List Processing

Lists (singly linked lists) are the main data structure used to represent code in Lisp. There are many ways to create lists:

(cons 1 nil)  # concatenate to the empty list
(list 1)  # use the list constructor
'(1)  # quote code

Lists are immutable. In order to create modify a list, you have to create a new one. Here are the main operations used to manipulate lists:

(car '(1 2 3)) # 1
(cdr '(1 2 3))) # (2 3)
(map (fn (x) (+ x 1))  # (2 3 4)
    '(1 2 3))
(filter (fn (x) false)  # ()
    '(1 2 3))
(reduce + '(1 2 3))  # 6
(reverse '(1 2 3))  # (3 2 1)
(is-list '(1 2 3))  # true

Note: list processing is often used in metaprogramming