point-min))
    (while (search-forward "Hello")
      (replace-match "Bonjour"))
    (other-window 1))


Moving around

(goto-char (point-min)) goes to the beginning of the buffer. (search-forward "Hello") searches for the string "Hello". (while x y) evaluates the y sexp(s) while x returns something. If x returns nil (nothing), we exit the while loop. 


(replace-hello-by-bonjour)

You should see all occurrences of "Hello" in the *test* buffer replaced by "Bonjour".

You should also get an error: "Search failed: Hello".

To avoid this error, you need to tell `search-forward' whether it should stop searching at some point in the buffer, and whether it should silently fail when nothing is found:

(search-forward "Hello" nil t) does the trick:

The nil argument says: the search is not bound to a position. The t argument says: silently fail when nothing is found.

We use this sexp in the function below, which doesn't throw an error: 


(defun hello-to-bonjour ()
    (switch-to-buffer-other-window "*test*")
    (erase-buffer)
    ;; Say hello to names in `list-of-names'
    (mapcar 'hello list-of-names)
    (goto-char (point-min))
    ;; Replace "Hello" by "Bonjour"
    (while (search-forward "Hello" nil t)
      (replace-match "Bonjour"))
    (other-window 1))

(hello-to-bonjour)

Change fonts

Let's colorize the names: 


(defun boldify-names ()
    (switch-to-buffer-other-window "*test*")
    (goto-char (point-min))
    (while (re-search-forward "Bonjour \\(.+\\)!" nil t)
      (add-text-properties (match-beginning 1)
                           (match-end 1)
                           (list 'face 'bold)))
    (other-window 1))

Searching regexps

This functions introduces `re-search-forward': instead of searching for the string "Bonjour", you search for a pattern, using a "regular expression" (abbreviated in the prefix "re-").

The regular expression is "Bonjour \\(.+\\)!" and it reads: the string "Bonjour ", and a group of (this is the \\( ... \\) construct) any character (this is the .) possibly repeated (this is the +) and the "!" string.

Ready? Test it! 


(boldify-names)

`add-text-properties' adds... text properties, like a face.

OK, we are done. Happy hacking!

How to get documentation

If you want to know more about a variable or a function:

C-h v a-variable RET C-h f a-function RET

To read the Emacs Lisp manual with Emacs:

C-h i m elisp RET

To read an online introduction to Emacs Lisp: https://www.gnu.org/software/emacs/manual/html_node/eintr/index.html

Thanks to these people for their feedback and suggestions:

  • Wes Hardaker
  • notbob
  • Kevin Montuori
  • Arne Babenhauserheide
  • Alan Schmitt
  • LinXitoW
  • Aaron Meurer
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