I'm using Lamina to implement Functional reactive programming (FRP).
As a starter, I try to code a very simple clock in order to understand the library basics.
According to the Lamina 0.5.0-rc4 API documentation, there is lamina.time API:
http://ideolalia.com/lamina/lamina.time.html#var-now
I want to implement a very simple clock where:
Interval of every second as Observable time Streaming Collection/List/Seq (I don't quite understand the difference yet) （EDIT: now I understood it's called Channels on Lamina)
Now as Observable Streaming data
Println Now on every second (subscribe or for-each Observable time Collection)
Any feedback is welcome. Thanks.
EDIT: I quit.
After some research, I conclude the best way to code FRP is ClojureScript with RxJs(ReactiveExtention from MS).
See the example Code for ClojureScript + RxJs + node.js in my related Qestion Here:
ClojureScript on node.js, code
You could try Bacon.js, a successor to Rx.js created after its author complained about its obscure API. Bacon.js is very clean and can integrate with jQuery, Backbone.js, AngularJS, and even Node.js. Basically you can wrap any kind of events into a Bacon's event stream and start do wonderful things with them.
Bonus : you could even try yolk, described as "A thin Clojurescript wrapper around bacon.js".
-- edit : quick typo fixes
I found a good article:
http://adambard.com/blog/why-clojure-part-2-async-magic/
-LAMINA: ADVANCED ASYNCHRONOUS PROCESSING
(ns example.helloperiodically
(:require [lamina.core :as lamina]))
(def ch (lamina/periodically 1000 (fn [] "Hello World!")))
(def loop-forever (comp doall repeatedly))
(defn consumer []
(loop-forever
(fn [] (println #(lamina/read-channel ch)))))
(defn main []
(-> (Thread. consumer) .start))
Why not try ProAct.js - it implements both the functional and object-oriented approaches. It has a package for node.js (proact.js) - it stable and well tested.
Related
What does at do actually in clojure? I saw this example in internet making a loop for overtone and cant get the meaning of at:
(defn looper [nome sound]
(let [beat (nome)]
(at (nome beat) (sound))
(apply-at (nome (inc beat)) looper nome sound [])))`
The at command is a part of overtone, not Clojure.
The overtone cheatsheet shows that it is a timing function:
https://github.com/overtone/overtone/raw/master/docs/cheatsheet/overtone-cheat-sheet.pdf
Also you can look at both github and the wiki:
https://github.com/overtone/overtone
https://github.com/overtone/overtone/wiki
http://overtone.github.io/
I have a rabbitMQ connection that seems to be started at compile time (when I type lein compile) and then blocks the building of my project. Here are more details on the problem. Let us say this is the clojure file bla_test.clj
(import (com.rabbitmq.client ConnectionFactory Connection Channel QueueingConsumer))
;; And then we have to translate the equivalent java hello world program using
;; Clojure's excellent interop.
;; It feels very strange writing this sort of ceremony-oriented imperative code
;; in Clojure:
;; Make a connection factory on the local host
(def connection-factory
(doto (ConnectionFactory.)
(.setHost "localhost")))
;; and get it to make you a connection
(def connection (.newConnection connection-factory))
;; get that to make you a channel
(def channel (. connection createChannel))
;;HERE I WOULD LIKE TO USE THE SAME CONNECTION AND THE SAME CHANNEL INSTANCE AS OFTEN AS
;; I LIKE
(dotimes [ i 10 ]
(. channel basicPublish "" "hello" nil (. (format "Hello World! (%d)" i) getBytes)))
The clojure file above is part of a bigger clojure program that I build using lein. My problem is that when I compile with "lein compile", a connection is done because of the line (def connection (.newConnection connection-factory)) and then the compilation is stopped! How can I avoid this? Is there a way to compile without building connection? How can I manage to use the same instance of channel over several calls coming from external components?
Any help would be appreciated.
Regards,
Horace
The Clojure compiler must evaluate all top-level forms, because it can be required to run arbitrary code when expanding calls to macros.
The usual solution to issues like the one you describe is to define a top-level Var holding an object of a dereferenceable type, for example an atom or a promise, and have an initialization function provide the value at runtime. (You could also use a delay and specify the value inline; this is less flexible, since it makes it more difficult to use a different value for testing etc.)
