11.1 Lists and Cons Cells

Lists in Lisp are not a primitive data type; they are built up from cons cells. A cons cell is a data object that represents an ordered pair. It records two Lisp objects, one labeled as the CAR, and the other labeled as the CDR. These names are traditional; see Cons Cell Type. CDR is pronounced “could-er.”

A list is a series of cons cells chained together, one cons cell per element of the list. By convention, the CARs of the cons cells are the elements of the list, and the CDRs are used to chain the list: the CDR of each cons cell is the following cons cell. The CDR of the last cons cell is nil. This asymmetry between the CAR and the CDR is entirely a matter of convention; at the level of cons cells, the CAR and CDR slots have the same characteristics.

Because most cons cells are used as part of lists, the phrase list structure has come to mean any structure made out of cons cells.

The symbol nil is considered a list as well as a symbol; it is the list with no elements. For convenience, the symbol nil is considered to have nil as its CDR (and also as its CAR).

The CDR of any nonempty list l is a list containing all the elements of l except the first.