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02-async-await.md

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Future-based asynchronous processes, aka async/await

In this guide, we'll implement async/await syntax on top of java.util.concurrent.CompletableFuture. Unlike clojure's future, java's CompletableFuture allows nonblocking composition at the price of additional syntax complexity.

This syntax burden is familiar to javascript developers and leads to the famous callback-hell when improperly controlled. The async/await syntax, a popular answer to this problem, allows synchronous-looking code to be converted into an asynchronous process.

As we'll see, coroutines can easily emulate async/await, and this technique can be transposed to any callback-based asynchronous primitive, including javascript promises in clojurescript where blocking is not an option.

(require '[cloroutine.core :refer [cr]])
(import '(java.util.concurrent CompletableFuture))

First, let's define our break points and their associated thread-local context. The asynchronous processes we'll build with async blocks are lightweight threads scheduled in user space, we'll refer to them as *fiber*s and model them with java.util.function.BiConsumer. The await function will suspend the fiber and register it on given CompletableFuture. When the future resolves, a *value* or an *error* becomes available to be consumed by thunk, the resume function associated with await.

(def ^:dynamic *fiber*)
(def ^:dynamic *value*)
(def ^:dynamic *error*)
(defn await [cf] (.whenComplete ^CompletableFuture cf *fiber*))
(defn thunk [] (if-some [e *error*] (throw e) *value*))

We can now define async blocks as a macro wrapping its body in a coroutine, passing its result or error to a fresh future. The fresh coroutine is then wrapped in a fiber and immediately started, and the future is returned.

(defmacro async [& body]
  `(let [cf# (CompletableFuture.)
         cr# (cr {await thunk}
               (try (.complete cf# (do ~@body))
                    (catch Throwable e#
                      (.completeExceptionally cf# e#))))]
     (binding [*fiber* (reify java.util.function.BiConsumer
                         (accept [f# v# e#]
                           (binding [*fiber* f#
                                     *value* v#
                                     *error* e#]
                             (cr#))))]
       (cr#)) cf#))

We can now define async blocks and use await inside to park on a future result.

(def six (async 6))                                                      ;; a future of 6
(def seven (async (inc (await six))))                                    ;; a future of 7
(def failed (async (throw (ex-info "this is fine." {}))))                ;; a failed future
(def recovered (async (try (await failed) (catch Exception e :failed)))) ;; a future of :failed