Perhaps a possible solution to use (<! c) outside go macro could be done with macro and its macro expansion time :
This is my example:
(ns fourclojure.asynco
(require [clojure.core.async :as async :refer :all]))
(defmacro runtime--fn [the-fn the-value]
`(~the-fn ~the-value)
)
(defmacro call-fn [ the-fn]
`(runtime--fn ~the-fn (<! my-chan))
)
(def my-chan (chan))
(defn read-channel [the-fn]
(go
(loop []
(call-fn the-fn)
(recur)
)
))
(defn paint []
(put! my-chan "paint!")
)
And to test it:
(read-channel print)
(repeatedly 50 paint)
I've tried this solution in a nested go and also works. But I'm not sure if it could be a correct path
The reason about this question is releated to this other question Isn't core.async contrary to Clojure principles?, #aeuhuea comment that "It seems to me that this prevents simplicity and composability. Why is it not a problem?" and #cgrand response "The limitation of the go macro (its locality) is also a feature: it enforces source code locality of stateful operations."
But force to localize your code is not the same as "complect"?
Regarding the title of your question:
>!must be called in a go block because it's designed to. If you are interested in the go-block state-machine mechanics, I can highly recommend Timothy Baldridges Youtube videos on that http://www.youtube.com/channel/UCLxWPHbkxjR-G-y6CVoEHOw
Remember that there is always blocking take and put >!! and <!!. I don't know which part of your code is supposed to provide a "solution" for not being able to use <! and >! outside of a go block, however looping around events dispatched from a single channel is common practice. Here is a modified version of read-channel
(defn do-channel [f ch]
(go-loop []
(when-let [v (<! ch)]
(f v)
(recur))))
put! puts asynchronously, an effect that you usually don't intend. In your example, to put the string "paint" into the channel 50 times, I'd recommend a one-liner like this one:
(do-channel println (to-chan (repeat 50 "print")))
Here is a comment as an answer to your edit:
Channels are not designed to be used as mutable data-structures, period. They have a buffer and that buffer can be thought of as a mutable queue. However we don't use channels to store a value in there, just to take it out a few lines later again.
We use channels as helping construct that may be used to bring execution of two or more different pieces of source-code in two or more different places in line. E.g. a go-block here does not continue to execute until it has received a value produced by another go-block. >! and >!! help us to distinguish whether they are used in a thread-blocking context or in a go-block (blocking a spawned process).
Also, please refer to this answer: Clojure - Why does execution hang when doing blocking insert into channel? (core.async)
You should not use >!! or <!! inside of a go-block, neither transparently or nested in a function call. Rich Hickey himself has commented on that in a recent bug report (http://dev.clojure.org/jira/browse/ASYNC-29?focusedCommentId=32414&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-32414).
Looking at the source-code of >! you will see that it only throws an exception. As a matter of fact, go will replace >! with different source-code. go spawns a state-machine controlled process. Depending on the context you may want to make this explicitly known or nest the go block inside of a macro or function (like in the code examples that you have provided).
Regarding David Nolens (swannodettes) helpers: They have been implemented by Rich Hickey and Nolen himself into the core.async library. Nolen said himself that they are superseded in this presentation (http://www.youtube.com/watch?v=AhxcGGeh5ho). Notice that go-loop has been implemented since after Nolens commit.
I'm confused by how calls with carmine should be done. I found the wcar macro described in carmine's docs:
(defmacro wcar [& body] `(car/with-conn pool spec-server1 ~#body))
Do I really have to call wcar every time I want to talk to redis in addition to the redis command? Or can I just call it once at the beginning? If so how?
This is what some code with tavisrudd's redis library looked like (from my toy url shortener project's testsuite):
(deftest test_shorten_doesnt_exist_create_new_next
(redis/with-server test-server
(redis/set "url_counter" 51)
(shorten test-url)
(is (= "1g" (redis/get (str "urls|" test-url))))
(is (= test-url (redis/get "shorts|1g")))))
And now I can only get it working with carmine by writing it like this:
(deftest test_shorten_doesnt_exist_create_new_next
(wcar (car/set "url_counter" 51))
(shorten test-url)
(is (= "1g" (wcar (car/get (str "urls|" test-url)))))
(is (= test-url (wcar (car/get "shorts|1g")))))
So what's the right way of using it and what underlying concept am I not getting?
Dan's explanation is correct.
Carmine uses response pipelining by default, whereas redis-clojure requires you to ask for pipelining when you want it (using the pipeline macro).
The main reason you'd want pipelining is for performance. Redis is so fast that the bottleneck in using it is often the time it takes for the request+response to travel over the network.
