append

append takes two or more lists and constructs a new list with all of their elements. For example,

(append '(1 2) '(3 4))

returns a list (1 2 3 4).

Notice that append works differently from the way list does:

(list '(1 2) '(3 4))

returns ((1 2) (3 4)), a two element list of the lists it was given. list makes its arguments elements of the new list, independent of whether the arguments are lists or something else.

append requires that its arguments are lists, and makes a list whose elements are the elements of those lists--in this case, a four-element list. Intuitively, it concatenates the lists it is given. It only concatenates the top-level structure, however--it doesn't "flatten" nested structures. For example

(append '((1 2) (3 4))
        '((5 6) (7 8)))

returns ((1 2) (3 4) (5 6) (7 8))

append doesn't modify any of its arguments, but the result of append generally shares structure with the last list it's given. (It effectively conses the elements of the other lists onto the last list to create the result list.) It's therefore dangerous to make a "new" list with append and then modify the "old" list. This type of structure sharing is one of the reasons side effects are discouraged in Scheme.