2025-02-24 missionary | Clojure Slack Archive

missionary

Sam Williams 2025-02-24T21:59:23.316529Z

I'm trying to create a flow that dynamically pulls from other flows based on an input flows latest value. I'd like to cancel its subscription to any flow that it was listening to for a previous value of the input stream. The below is clearly wrong, but I hope conveys the basic meaning I'm after ... is there a clean way of doing what I'm attempting to do, or am I barking up the wrong tree?

(defn switch-between-flows
  [inputs]
  (ap 
    (let [n (?> inputs)]
      (amb (case n
             1 (?> flow-1)
             2 (?> flow-2))))))

leonoel 2025-02-25T06:55:15.114319Z

Use ?< in ap (if discrete) or cp (if continuous). Note - with ap you'll need to guard the switch with try/catch, this is a legacy quirk that will eventually be fixed

❤️ 1

Sam Williams 2025-02-25T10:44:23.549669Z

oh wow, that is perfect and does exactly what I wanted.

Sam Williams 2025-02-25T10:47:09.370469Z

I was expecting to need to clean up the old flow (cancel it) whenever the input value changed... this is so much simpler.

Sam Williams 2025-02-25T10:48:13.898239Z

I think I need to go read more about the difference between ?> and ?<

Sam Williams 2025-02-25T10:49:14.220419Z

For anyone coming along later, here is the working code

(defn switch-between-flows
  [inputs]
  (cp 
    (let [n (?< inputs)]
      (case n
        1 (?< flow-1)
        2 (?< flow-2)))))

🙏 1

2025-02-24T02:33:46.288419Z

Probably a dumb question; trying to figure out how m/ap works with recursion. Is loop handled specially, or am I missing something? I've got an example I've been playing with that works when using loop but doesn't when using a named function. I've unwrapped the m/amb I was using into the m/?> (m/seed (range 2)) and case to try to figure out the source of my No matching clause: missionary.impl.Ambiguous in the second example.

(m/?
 (m/reduce
  #(println %2) nil
  (m/ap
    (loop [x 0]
      (if (> x 10)
        (m/?> m/none)
        (let [y (m/?> (m/seed (range 2)))]
          (println y)
          (case y
            0 x
            1 (recur (inc x)))))))))

(m/?
 (m/reduce
  #(println %2) nil
  (m/ap
    ((fn example [x]
       (if (> x 10)
         (m/?> m/none)
         (let [y (m/?> (m/seed (range 2)))]
           (println y)
           (case y
             0 x
             1 (example (inc x))))))
     0))))

;; y = #object[missionary.impl.Ambiguous$Branch 0x797ee69c missionary.impl.Ambiguous$Branch@797ee69c]
;; Execution error (IllegalArgumentException) at scratch/eval15485$cr15488-block-0$example (REPL:60).
;; No matching clause: missionary.impl.Ambiguous$Branch@797ee69c

2025-02-24T12:14:41.603209Z

That makes sense; thanks for explaining! 🙂

2025-02-24T02:36:05.794979Z

Curious if it'd make sense to make m/amb expand to have a default value that throws a more meaningful error in situations like this?

leonoel 2025-02-24T07:19:08.563769Z

The second example is illegal because m/?> is an indirect call. m/ap doesn't rewrite code inside lambdas. I agree about bad DX, it should be an error in this case https://github.com/leonoel/missionary/issues/127

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