# When does PEngines return args/functor format and when does it return plain format?

**URL:** <https://swi-prolog.discourse.group/t/when-does-pengines-return-args-functor-format-and-when-does-it-return-plain-format/755>\
**Category:** Help!\
**Created:** [June 3, 2019, 4:05am UTC](https://swi-prolog.discourse.group/t/when-does-pengines-return-args-functor-format-and-when-does-it-return-plain-format/755 "2019-06-03T04:05:35Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![prodog](https://avatars.discourse-cdn.com/v4/letter/p/57b2e6/32.png) [@prodog](https://swi-prolog.discourse.group/u/prodog)\
**Post date:** [June 3, 2019, 4:05am UTC](https://swi-prolog.discourse.group/t/when-does-pengines-return-args-functor-format-and-when-does-it-return-plain-format/755/1 "2019-06-03T04:05:35Z")

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I’m using: SWI-Prolog version 8.0.2 on Ubuntu Linux 18.04.

I use the predicate below to create the response I send back to Node.JS queries to my PEngines server.

```prolog
create_response(EventList) :-
		% Accumulate all the temporary data generated during this volley.
		bagof(Event, u_query(temp(Event)), EventList),
		!.
% If the query did not generate any temp elements, then it is an empty report.
create_response([empty_report]) :- !.

```

It gathers up all the facts I asserted to the Prolog data that were generated during the query and returns that information as the result of the query. As you can see, if there were no facts asserted to the database, the catch-all response `[empty_report]` is returned.

What I don’t understand is the following: If the `bagof()` goal succeeds, I get back a list of objects in a list. As I mentioned in another thread, that list objects reaches my NodeJS code in some custom format I call the `args/functor` format that appears to be some kind of `meta` description of the list of objects `bagof()` returned. This is _not_ my format. It is something that PEngines appears to do on its own. Because of this I wrote a parser on the NodeJS side to turn that format back into the original object llist without the `args/functor` decorations. This works fine.

However, for some reason the simpler return that is the result of the second `create_response()` clause, that of the value `[empty_report]`, does **not** reach my NodeJS code in `args/functor` format. It shows up in the result object received by my `on-success` event handler as just a simple list with the variable name I assigned the response to -\> _Report: [empty\_report]_. This causes my parsing code to fail since it is not in `args/functor` format and that code expects it to be in that format.

Can someone tell me the rules for what causes PEngines to use the `args/functor` format and at other times, not use it?

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**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\
**Post date:** [June 3, 2019, 7:19am UTC](https://swi-prolog.discourse.group/t/when-does-pengines-return-args-functor-format-and-when-does-it-return-plain-format/755/2 "2019-06-03T07:19:45Z")

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First of all, why not simply `create_response([])`?. Second, stay away from bagof/setof unless you really understand them and need them. They are logically sweet, but often surprise people. For most cases simply use findall/3, which gives far less surprises.

Finally, just call `u_query(temp(Event))`. That gives you the events one-by-one. If there are many this gets slow, but if you give `chunk:1000` as option to _ask_, you get them in chunks of 1,000 with practically the same performance. If there are really a lot, the UI can indicate such instead of going ballistic.