s c h e m a t i c s : c o o k b o o k

; parallel-or : thunk thunk -> value
;   evaluate thunk1 and thunk2 in parallel,
;   return the return value from the first thunk
;   to end its evaluation and stop evaluation of the
;   other thread.
(define (parallel-or thunk1 thunk2)
  (let* ([result1 #f]
         [result2 #f]
         [thread1 (thread (lambda () (set! result1 (thunk1))))]
         [thread2 (thread (lambda () (set! result2 (thunk2))))]
         [finished-thread (object-wait-multiple #f thread1 thread2)])
    (cond
      [(eq? finished-thread thread1) (kill-thread thread2)
                                     result1]
      [else                          (kill-thread thread1)
                                     result2])))

(define (infinite-loop) (infinite-loop))

(parallel-or (lambda () (infinite-loop))
             (lambda () (sleep 3) 41))

(parallel-or (lambda () (sleep 3) 42)
             (lambda () (infinite-loop)))