Ah, thanks for pointing that out. I must've overlooked your example,
because I'd already written off futures.
It seems like what you are suggesting, catch and print, might be about as
good as I could hope for. If I don't want to block the main thread, then I
don't see what else I could possibly do but print the exception. I guess I
could store it somewhere, but in any case, I'd use this same pattern.
Thanks, Justin.
On Saturday, January 2, 2021 at 1:44:55 PM UTC-8 noise...@gmail.com wrote:
> to be clear, in my second example you see the error from the future
> without using deref
>
> good luck finding your solution
>
> On Sat, Jan 2, 2021 at 12:50 PM Austin Haas <aus...@pettomato.com> wrote:
>
>> Thank you very much for the explanation, Justin.
>>
>> I don't see how I can use futures, though, without blocking on the main
>> thread (to get the exception when it occurs). I'm spawning a long-running
>> process that never returns a value.
>> On Saturday, January 2, 2021 at 12:43:14 AM UTC-8 noise...@gmail.com
>> wrote:
>>
>>> By the time the exception is caught, you are already outside the context
>>> of the Thread which the repl client is interacting with. The default
>>> exception handler has no information tying the executing thread to the repl
>>> process (not to mention the dynamic variables clojure is using to associate
>>> output from code your client runs with that socket connection).
>>>
>>> You probably don't want to rebind the root exception handler to show
>>> *all* exceptions to your client socket. Which means that you need to set up
>>> some soft of infrastructure connecting the information about the failed
>>> function to the socket you are listening to.
>>>
>>> I find "future" very convenient for this, it uses a pool which will
>>> perform better than creating Threads ad-hoc, and will capture Exceptions
>>> and re-throw when you deref (of course, it's up to you to ensure you deref,
>>> or use try/catch and otherwise forward the failure information via the
>>> catch block). Also, it conveys dynamic bindings for things like
>>> clojure.core/*out* and clojure.core/*err* that java classes don't know
>>> about.
>>>
>>> (ins)user=> (def fut (future (throw (Exception. "oops"))))
>>> #'user/fut
>>> (ins)user=> @fut ; waits until deref to raise the error
>>> Execution error at user/fn (REPL:11).
>>> oops
>>> (ins)user=> (def fut2 (future (try (throw (Exception. "oops")) (catch
>>> Exception e (println "wat\n" e))))) ; prints instead of raising
>>> #'user/fut2
>>> user=> wat
>>> #error {
>>> :cause oops
>>> :via
>>> [{:type java.lang.Exception
>>> :message oops
>>> :at [user$fn__165 invokeStatic NO_SOURCE_FILE 13]}]
>>> :trace
>>> [[user$fn__165 invokeStatic NO_SOURCE_FILE 13]
>>> [user$fn__165 invoke NO_SOURCE_FILE 13]
>>> [clojure.core$binding_conveyor_fn$fn__5754 invoke core.clj 2030]
>>> [clojure.lang.AFn call AFn.java 18]
>>> [java.util.concurrent.FutureTask run FutureTask.java 264]
>>> [java.util.concurrent.ThreadPoolExecutor runWorker
>>> ThreadPoolExecutor.java 1128]
>>> [java.util.concurrent.ThreadPoolExecutor$Worker run
>>> ThreadPoolExecutor.java 628]
>>> [java.lang.Thread run Thread.java 834]]}
>>>
>>>
>>>
>>> On Thu, Dec 31, 2020 at 1:48 PM Austin Haas <aus...@pettomato.com>
>>> wrote:
>>>
>>>>
>>>> Problem: When I connect to a socket server and create a thread,
>>>> exceptions in the thread are printed in the server's process, not the
>>>> client's. I'd like them to appear in the client's process, where the
>>>> thread
>>>> was created.
>>>>
>>>> (I'm using the term "process" very generally here, because I don't
>>>> understand what is going on.)
>>>>
>>>> From searching, I understand that there are some other things at play,
>>>> like System/err, but I don't understand what is happening or how I can
>>>> work
>>>> around it. Why does an exception thrown in the client process show in the
>>>> client process, but an exception thrown in a thread created by the client
>>>> process shows in the server process? Why doesn't binding *err* in a thread
>>>> seem to have any effect? Any suggestions or workarounds?
>>>>
>>>> I'm not using futures, because this is a long-running process that
>>>> never returns a value.
>>>>
>>>> Example transcript:
>>>>
>>>> # Socker server
>>>>
>>>> (The only command entered is the first one, which begins with clj.
>>>> Everything after "user=>" is due to the client below.)
>>>>
>>>> $ clj -J-Dclojure.server.myrepl='{:port
>>>> 5555,:accept,clojure.core.server/repl}'
>>>> Clojure 1.10.1
>>>> user=> My second message.
>>>> Exception in thread "Thread-0" clojure.lang.ExceptionInfo: My second
>>>> exception {}
>>>> at user$eval5$fn__141.invoke(NO_SOURCE_FILE:7)
>>>> at clojure.lang.AFn.run(AFn.java:22)
>>>> at java.lang.Thread.run(Thread.java:745)
>>>> Exception in thread "Thread-1" clojure.lang.ExceptionInfo: My third
>>>> exception {}
>>>> at user$eval144$fn__145.invoke(NO_SOURCE_FILE:16)
>>>> at clojure.lang.AFn.run(AFn.java:22)
>>>> at java.lang.Thread.run(Thread.java:745)
>>>>
>>>> # Client
>>>>
>>>> $ nc localhost 5555
>>>> user=> (println "My first message.")
>>>> My first message.
>>>> nil
>>>> user=> (throw (ex-info "My first exception." {}))
>>>> Execution error (ExceptionInfo) at user/eval3 (REPL:2).
>>>> My first exception.
>>>> user=> (.start
>>>> (Thread.
>>>> (fn []
>>>> (println "My second message.")
>>>> (throw (ex-info "My second exception" {})))))
>>>> nil
>>>> user=> (.start
>>>> (Thread.
>>>> (let [out *out*
>>>> err *err*]
>>>> (fn []
>>>> (binding [*out* out
>>>> *err* err]
>>>> (println "My third message.")
>>>> (throw (ex-info "My third exception" {})))))))
>>>> nil
>>>> My third message.
>>>>
>>>> Any clues would be appreciated. Thanks!
>>>>
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