A cons cell is an object that consists of two pointers or slots, called the CAR slot and the CDR slot. Each slot can point to or hold to any Lisp object. We also say that the "the CAR of this cons cell is" whatever object its CAR slot currently points to, and likewise for the CDR.
A list is a series of cons cells, linked together so that the CDR slot of each cons cell holds either the next cons cell or the empty list. See section Lists, for functions that work on lists. Because most cons cells are used as part of lists, the phrase list structure has come to refer to any structure made out of cons cells.
The names CAR and CDR derive from the history of Lisp. The
original Lisp implementation ran on an IBM 704 computer which
divided words into two parts, called the "address" part and the
"decrement"; CAR was an instruction to extract the contents of
the address part of a register, and CDR an instruction to extract
the contents of the decrement. By contrast, "cons cells" are named
for the function cons that creates them, which in turn is named
for its purpose, the construction of cells.
Because cons cells are so central to Lisp, we also have a word for "an object which is not a cons cell". These objects are called atoms.
The read syntax and printed representation for lists are identical, and consist of a left parenthesis, an arbitrary number of elements, and a right parenthesis.
Upon reading, each object inside the parentheses becomes an element
of the list. That is, a cons cell is made for each element. The
CAR slot of the cons cell points to the element, and its CDR
slot points to the next cons cell of the list, which holds the next
element in the list. The CDR slot of the last cons cell is set to
point to nil.
A list can be illustrated by a diagram in which the cons cells are
shown as pairs of boxes, like dominoes. (The Lisp reader cannot read
such an illustration; unlike the textual notation, which can be
understood by both humans and computers, the box illustrations can be
understood only by humans.) This picture represents the three-element
list (rose violet buttercup):
--- --- --- --- --- ---
| | |--> | | |--> | | |--> nil
--- --- --- --- --- ---
| | |
| | |
--> rose --> violet --> buttercup
In this diagram, each box represents a slot that can point to any Lisp object. Each pair of boxes represents a cons cell. Each arrow is a pointer to a Lisp object, either an atom or another cons cell.
In this example, the first box, which holds the CAR of the first
cons cell, points to or "contains" rose (a symbol). The second
box, holding the CDR of the first cons cell, points to the next
pair of boxes, the second cons cell. The CAR of the second cons
cell is violet, and its CDR is the third cons cell. The
CDR of the third (and last) cons cell is nil.
Here is another diagram of the same list, (rose violet
buttercup), sketched in a different manner:
--------------- ---------------- ------------------- | car | cdr | | car | cdr | | car | cdr | | rose | o-------->| violet | o-------->| buttercup | nil | | | | | | | | | | --------------- ---------------- -------------------
A list with no elements in it is the empty list; it is identical
to the symbol nil. In other words, nil is both a symbol
and a list.
Here are examples of lists written in Lisp syntax:
(A 2 "A") ; A list of three elements.
() ; A list of no elements (the empty list).
nil ; A list of no elements (the empty list).
("A ()") ; A list of one element: the string "A ()".
(A ()) ; A list of two elements: A and the empty list.
(A nil) ; Equivalent to the previous.
((A B C)) ; A list of one element
; (which is a list of three elements).
Here is the list (A ()), or equivalently (A nil),
depicted with boxes and arrows:
--- --- --- ---
| | |--> | | |--> nil
--- --- --- ---
| |
| |
--> A --> nil
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