Difference between revisions of "Emacs Lisp in 15 minutes"

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

Latest revision as of 22:27, 16 April 2014

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

Author: Bastien / @bzg2 / http://bzg.fr

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[edit]

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[edit]

setq stores a value into a variable:

(setq my-name "Bastien")

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

Insert text[edit]

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[edit]

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[edit]

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[edit]

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[edit]

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[edit]

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[edit]

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[edit]

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[edit]

(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[edit]

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[edit]

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[edit]

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