clojure.core/promise

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Type: function Added: Clojure 1.1 Examples: 3 Runnable: 0

([])

Returns a promise object that can be read with deref/@, and set, once only, with deliver. Calls to deref/@ prior to delivery will block, unless the variant of deref with timeout is used. All subsequent derefs will return the same delivered value without blocking. See also - realized?.

Examples

by gstamp on 2010-08-02 . May have evaluation errors. ▶

(def x (promise)) #'user/x ;; Trying to deref at this point will make your repl wait forever (deliver x 100) #<core$promise$reify__5534@4369a50b: 100> ;; the promise has been delivered, deref x will return immediately @x 100

by neotyk on 2011-12-27 . May have evaluation errors. ▶

;; Create a promise (def p (promise)) #'user/p ; p is our promise ;; Check if was delivered/realized (realized? p) false ; No yet ;; Delivering the promise (deliver p 42) #<core$promise$reify__5727@47122d: 42> ;; Check again if it was delivered (realized? p) true ; Yes! ;; Deref to see what has been delivered @p 42 ;; Note that @ is shorthand for deref (deref p) 42

by JoshAaronJones on 2016-09-20 . May have evaluation errors. ▶

;; Illustrates how threads can work together via promises ;; First, an example to show a future that delivers (def p (promise)) #'user/p ;; future that will deliver the promise from another thread after 10 sec delay (future (Thread/sleep 10000) (deliver p 123)) #future[{:status :pending, :val nil} 0x9a51df1] ;; within 10 seconds dereference p, and wait for delivery of the value @p 123 ;; Now, an example to show a future that blocks while waiting for a promise ;; to be delivered -- this is used to achieve callback-style functionality ;; redefine p (def p (promise)) #'user/p ;; create a new callback thread that will wait for a promise to be delivered (future (println "About to block while waiting for 'p'") (println "Now I can do some work with the value " @p)) About to block while waiting for 'p' #future[{:status :pending, :val nil} 0x1737df29] ;; deliver the promise, triggering the blocking callback thread (deliver p 123) Now I can do some work with the value 123 #promise[{:status :ready, :val 123} 0x674a4c4a]

See also

• clojure.core/realized? (1)
• clojure.core/future (1)
• clojure.core/deliver (1)

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