A cons cell is an object that consists of two slots, called the car slot and the cdr slot. Each slot can hold or refer to any Lisp object. We also say that “the car of this cons cell is” whatever object its car slot currently holds, and likewise for the cdr.
A note to C programmers: in Lisp, we do not distinguish between “holding” a value and “pointing to” the value, because pointers in Lisp are implicit.
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. The empty list is actually the symbol
See 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.
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. Here are examples of lists:
(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:
Aand the empty list. (A nil) ; Equivalent to the previous. ((A B C)) ; A list of one element ; (which is a list of three elements).
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 holds the element, and its cdr
slot refers 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
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 was named
for its purpose, the construction of cells.