Comment by eduction | Hacker News Reader

Comment by eduction | Hacker News Reader

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Yes, core.async is CSP style async (not to be confused with CPS programming style, which this article is about) and there is a learning curve. Particularly as the go macro cannot see across function boundaries. (Some of Rich Hickey's videos on it give an overview similar to what you wrote above.)

My confusion was on the OP's statement about "sigils": "if you use the wrong sigil at the wrong place, you are dead in the water."

So don't use the wrong sigil? There are all of two of them; I think OP means the parking take and blocking take macros. One is used inside go blocks and one outside. That was the easy part. The hard part was wrapping my head around how to efficiently program within the constraints imposed by core.async. But the machinery of how to do things (macros, functions) was very simple and easy to learn. You basically just need to learn "go", "<!" and "<!!". Eventually you may need ">!", "alts!", and "chan".

    (defn test-dbg7 [] ;; test buffers
        (record "test-dbg.svg"
                (let [c ^{:name "chan"} (async-dbg/chan 1)]
                  ^{:name "thread"}
                  (async-dbg/thread
                    (dotimes [n 3]
                      ^{:name "put it!"} (async-dbg/>!! c n))
                    ;; THE BUG IS HERE. FORGOT TO CLOSE GODAMNIT
                    #_(async-dbg/close! c))
                  (loop [x (async-dbg/<!! c)]
                    (when x
                      (println "-->" x)
                      (recur ^{:name "take it!"} (async-dbg/<!! c)))))))

The code above produces the following before hanging:

    --> 0
    --> 1
    --> 2

https://pasteboard.co/L4WjXavcFKaM.png

In this test case, everything sits nicely within the same let statement, but these puts and reads to the same channel could be in different source files, making the bug hard to track.

Once the bug is corrected the sequence diagram should look like this:

https://pasteboard.co/CCyGZKUUkVFL.png

Ya I also needed some time to wrap my head around async programming but OP was talking about "use[ing] the wrong sigil at the wrong place" - that's not what your stumbling block is here, you forgot to close the channel and you have a loop statement that by design is going to read eternally from the channel so as long as the channel is open you're going to "hang". Doesn't have anything to do with mixing up "sigils", it's just that async programming has unique challenges.

I am with you on this - it's not impossible, but is it maintainable? what if I break a leg? and when something blocks and you don't know why, who can debug it? that's why we went for a different approach.

The problem with core.async is that it is an excellent PoC, but does not actually solve the underlying problem, that is "Hey! I want a new thread here! And I want it cheap.". Project Loom solves it. Of course, the problem is not something that could be solved within the land of bytecode.

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