pdf version page 165: "
Promises don’t detect cyclic dependencies This means that (deliver p @p), where p is a promise, will block indefinitely. However, such blocked promises are not locked down, and the situation can be resolved: (def a (promise)) (def b (promise)) (future (deliver a @b)) ;-------1 (future (deliver b @a)) ;--------2 (realized? a) ;= false (realized? b) ;= false (deliver a 42) ;--------3 ;= #<core$promise$reify__5727@6156f1b0: 42> @a ;= 42 @b ;= 42 1----Futures are used there to not block the REPL. 2-----a and b are not delivered yet. 3-----Delivering a allows the blocked deliveries to resume—obviously (deliver a @b) is going to fail (to return nil) but (deliver b @a) proceeds happily. " why " deliver a @b is going to fail " ? 1 and 2 are just two independent threads , "deliver a 42" will unblock thread 2 , and then thread 1 will be unblocked . pdf version page 165: " An immediately practical application of promises is in easily making callback-based APIs synchronous. Say you have a function that takes another function as a callback: (defn call-service [arg1 arg2 callback-fn] ; ...perform service call, eventually invoking callback-fn with results... (future (callback-fn (+ arg1 arg2) (- arg1 arg2)))) Using this function’s results in a synchronous body of code requires providing a call- back, and then using any number of different (relatively unpleasant) techniques to wait for the callback to be invoked with the results. Alternatively, you can write a simple wrapper on top of the asynchronous, callback-based API that uses a promise’s blocking behavior on deref to enforce the synchronous semantics for you. Assuming for the moment that all of the asynchronous functions you’re interested in take the callback as their last argument, this can be implemented as a general-purpose higher-order function: (defn sync-fn [async-fn] (fn [& args] (let [result (promise)] (apply async-fn (conj (vec args) #(deliver result %&))) @result))) ((sync-fn call-service) 8 7) ;= (15 1) " what is the pointer of function sync-fn ? what is the execution sequence of functions such as sync-fn,call-servervice, and callback-fn ? statement "((sync-fn call-service) 8 7) ", where is the callback-fn parameter ? -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
