One shot synchronizable variables.

ivar.rkt

#lang racket/base

(require racket/contract
         racket/undefined)

(provide ivar?
         exn:fail:ivar?
         make-ivar
         (contract-out
          [ivar-put!    (-> ivar? any/c any)]
          [ivar-get-evt (-> ivar? any)]
          [ivar-get     (-> ivar? any)]))

(struct exn:fail:ivar exn:fail ())

(struct ivar (box semaphore)
  #:property prop:evt (lambda (an-ivar) (ivar-get-evt an-ivar)))

(define (make-ivar)
  (ivar (box undefined) (make-semaphore)))

(define (ivar-put! an-ivar value)
  (cond
    [(box-cas! (ivar-box an-ivar) undefined value)
     (semaphore-post (ivar-semaphore an-ivar))]
    [else
     (raise (exn:fail:ivar "ivar-put!: ivar has already been assigned"
                           (current-continuation-marks)))]))

(define (ivar-get-evt an-ivar)
  (wrap-evt (semaphore-peek-evt (ivar-semaphore an-ivar))
            (lambda (_ignore)
              (unbox (ivar-box an-ivar)))))

(define (ivar-get an-ivar)
  (sync an-ivar))

Here's a slight variation:

@LiberalArtist, this is great. Much better than the newer version I have done (unpublished) with an extra semaphore to lock write access. Thanks!