Flatten a list using common lisp

Question

I was reading the book On Lisp by Paul Graham. In Chapter 4, Utility Functions, he gives examples of small functions that operate on lists, which would be helpful while writing

Answer 1

You're actually very close. As Sylwester mentions, the issue is that (push el acc) only modifies the local binding of el (of which there's a new one for each call to rflatten. As Rainer mentions, it's not really an accumulator in the traditional sense, so I'm going not going to call it acc, but result. Since you're already defining a local function, there's no reason not to define result in a wider scope:

(defun flatten (lst)
  (let ((result '()))
    (labels ((rflatten (lst1)
               (dolist (el lst1)
                 (if (listp el)
                   (rflatten el)
                   (push el result)))))
      (rflatten lst)
      (nreverse result))))

There are actually a few ways to clean this up, too. The first is a matter of style and taste, but I'd use an &aux variable to bind result, so

(defun flatten (lst &aux (result '()))
  ...)

The next is that dolist can take a third argument, a form to evaluate as for the return value. This is often used in a "push to create a list, then reverse for the return value" idiom, e.g.,

(let ((result '()))
  (dolist (x list (nreverse result))
    ...
    (push ... result)))

You don't want to reverse after every dolist, but you can still return result from the dolist, and thus from rflatten. Then you can simply call nreverse with the result of rflatten:

(defun flatten (lst &aux (result '()))
  (labels ((rflatten (lst1)
             (dolist (el lst1 result)
               (if (listp el)
                 (rflatten el)
                 (push el result)))))
      (nreverse (rflatten lst))))