Emacs Lisp: Cons Pair
cons pair (aka cons cell or just cons) is a value type, and is a fundamental data structure in emacs lisp. List is made of cons pairs.
(cons val1 val2)- Creates a cons pair structure. This structure is used to make lists.
- Cons has 2 values, always. Otherwise it's syntax error.
- The values can be any lisp value type.
Cons Pair Can be Nested in Any Way
For example, the following are all valid:
;; cons can be nested in anyway ;; value can be any lisp value (cons 2 3) (cons (cons 2 3) 3) (cons (cons (cons 2 3) 3) 3) (cons (cons 2 3) (cons 2 3)) (cons 2 (cons 2 3)) (cons 2 (cons (cons 2 3) 3)) (cons 2 (cons 2 (cons 2 3)))
Cons can also contain nil, because any value is allowed.
(cons nil nil) (cons 3 nil) (cons nil 3)
Get First/Last of cons Pair
(car cons_pair)- Get the first element of a
(cons a b)pair. (cdr cons_pair)- Get the last element of a
(cons a b)pair.
(setq xx (cons 3 4)) ;; get first element (car xx) ; 3 ;; get last element (cdr xx) ; 4
Note: The weird names {car, cdr, cons} are like that for historical reasons.
List is Made of Cons
All lisp lists are built from cons. For example, this list:
(list "a" "b" "c")
it is identical to
(cons "a" (cons "b" (cons "c" nil)))
Dot Notation '(a . b)
(cons a b)
has a syntax shortcut
'(a . b)
;; all true (equal (cons 3 4) '(3 . 4)) (equal (cons 3 4) (quote (3 . 4)))
