Name of this function in built-in Emacs Lisp library?

Question

The following Emacs Lisp function takes a list of lists and returns a list in which the items of the inner lists have been concatenated to one big list. It is pretty straight-forward and I am convinced something like this must already be part of the standard function library.

(defun flatten (LIST)
  (if LIST
      (append (car LIST) (flatten (cdr LIST)))
    nil))

I am looking for a function that will take a single list of lists as its argument and then append all the inner lists.

(flatten '((a b) (c d)))

will give

(a b c d)

Does anyone know whether this function is already built in, and if so, under which name?

Thanks!

Answer 1

You're either looking for append:

(defun flatten (list-of-lists)
  (apply #'append list-of-lists))

If (and only if) you know that you'll always have a list of lists.

Otherwise:

(defun flatten (list)
  (mapcan (lambda (x) (if (listp x) x nil)) list))