# Pengine Event Loop fails to terminate **URL:** \ **Category:** Help!\ **Created:** [September 5, 2019, 4:29pm UTC](https://swi-prolog.discourse.group/t/pengine-event-loop-fails-to-terminate/1177 "2019-09-05T16:29:59Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![PaulBrownMagic](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/paulbrownmagic/32/211_2.png) [@PaulBrownMagic](https://swi-prolog.discourse.group/u/PaulBrownMagic)\ **Post date:** [September 5, 2019, 4:29pm UTC](https://swi-prolog.discourse.group/t/pengine-event-loop-fails-to-terminate/1177/1 "2019-09-05T16:29:59Z")
I was looking to update the Pengine’s docs, particularly the examples but I think I’ve found a bug. In the example (updated to https from the one online) : ```prolog :- use_module(library(pengines)). main :- pengine_create([ server('https://pengines.swi-prolog.org'), src_text(" q(X) :- p(X). p(a). p(b). p(c). ") ]), pengine_event_loop(handle, []). handle(create(ID, _)) :- pengine_ask(ID, q(_X), []). handle(success(_ID, [X], false)) :- writeln(X). handle(success(ID, [X], true)) :- writeln(X), pengine_next(ID, []). ``` The loop fails to terminate, after writing `q(c)` it just hangs waiting for more responses. On tracing it there seems to be an `Continue = true ->` branch that is succeeding when it should fail? ---
**Author:** ![torbjorn.lager](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/torbjorn.lager/32/7890_2.png) [@torbjorn.lager](https://swi-prolog.discourse.group/u/torbjorn.lager)\ **Post date:** [September 5, 2019, 6:51pm UTC](https://swi-prolog.discourse.group/t/pengine-event-loop-fails-to-terminate/1177/2 "2019-09-05T18:51:53Z")
Hmm, it works if I run it from SWISH. See [https://swish.swi-prolog.org/p/mHpuRmLv.pl](https://swish.swi-prolog.org/p/mHpuRmLv.pl) From where do you run it? ---
**Author:** ![torbjorn.lager](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/torbjorn.lager/32/7890_2.png) [@torbjorn.lager](https://swi-prolog.discourse.group/u/torbjorn.lager)\ **Post date:** [September 5, 2019, 8:17pm UTC](https://swi-prolog.discourse.group/t/pengine-event-loop-fails-to-terminate/1177/4 "2019-09-05T20:17:33Z")
Since I cannot stop nagging people about Web Prolog, I’d like to take this opportunity to show how the program above can be written in it: ```nohighlight main :- pengine_spawn(Pid, [ node('http://localhost:3060'), src_text(" q(X) :- p(X). p(a). p(b). p(c). ") ]), pengine_ask(Pid, q(_X)), collect_and_print(Pid). collect_and_print(Pid) :- receive({ success(Pid, [X], false) -> writeln(X); success(Pid, [X], true) -> writeln(X), pengine_next(Pid), collect_and_print(Pid) }). ``` Now, let’s compare this program with the program using `library(pengines)`: 1. Web Prolog’s Erlang-ish style is much easier to understand (and especially easy for Erlangers). 2. Using `library(pengines)`, one has to wait for a `create` message before doing anything else. In Web Prolog, `pengine_spawn/1-2` doesn’t return until the Pid has a value. Just like in Erlang. 3. In the Web Prolog program, we are checking that the `success` messages really are coming from the actor named Pid. (After all, in another context, they could have come from anywhere.) This is not done in the program using `library(pengines)`. (It can be done, but it isn’t. The handler would need one more argument.) 4. Suppose that instead of printing the value of each X, we want to collect them in a list. This would be a bit tricky using `library(pengines)`, but is easy using Web Prolog: ```nohighlight main(List) :- pengine_spawn(Pid, [ node('http://localhost:3060'), src_text(" q(X) :- p(X). p(a). p(b). p(c). ") ]), pengine_ask(Pid, q(_X)), collect_in_list(Pid, List). collect_in_list(Pid, List) :- receive({ success(Pid, [X], false) -> List = [X]; success(Pid, [X], true) -> List = [X|Rest], pengine_next(Pid), collect_in_list(Pid, Rest) }). ``` It runs like this in the PoC demonstrator: ```nohighlight Welcome to SWI Web Prolog! ?- main(List). List = [q(a),q(b),q(c)]. ?- ``` 1. There are other problems with `library(pengines)`, but nothing more (I think) that shows up in this example. Ergo, due to DNA from Erlang, Web Prolog has a much better design. I’m allowed to say this, because I designed `library(pengines)`. Well, Jan helped too, but his design ideas were good, mine were not, and I feel bad about them. Naturally, I want you all to have the best! 🙂 Of course, Web Prolog is under development, and the PoC demonstrator is not secure, so if you need something that is stable and secure, you have to stay with `library(pengines)`. ---
**Author:** ![PaulBrownMagic](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/paulbrownmagic/32/211_2.png) [@PaulBrownMagic](https://swi-prolog.discourse.group/u/PaulBrownMagic)\ **Post date:** [September 6, 2019, 11:27am UTC](https://swi-prolog.discourse.group/t/pengine-event-loop-fails-to-terminate/1177/5 "2019-09-06T11:27:33Z")
I was running it in my terminal, but it’s weird, it now runs this morning?! No idea what happened there! ---
**Author:** ![PaulBrownMagic](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/paulbrownmagic/32/211_2.png) [@PaulBrownMagic](https://swi-prolog.discourse.group/u/PaulBrownMagic)\ **Post date:** [September 6, 2019, 11:29am UTC](https://swi-prolog.discourse.group/t/pengine-event-loop-fails-to-terminate/1177/6 "2019-09-06T11:29:17Z")
The design certainly seems like an improvement. I’m sure the iterative development process will result in a truly powerful tool. ---
**Author:** ![torbjorn.lager](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/torbjorn.lager/32/7890_2.png) [@torbjorn.lager](https://swi-prolog.discourse.group/u/torbjorn.lager)\ **Post date:** [September 6, 2019, 11:32am UTC](https://swi-prolog.discourse.group/t/pengine-event-loop-fails-to-terminate/1177/7 "2019-09-06T11:32:02Z")
Yes, I really do think it’s an improvement. And I’m sure that with the help of you and others in the community, it can be improved even further! ---
**Author:** ![EricGT](https://avatars.discourse-cdn.com/v4/letter/e/f1d935/32.png) [@EricGT](https://swi-prolog.discourse.group/u/EricGT)\ **Post date:** [September 6, 2019, 1:10pm UTC](https://swi-prolog.discourse.group/t/pengine-event-loop-fails-to-terminate/1177/8 "2019-09-06T13:10:43Z")
Split this topic on September 6, 2019, 1:10pm UTC 3 posts were split to a new topic: [Web prolog error handling](https://swi-prolog.discourse.group/t/web-prolog-error-handling/1187)