iterators.scm

; create iterator from list
(define (iter lst)
  (define state lst)
  (lambda ()
    (if (null? state)
      '()
      (let ((v (car state)))
        (set! state (cdr state))
        v))))

; map over iterator
(define (map f it)
  (lambda ()
    (let ((v (it)))
      (if (null? v)
        '()
        (f v)))))

; skip the first n elements
(define (skip n it)
  (define i 0)
  (define (loop)
    (if (< i n)
      (begin
        (set! i (+ i 1))
        (it)
        (loop))))
  (lambda ()
    (if (= i 0)
      (loop))
    (it)))

; take a fixed amount of values
(define (take n it)
  (define i 0)
  (lambda ()
    (if (>= i n)
      '()
      (begin
        (set! i (+ i 1))
        (it)))))

; collect the values
(define (collect it)
  (define (loop lst)
    (let ((v (it)))
      (if (null? v)
        lst
        (loop (append lst (list v))))))
  (loop '()))

; example
; from '(1 2 3 4 5 6), skip 2, take 3, map, square, collect
; '(16 25)
(collect (map (lambda (n) (* n n)) (take 2 (skip 3 (iter '(1 2 3 4 5 6))))))