scheme tail recursion

Question

I am trying to create a scheme tail recursive function flatten-tl-rec that flattens a nested list of lists.

(define flatten-tl-rec
  (lambda (xs)
    (letrec ([flatten-tl-rec-acc
              (lambda (xs acc)
                (cond ((empty? xs) acc)
                      ((list? (first xs)) (flatten-tl-rec-acc (rest xs) (append (flatten-tl-rec-acc (first xs) '()) acc)))
                      (else (flatten-tl-rec-acc (rest xs) (cons (first xs) acc))))
                )])
      (flatten-tl-rec-acc xs '()))))

(flatten-tl-rec '(1 2 3 (4 5 6) ((7 8 9) 10 (11 (12 13))))) 

But I am getting (13 12 11 10 9 8 7 6 5 4 3 2 1) instead of (1 2 3 4 5 6 7 8 9 10 11 12 13). Whats wrong here?

Answer 1

You're accumulating the elements at the wrong end of the list. You can either append them at the correct end of the list:

(define flatten-tl-rec
  (lambda (xs)
    (letrec ([flatten-tl-rec-acc
              (lambda (xs acc)
                (cond ((empty? xs) acc)
                      ((list? (first xs))
                       (flatten-tl-rec-acc
                        (rest xs)
                        (append acc (flatten-tl-rec-acc (first xs) '()))))
                      (else (flatten-tl-rec-acc
                             (rest xs)
                             (append acc (list (first xs)))))))])
      (flatten-tl-rec-acc xs '()))))

... Or simply reverse the list at the end:

(define flatten-tl-rec
  (lambda (xs)
    (letrec ([flatten-tl-rec-acc
              (lambda (xs acc)
                (cond ((empty? xs) acc)
                      ((list? (first xs))
                       (flatten-tl-rec-acc
                        (rest xs)
                        (append (flatten-tl-rec-acc (first xs) '()) acc)))
                      (else (flatten-tl-rec-acc
                             (rest xs)
                             (cons (first xs) acc)))))])
      (reverse (flatten-tl-rec-acc xs '())))))