In computer programming, cons ( or /ˈkɒns/) is a fundamental function in most dialects of the Lisp programming language. cons constructs (hence the name) memory objects which hold two values or pointers to values. These objects are referred to as (cons) cells, conses, 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 cdr.

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, which adds an element to the beginning of a list.)

Use

Although cons cells can be used to hold ordered pairs of simplex data, they are more commonly used to construct more complex compound data structures, notably lists and binary trees.

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:

Note the dot between 1 and 2.

Lists

In Lisp, lists are implemented on top of cons pairs. More specifically, any list structure in Lisp is either:

1. An empty list (), which is a special object usually called nil.
2. A cons cell whose car is the first element of the list and whose cdr is 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 cons, car, and 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:

1. Cons 3 onto nil, the empty list
2. Cons 2 onto the result
3. Cons 1 onto the result

which is equivalent to the single expression:

or its shorthand:

The resulting value is the list:

i.e.

which is generally abbreviated as:

Thus, 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:

Another useful list procedure is append, which concatenates two existing lists (i.e. combines two lists into a single list).