Lists in

About Lists

Lists are a very common data type in Common Lisp. They are made up of a sequence of cons cells. Each car is an element of the list and every cdr is a either the next cons cell or a terminating atom.

A list which terminates with the empty list is called a "proper list".

A list which terminates with an atom that is not the empty list is called a "dotted list" (based upon how it is printed: (cons 'a 'b) ;=> (a . b)).

A list can also be circular if some cons cell in the list cdr of a later cons cell. For example: (let ((x (list 1))) (setf (cdr x) x) (write x :stream t :circle t) :done). Take care when working with circular lists as allowing them to be printed out without binding *print-circle* to t will cause an infinite loop and will lock up the REPL (in this example by specifying :circle t to write, *print-circle will be bound to t).

Combining lists

You can call append with any number of lists, including 1 or 0:

(append '(a b c) '(1 2 3)) ; => (A B C 1 2 3)
(append '(a b c) '()))     ; => (A B C)
(append '(a b c))          ; => (A B C)
(append)                   ; => ()

As an alternative to append there is nconc which has the same result but has the side-effect of modifying the lists. You should be careful when using nconc as it may have surprising effects.