#lang scheme
(require scheme/generator)

(define (sequence-ref s i)
  (define-values (more? get) (sequence-generate s))
  (let loop ([n i])
    (if (more?)
        (if (zero? n)
            (get)
            (begin
              (get)
              (loop (sub1 n))))
        (error 'sequence-ref "Sequence does not contain ~e elements" i))))

(define (sequence-count s)
  (define-values (more? get) (sequence-generate s))
  (let loop ([i 0])
    (if (more?)
        (begin (get) (loop (add1 i)))
        i)))

(define (sequence->list s)
  (for/list ([e s]) e))

(define (sequence-map f s)
  (make-do-sequence
   (lambda ()
     (define-values (more? get) (sequence-generate s))
     (values
      (lambda (pos) (f (get)))
      (lambda (pos) pos)
      0
      (lambda (pos) (more?))
      (lambda (val) #t)
      (lambda (pos val) #t)))))

; XXX
(define (sequenceof c)
  sequence?)

(provide (all-defined-out))