vlist))

(define (vlist-length vlist)
  "Return the length of VLIST."
  (assert-vlist vlist)
  (let loop ((base (vlist-base vlist))
             (len  (vlist-offset vlist)))
    (if (eq? base block-null)
        len
        (loop (block-base base)
              (+ len 1 (block-offset base))))))

(define* (vlist-unfold p f g seed
                       #:optional (tail-gen (lambda (x) vlist-null)))
  "Return a new vlist.  See the description of SRFI-1 `unfold' for details."
  (let uf ((seed seed))
    (if (p seed)
        (tail-gen seed)
        (vlist-cons (f seed)
                    (uf (g seed))))))

(define* (vlist-unfold-right p f g seed #:optional (tail vlist-null))
  "Return a new vlist.  See the description of SRFI-1 `unfold-right' for
details."
  (let uf ((seed seed) (lis tail))
    (if (p seed)
        lis
        (uf (g seed) (vlist-cons (f seed) lis)))))

(define (vlist-append . vlists)
  "Append the given lists."
  (if (null? vlists)
      vlist-null
      (fold-right (lambda (vlist result)
                    (vlist-fold-right (lambda (e v)
                                        (vlist-cons e v))
                                      result
                                      vlist))
                  vlist-null
                  vlists)))

(define (vlist-for-each proc vlist)
  "Call PROC on each element of VLIST.  The result is unspecified."
  (vlist-fold (lambda (item x)
                (proc item))
              (if #f #f)
              vlist))