Flattening a List of Lists

Question

I\'m new to Scheme and functional programming in general. Can someone explain this code — specifically what kons and knil are? The goal is to flat

Answer 1

Following code using 'named let' and 'for' loop can be used to flatten the list of elements which themselves may be lists:

(define (myflatten ll)
  (define ol '())
  (let loop ((ll ll))
    (for ((i ll))
      (if (list? i)
          (loop i)
          (set! ol (cons i ol)))))
  (reverse ol))


(myflatten '(a () (b e (c)) (((d))))) 

Output:

'(a b e c d)

However, it uses 'set!' which is generally not preferred.

The 'for' loop can also be replaced by 'named let' recursion:

(define (myflatten ll) 
  (define ol '())
  (let outer ((ll ll))
    (let inner ((il ll))
      (cond
        [(empty? il)]
        [(list? (first il))
         (outer (first il))
         (inner (rest il))]
        [else
         (set! ol (cons (first il) ol))
         (inner (rest il))])))
  (reverse ol))