Learning emacs elisp - Functions 3/4

August, 2023

This is a guide to learn by examples. There are two ways I run the commands.

Open a new buffer and run `C-x C-e` after each expression

Using interactive expression evaluation `M-x ielm`

List function and iterations

(defun greet (name)
  "greet someone"
  (message "Hello, %s" name))

(greet "Alice") ; function call

(defun greetHi (name last-name &optional middle-name)
  "Optional params"
  (interactive)
  (message "%s %s %s" name middle-name last-name))

(greetHi "Alice" "Cooper" "Wayne") ;
(greetHi "Alice" "Cooper") ; will print `nil` and learn how to get rid of it

(defun greetAll (word &optional me &rest people)
  "Greet all"
  (message "%s! %s" word people)) ; optional and a list of params are given in that order


;; Emacs lisp does not support named parameters, default values for params, and type checking
;; Use a alist (association list) to emulate them

(defun doIt (x &optional xOpt)
  "Add and print the values"

Details

X is a number

xOpt is a list of numbers like `ARG1`

See URL 'http://example.org'
See 'dired'"

  (let ((arg1 (or (cdr (assoc "arg1" xOpt)) 1)))
  (message "%d" (+ arg1 x))))


(doIt 2 '(("arg1" . 40)))


;; throw exit

(defun catch-throw-exit ()
  "example. using catch/throw to exit function"
  (interactive)

  (catch 'aaa
    (if (y-or-n-p "exit?")
        (progn
          (message "exited")
          (throw 'aaa 3) ; return 3 to catch
          )
      (progn ; else
        (message "continued")
        4 ; return 4
        ))))


;; Apply list to args

(defun addAll (x y z)
  "add X Y Z"

  (+ x y z))

(apply 'addAll '(1 2 3))


((lambda (x)
  (1+ x))  41) ; apply lambda to a value

(lambda (x)
  (let (a)
    (setq a)
    (list a '(2))))

(mapcar
 (lambda (x) (aref x 1))
 [[1 1] [2 4] [3 9]]
 )

(fset 'inc (lambda (x) "add 1 to arg" (1+ x)))

(defun exit-with-error ()
  (if (y-or-n-p "really exit?")
      (user-error "Error, 'cause you exited")
    (progn
      (message "continue"))))

(catch 'found-three
  (while (setq i (random 5))
   (message "%s" i)
   (when (eq 3 i) (throw 'found-three t))))

;; function types
(special-form-p 'if) ; f, cond, and, or, while, progn
(macrop 'when)
(commandp 'count-words)
(functionp 'file-name-directory)
(boundp 'buffer-filename)
(fboundp 'not-defined-func-yet) ; is it defined

;; Similar to a finally
(unwind-protect
    (progn
      (do-something)
      (do-something-else))
  (first-finally-expr)
  (second-finally-expr))

;; Ignore errors
(ignore-errors
  (do-something)
  (do-something-else))