threaded box thingy

holder.rkt

#lang racket/base

(require racket/match)

(struct holder (thd req-ch))

(define (make-holder initial-value)
  (define req-ch (make-channel))
  (define thd
    (thread
     (lambda ()
       (define (run current-value)
         (match-define (list update return) (channel-get req-ch))
         (define new-value (update current-value))
         (return current-value new-value)
         (run new-value))
       (run initial-value))))
  (holder thd req-ch))

(define (holder-send h update return)
  (channel-put (holder-req-ch h) (list update return)))

(define (holder-send/sync h update)
  (define rsp-ch (make-channel))
  (define (reply old new)
    (channel-put rsp-ch (list old new)))
  (holder-send h update reply)
  (channel-get rsp-ch))

(define (holder-send/async h update)
  (holder-send h update void))