emacs-lisp-aez-notes

Emacs Lisp the language
Emacs Lisp Packages
- JSON
- String

Emacs Lisp the language

Variables

Use defvar to define a variable along with optionally an initial value and a docstring. Use setq to set the value of an existing variable.

Nonstandard evaluation

If you want to evaulate an element of a quoted list you can use , as in the following example:

`(a list of ,(+ 2 3) elements)
     => (a list of 5 elements)

If you want to expand a list out in a quoted list you can use ,@ as in the following example:

(setq some-list '(2 3))
     => (2 3)

(cons 1 (append some-list '(4) some-list))
     => (1 2 3 4 2 3)

`(1 ,@some-list 4 ,@some-list)
     => (1 2 3 4 2 3)

Program flow

progn to sequence commands and return the value of the last one.

String handling

The s.el library provides sensible string functions, see the notes below.

string interpolation

(s-format "foo ${bar}" 'aget `(("bar" . ,(+ 1 1))))

Working with dates and times

format-time-string to get datetime as a string
current-time for the current time as a list of integers
time-subtract and time-add for datetime arithmetic

Working with files

Function	Use
`directory-files`	list files in a directory

Working with passwords

(read-passwd "magic word: ")

More details are available in the docs.

System command

To run a system command/shell command there is shell-command.

SICP

Exercise 1.3

Define a procedure that takes three numbers as arguments and returns the sum of the squares of the two larger numbers.

Answer

(defun square (x)
  (* x x))

(defun sum-of-squares (x y)
  (+ (square x)
     (square y)))

(defun biggest-two (a b c)
  (cond
   ((and (< a b)
         (< a c))
    (list b c))
   ((and (< b a)
         (< b c))
    (list a c))
   ((and (< c a)
         (< c b))
    (list a b))))

(defun foo (a b c)
  (apply 'sum-of-squares
         (biggest-two a b c)))

Emacs Lisp Packages

JSON

For parsing and encoding to JSON there is the json.el package.

Examples to and from a string

Open up a REPL in emacs and try the following

(json-encode '(("a" . 1) ("b" . "c")))
(json-read-from-string "{\"a\":1,\"b\":\"c\"}")

Examples reading from file

json-read-file to read from the given file.

String

The s.el library provides sensible string functions, see these notes for a table of commonly used functions.

Function	Use	Example
`s-format`	string interpolation
`s-concat`	string concatenation	`(s-concat "foo" "bar")`
`s-join`	string concatenation (with separator)	`(s-join "-" '("foo" "bar"))`
`s-matches?`	regex matching