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+---
+language: elisp
+contributors:
+ - ["Bastien Guerry", "http://bzg.fr"]
+filename: learn-emacs-lisp.el
+---
+
+```elisp
+;; This gives an introduction to Emacs Lisp in 15 minutes (v0.2a)
+;;
+;; First make sure you read this text by Peter Norvig:
+;; http://norvig.com/21-days.html
+;;
+;; Then install GNU Emacs 24.3:
+;;
+;; Debian: apt-get install emacs (or see your distro instructions)
+;; MacOSX: http://emacsformacosx.com/emacs-builds/Emacs-24.3-universal-10.6.8.dmg
+;; Windows: http://ftp.gnu.org/gnu/windows/emacs/emacs-24.3-bin-i386.zip
+;;
+;; More general information can be found at:
+;; http://www.gnu.org/software/emacs/#Obtaining
+
+;; Important warning:
+;;
+;; 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 responsability. Have fun!
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;
+;; 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 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))
+;; ^ cursor here
+;; `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.
+
+;; `setq' stores a value into a variable:
+(setq my-name "Bastien")
+;; `C-xC-e' => "Bastien" (displayed in the mini-buffer)
+
+;; `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"
+
+;; 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.
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;
+;; 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.
+
+;; 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))
+
+;; 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.
+
+;; Let's format a string:
+(format "Hello %s!\n" "visitor")
+
+;; %s is a place-holder for a string, replaced by "Alice".
+;; \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")
+
+;; 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.
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;
+;; 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))
+
+;; (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)
+
+;; 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))
+
+;; 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!
+
+;; 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
+```