Emacs Lisp: Cons Pair

By Xah Lee. Date: . Last updated: .

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 x (cons 3 4))

;; get first element
(car x) ; 3

;; get last element
(cdr x) ; 4

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)))

Proper List

A cons structure whose last tail is nil, is called Proper List, or nil-terminated list.

;; proper lists
(cons nil nil)
(cons 3 nil)
(cons 3 (cons 3 nil))
(cons 3 (cons 3 (cons 3 nil)))
(cons 3 (cons 3 (cons 3 (cons 3 nil))))

;; also proper lists. right-most tail is nil
(cons (cons 3 4) nil)
(cons (cons 3 4) (cons 3 nil))
(cons (cons (cons 3 4) 4) nil)
;; all true
(proper-list-p (cons nil nil))
(proper-list-p (cons 3 nil))
(proper-list-p (cons 3 (cons 3 nil)))
(proper-list-p (cons 3 (cons 3 (cons 3 nil))))
(proper-list-p (cons 3 (cons 3 (cons 3 (cons 3 nil)))))

;; true
(proper-list-p (cons (cons 3 4) nil))
(equal (list "a")
       (cons "a" nil))
;; t

(equal (list "a" "b")
       (cons "a" (cons "b" nil)))
;; t

(equal (list "a" "b" "c")
       (cons "a" (cons "b" (cons "c" nil))))
;; t

Improper List

A cons that's not proper list is called Improper List .

;; improper list
(cons nil 3)
(cons (cons 3 nil) 3)
;; improper list
;; all false

;; tail is not nil
(proper-list-p (cons nil 3))

;; tail is not nil
(proper-list-p (cons (cons 3 nil) 3))

Proper List vs Improper List

Most list functions, assume you have a proper list. That is, they assume you have a nil as the last tail in the cons structure. Practically all functions that return a list, return a proper list. This is critical to know.

Improper list is rarely used.

Dot Notation '(a . b)

(cons a b) has a syntax shortcut '(a . b)

(equal (cons 3 4)
       '(3 . 4)) ; t

(equal (cons 'a 'b)
       '(a . b)) ; t

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