Question

My implementation of flatten looks like this:

(define flatten
  (lambda (lst)
    (if (null? lst)
        lst
        (append
          (rtn-lst (car lst))
          (flatten (cdr lst))))))

(define rtn-lst
  (lambda (lst)
    (cond 
      ((null? lst) 
        empty)
      ((atom? lst)
        (list lst))
      (else 
        (flatten lst)))))

While standard implementation is:

(define (flatten lst)
  (cond 
    ((null? list)
      empty)
    ((list? (car lst))
      (append (flatten (car lst)) (flatten (cdr lst))))
    (else
      (cons (car lst) (flatten (cdr lst))))))

Apart from the obvious verboseness, what else is wrong with my code?

Answer 1

I’d try this:

(define rtn-lst
  (lambda (lst)
    (cond 
      ((list? lst)
        (if (null? lst)
            empty
            (flatten-list lst)))
      ((atom? lst)
        (list lst))
      (else 
        (flatten-list lst)))))

Probably we have different implementations of Scheme.

EDIT:

With modified else branch:

(define rtn-lst
  (lambda (lst)
    (cond 
      ((list? lst)
        (if (null? lst)
            empty
            (flatten-list lst)))
      (else 
        (list lst)))))