[英]core.async channels - tracing what happens when (example)
我在Ch。 Paul Butcher 7 周内的7 个并发模型中的 6 个,重点是core.async
。
我们有以下功能
(defn map-chan [f from]
(let [to (chan)]
(go-loop []
(when-let [x (<! from)]
(>! to (f x))
(println "parking channel write.")
(recur))
(close! to))
(println "map-chan done.")
to))
我自己添加了printlns
,以探索计算的确切顺序,我想在这里询问。
我们可以这样运行
(def ch (to-chan (range 10))) ; [1]
(def mapped (map-chan (partial * 2) ch)) ; [2]
(<!! (async/into [] mapped)) ; [3]
;; [1] Create & rtn a channel from els of seq, closing it when seq fin.
;; [2] map-chan returns immediately, with blocked go blocks inside of it.
;; [3] calling async/into finally triggers the parked channel writes, as seen below.
在回复中:
channels.core=> (def ch (to-chan (range 10)))
#'channels.core/ch
channels.core=> (def mapped (map-chan (partial * 2) ch))
map-chan done.
#'channels.core/mapped
channels.core=> (<!! (async/into [] mapped))
parking channel write.
parking channel write.
parking channel write.
parking channel write.
parking channel write.
parking channel write.
parking channel write.
parking channel write.
parking channel write.
parking channel write.
[0 2 4 6 8 10 12 14 16 18]
channels.core=>
题
我们这里有一个(同步)(即无缓冲)通道,它准备好了写入器和读取器。 为什么在调用async/into
之前不会触发上面的“停放通道写入”? (触发它的不是使用<!!
读取的通道,它是async/into
自身 - 易于检查)。 我不是在抱怨这个,只是想了解为什么痕迹是这样的。 频道实际上是否也很懒惰? 他还没有在书中提到这一点。
请注意,对这段代码的依赖是org.clojure/core.async "0.1.267.0-0d7780-alpha"
,如果这有什么不同的话。
此外,在书中,他使用了长度为 10 的缓冲通道。然而,我也尝试使用无缓冲(同步)通道,结果似乎相同。
您的输出通道to
的大小为零,因此在请求相应的镜头之前无法进行写入。 查看代码的修改版本:
(ns tst.demo.core
(:use tupelo.core tupelo.test )
(:require
[clojure.core.async :as async]
))
(defn map-chan [f from]
(let [to (async/chan)]
(async/go
(loop []
(when-let [x (async/<! from)]
(println "put - pre")
(async/>! to (f x))
(println "put - post")
(recur)))
(async/close! to))
(println "map-chan returns output buffer")
to))
(dotest
(println :1)
(spyx
(def ch (async/to-chan (range 10)))) ; [1]
(Thread/sleep 2000) (println :2)
(spyx
(def mapped (map-chan (partial * 2) ch))) ; [2]
(Thread/sleep 2000) (println :3)
(spyx
(async/<!! (async/into [] mapped))) ; [3]
)
结果:
-------------------------------
Clojure 1.10.1 Java 13
-------------------------------
lein test tst.demo.core
:1
(def ch (async/to-chan (range 10))) => #'tst.demo.core/ch
:2
map-chan returns output buffer
(def mapped (map-chan (partial * 2) ch)) => #'tst.demo.core/mapped
put - pre
:3
put - post
put - pre
put - post
put - pre
put - post
put - pre
put - post
put - pre
put - post
put - pre
put - post
put - pre
put - post
put - pre
put - post
put - pre
put - post
put - pre
put - post
(async/<!! (async/into [] mapped)) => [0 2 4 6 8 10 12 14 16 18]
因此,go 循环确实立即开始运行,但第一个 put 操作会阻塞,直到步骤 [3] 中的async/into
发生。
如果我们使用长度为 20 的缓冲输出通道,我们会看到在步骤 [3] 发生之前运行的 go 循环:
...
(let [to (async/chan 20)]
...
结果:
:1
(def ch (async/to-chan (range 10))) => #'tst.demo.core/ch
:2
map-chan returns output buffer
(def mapped (map-chan (partial * 2) ch)) => #'tst.demo.core/mapped
put - pre
put - post
put - pre
put - post
put - pre
put - post
put - pre
put - post
put - pre
put - post
put - pre
put - post
put - pre
put - post
put - pre
put - post
put - pre
put - post
put - pre
put - post
:3
(async/<!! (async/into [] mapped)) => [0 2 4 6 8 10 12 14 16 18]
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