Emacs Lisp

Emacs Lisp is a dialect of Lisp used in Emacs.

• Everything in Lisp are represented as lists (made of “atoms” and nested lists).
• Function call: (print "Hello World!")
• To evaluate something means to “process it and find out its value.”
• A quoted list looks like this: '(a b c d () (lavender aster tulip) e)
  • Lisp won’t evaluate anything within a quoted list (' or (quote (...))). The quoted list (and nested ones) will not be treated as a function call. Symbols that are not the first element will not be evaluated as variables either.
  • a, b, c, d, and e are “symbols”
  • Empty lists (()) (and only empty ones) are atoms as well.
  • Printed expressions of both lists and atoms are symbolic expressions or sexps. Expressions are sometimes also called “forms.”
• Types of valid atoms:
  • numbers (85, 642, 79)
  • symbols (+, next-line, previous-line)
  • strings (“hey”, "", “This is a short sentence.“)
• All elements are whitespace-delimitted and Lisp doesn’t care about how much whitespaces is between elements of the list.
• Use C-x C-e to evaluate a Lisp expression at point (on or after the closing parenthesis)
• Multiple symbols may refer to the same (function) definition. Likewise, a symbol may have one or two (function or variable) definitions.
• Emacs Lisp does not support namespaces, so prepend the variable to identify where the variable comes from (e.g. org-agenda-type)
• Expressions (forms) that evaluated in an unusual manner are called “special forms,” like function definitions.
• A macro translates itself into another expression to be evaluated in its place.
• The innermost lists get evaluated first, otherwise everything is evaluated left to right. For example, (+ 2 (+ 3 3)) evaluates to 8.
• Lisp code (.el files) can be byte-compiled (into .elc files) to improve performance.
• When an expression is evaluated, the Lisp interpreter returns a value and carry out its side effects (like printing text, or moving files).
• A symbol can be bound to any value: to a number, to a string, to a list, or to a function definition.
• Error void-function xxx: symbol xxx does not have a function definition
• Error void-variable xxx: symbol xxx does not have a variable definition
• (concat "abc" "def") = "abcdef"
• (substring "aaabbb" 1 5) = "aabb"
• (number-to-string 42) = "42" (same thing for (string-to-number))
• Arithmetic: (+ ...), (- ...), (* ...), (/ ...)
• Error (wrong-type-argument number-or-marker-p hello) = “Argument with incorrect type: expected a number or marker, found symbol ‘hello‘“
• Functions with -p or p suffix are predicate functions (e.g. they test if the argument satisfies a condition, like belonging to a type).
• (message FORMAT-STRING &rest ARGS) prints stuff to the echo area (area below the modeline used for output)
• (set 'var-name value), or more conveniently, (setq var-name value)
  • (setq var-a value-a var-b value-b ...); the number of arguments must be even
• Increment variable: (setq num (+ num 1)) or (1+ num) or (incf num)
• Increment variable: (setq num (- num 1)) or (1- num) or (decf num)
• incf and decf also accepts a number argument. (incf num 2) increases num by 2.
• (buffer-name), (buffer-file-name), (current-buffer), (other-buffer), (switch-to-buffer), (set-buffer), (point)

(defun func-name (reqd-args... &optional opt-args... &rest rest-args)
  "optional documentation..."
  (interactive ...) ; optional
  func-body...)

• To return a value, simply place the expression at the end of the function

let expression: local variables

(let ((var1 value1)
      (var2 value2)
      var3 ; default value nil
      ...)
  body...)

if statement

(if condition
    if-clause
  else-clause-body...)