Future doesn't stop running

Fri, 03 Feb 2023 15:32:19 +0000

(defn get-result-from-first-engine
"Performs search on Powells and Cultura and returns the results
which comes first"
[query]
(let [result (promise)]
    (doseq [engine ["powells" "cultura"]]
        (future (if-let [engine-result (search query engine)]
            (deliver result engine-result)
        ))
    )
    (println "First result: " (deref result 2000 "timed out"))
))

(defn -main
    [& args]

    (time (do
        (println (search "clojure" "powells"))
        (println (search "clojure" "cultura"))
    ))

    (time (get-result-from-first-engine "clojure"))
)

This code is part of my solution to the first exercise from chapter 9 of "Clojure for the Brave and True". I want to perform the two requests in parallel and get the response of the one who process first.

Two weird things are happening:

The time from the parallel is WAY lower than the sequential one, written in main. Result example below. The time difference is so discrepant, that i guess if clojure caches it someway. I didn't use memoize or anything like that.
The main doesn't stop running. It holds like if it was waiting for an input or processing something. My theory is that the future doesn't stop running, but i don't know why

Time difference between parallel and sequential:

https://www.powells.com/book/clojure-cookbook-9781449366179
https://www3.livrariacultura.com.br/clojure-94991222/p
"Elapsed time: 2185.2716 msecs"
First result:  https://www3.livrariacultura.com.br/clojure-94991222/p
"Elapsed time: 63.5558 msecs"

Thanks for the help in advance

Why is reified clojure.lang.IDeref wrapping a promise blocking?

Thu, 21 Jul 2022 20:01:55 +0000

Using clojure 1.10.3:

user> (def p (promise))
#'user/p
user> (def wrapper
        (reify
          clojure.lang.IDeref
          (deref [_] @p)))
#'user/wrapper
user> wrapper ;; => repl hangs here

In above session the repl blocks on the last statement. I do not see why that is. I would expect @wrapper to block, evaluating wrapper however I expected it to return an Object of some kind. Am I missing something here?

Clojure Q&A - Recent questions tagged promise

Future doesn't stop running

Why is reified clojure.lang.IDeref wrapping a promise blocking?