Clojure destructuring provides a convenient way of dealing with the pipelined response, but it does require writing your code differently to redis-clojure. The way I'd write your example is something like this (I'm assuming your shorten fn has side effects and needs to be called before the GETs):
(deftest test_shorten_doesnt_exist_create_new_next
(wcar (car/set "url_counter" 51))
(shorten test-url)
(let [[response1 response2] (wcar (car/get (str "urls|" test-url))
(car/get "shorts|1g"))]
(is (= "1g" response1))
(is (= test-url response2))))
So we're sending the first (SET) request to Redis and waiting for the reply (I'm not certain if that's actually necessary here). We then send the next two (GET) requests at once, allow Redis to queue the responses, then receive them all back at once as a vector that we'll destructure.
At first this may seem like unnecessary extra effort because it requires you to be explicit about when to receive queued responses, but it brings a lot of benefits including performance, clarity, and composable commands.
I'd check out Touchstone on GitHub if you're looking for an example of what I'd consider idiomatic Carmine use (just search for the wcar calls). (Sorry, SO is preventing me from including another link).
Otherwise just pop me an email (or file a GitHub issue) if you have any other questions.
Don't worry, you're using it the correct way already.
The Redis request functions (such as the get and set that you're using above) are all routed through another function send-request! that relies on a dynamically bound *context* to provide the connection. Attempting to call any of these Redis commands without that context will fail with a "no context" error. The with-conn macro (used in wcar) sets that context and provides the connection.
The wcar macro is then just a thin wrapper around with-conn making the assumption that you will be using the same connection details for all Redis requests.
So far this is all very similar to how Tavis Rudd's redis-clojure works.
So, the question now is why does Carmine need multiple wcar's when Redis-Clojure only required a single with-server?
And the answer is, it doesn't. Apart from sometimes, when it does. Carmine's with-conn uses Redis's "Pipelining" to send multiple requests with the same connection and then package the responses together in a vector. The example from the README shows this in action.
(wcar (car/ping)
(car/set "foo" "bar")
(car/get "foo"))
=> ["PONG" "OK" "bar"]
Here you will see that ping, set and get are only concerned with sending the request, leaving the receiving of response up to wcar. This precludes asserts (or any result access) from inside of wcar and leads to the separation of requests and multiple wcar calls that you have.
Context
I'm playing with ClojureScript, so Ajax works as follows for me:
(make-ajax-call url data handler);
where handler looks something like:
(fn [response] .... )
Now, this means when I want to say something like "fetch the new data, and update the left sidebar", my end ends up looking like:
(make-ajax-call "/fetch-new-data" {} update-sidebar!) [1]
Now, I'd prefer to write this as:
(update-sidebar! (make-ajax-call "/fetch-new-data" {})) [2]
but it won't work because make-ajax call returns immediately.
Question
Is there some way via monads, or macros, to make this work? So that [2] gets auto rewritten into [1] ? I believe:
there will be no performance penality, since it's rewritten into [1[
it's clearer for me to reason about since I can think in synchronous steps rather than async events
I suspect I'm not the first to run into this problem, so if this is a well known problem, answers of the form "Google for Problem Foo" is perfectly valid.
Thanks!
Since Jun 28, 2013, the time that is released clojure core.async lib, you can do it, more or less, in this way: https://gist.github.com/juanantonioruz/7039755
Here the code pasted:
(ns fourclojure.stack
(require [clojure.core.async :as async :refer :all]))
(defn update-sidebar! [new-data]
(println "you have updated the sidebar with this data:" new-data))
(defn async-handler [the-channel data-recieved]
(put! the-channel data-recieved)
)
(defn make-ajax-call [url data-to-send]
(let [the-channel (chan)]
(go
(<! (timeout 2000)); wait 2 seconds to response
(async-handler the-channel (str "return value with this url: " url)))
the-channel
)
)
(update-sidebar! (<!! (make-ajax-call "/fetch-new-data" {})))
More info in:
* http://clojure.com/blog/2013/06/28/clojure-core-async-channels.html
* https://github.com/clojure/core.async/blob/master/examples/walkthrough.clj
a macro would change the appearance of the code while leaving the Ajax call asynchronous.
it's a simple template macro. another approach would be to wrap the call to make-ajax-call in a function that waits for the result. while either of these could be made to work they may seem a bit awkward and "un ajax like". will the benefits be worth the extra layer of abstraction?
What about using the threading macro? Isn't good enough?
(->> update-sidebar! (make-ajax-call "/fetch-new-data" {}))
We had rough ideas about this in the async branch of seesaw. See in particular the seesaw.async namespace.