In computer programming,
cons (pron.: // or //) is a fundamental function in most dialects of the Lisp programming language.
cons constructs memory objects which hold two values or pointers to values. These objects are referred to as (cons) cells, conses, non-atomic s-expressions ("NATSes"), or (cons) pairs. In Lisp jargon, the expression "to cons x onto y" means to construct a new object with
(cons x y). The resulting pair has a left half, referred to as the
car (the first element), and a right half (the second element), referred to as the
It is loosely related to the object-oriented notion of a constructor, which creates a new object given arguments, and more closely related to the constructor function of an algebraic data type system.
The word "cons" and expressions like "to cons onto" are also part of a more general functional programming jargon. Sometimes operators that have a similar purpose, especially in the context of list processing, are pronounced "cons". (A good example is the :: operator in ML and Scala, which adds an element to the beginning of a list.)
For example, the Lisp expression
(cons 1 2) constructs a cell holding 1 in its left half (the so-called
car field) and 2 in its right half (the
cdr field). In Lisp notation, the value
(cons 1 2) looks like:
(1 . 2)
Note the dot between 1 and 2; this indicates that the S-expression is a "dotted pair," rather than a "list."
In Lisp, lists are implemented on top of cons pairs. More specifically, any list structure in Lisp is either:
- An empty list
(), which is a special object usually called
- A cons cell whose
caris the first element of the list and whose
cdris a list containing the rest of the elements.
This forms the basis of a simple, singly linked list structure whose contents can be manipulated with
cdr. Note that
nil is the only list that is not also a cons pair. As an example, consider a list whose elements are 1, 2, and 3. Such a list can be created in three steps:
- Cons 3 onto
nil, the empty list
- Cons 2 onto the result
- Cons 1 onto the result
which is equivalent to the single expression:
(cons 1 (cons 2 (cons 3 nil)))
or its shorthand:
(list 1 2 3)
The resulting value is the list:
(1 . (2 . (3 . nil)))
*--*--*--nil | | | 1 2 3
which is generally abbreviated as:
(1 2 3)
cons can be used to add one element to the front of an existing linked list. For example, if x is the list we defined above, then
(cons 5 x) will produce the list:
(5 1 2 3)
(cons (cons 1 2) (cons 3 4))
results in the tree:
((1 . 2) . (3 . 4))
* / \ * * / \ / \ 1 2 3 4
Technically, the list (1 2 3) in the previous example is also a binary tree, one which happens to be particularly unbalanced. To see this, simply rearrange the diagram:
*--*--*--nil | | | 1 2 3
to the following equivalent:
* / \ 1 * / \ 2 * / \ 3 nil
Use in conversation 
Cons can refer to the general process of memory allocation, as opposed to using destructive operations of the kind that would be used in an imperative programming language. For example:
I sped up the code a bit by putting in side effects instead of having it cons like crazy.
Not technically fundamental 
Since Lisp has first-class functions, all data structures, including cons cells, are not fundamentally necessary to the language, since all data structures can be implemented using functions. For example, in Scheme:
(define (cons x y) (lambda (m) (m x y))) (define (car z) (z (lambda (p q) p))) (define (cdr z) (z (lambda (p q) q)))
The above code re-implements the cons, car, and cdr operations, using a function as the "cons cell". This is the usual way of defining data structures in pure lambda calculus, an abstract, theoretical model of computation that is closely related to Scheme.
This implementation, while academically interesting, is impractical because it renders cons cells indistinguishable from any other Scheme procedure, as well as introducing unnecessary computational inefficiencies.
However, the same kind of encoding can be used for more complex algebraic data types with variants, where it may even turn out to be more efficient than other kinds of encoding. This encoding also has the advantage of being implementable in a statically typed language that doesn't have variants, such as Java, using interfaces instead of lambdas.
See also 
- Lisp (programming language)
- CAR and CDR
- Constructor (computer science)
- Algebraic data type
- Hash consing