(defstruct thread nil
  suspended
  cont
  (:method resume (self)
    [self.cont])
  (:method give (self item)
    [self.cont item])
  (:method get (self)
    (yield-from run nil))
  (:method start (self)
    (set self.cont (obtain self.(run)))
    (unless self.suspended
      self.(resume)))
  (:postinit (self)
    self.(start)))

(defstruct consumer thread
  (count 0)
  (:method run (self)
    (whilet ((item self.(get)))
      (prinl item)
      (inc self.count))))

(defstruct producer thread
  consumer
  (:method run (self)
    (whilet ((line (get-line)))
      self.consumer.(give line))))

(let* ((con (new consumer))
       (pro (new producer suspended t consumer con)))
  pro.(resume)
  (put-line `count = @{con.count}`))