#255 The Datalog engine hang on recursive query

[chi]

Hi,

I find the following program, which has a recursive query, makes the Datalog engine hang.

:- table sxud/1.

kuts(-28).

sxud(A) :- kuts(A).
huvd(B) :- kuts(B).
kuts(B) :- huvd(B).

I run it with the command:

xsb --noprompt --quietload --nobanner --nofeedback -e "example.P, sxud(A), writeln([A]), fail. halt."

But if I query the result of kuts, I can get the result, as follow:

:- table kuts/1.

kuts(-28).

sxud(A) :- kuts(A).
huvd(B) :- kuts(B).
kuts(B) :- huvd(B).

I run it with command:

xsb --noprompt --quietload --nobanner --nofeedback -e "example.P, kuts(A), writeln([A]), fail. halt."

the result is -28.

Did I do something wrong?

[Michael Kifer]

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you are tabling sxud/1 but the recursive predicate is kuts/1 or huvd/1, not sxud/1.

On 11/28/22 8:00 AM, chi wrote:

---

** [bugs:#255] The Datalog engine hang on recursive query**

**Status:** open
**Group:** wrong answer
**Created:** Mon Nov 28, 2022 01:00 PM UTC by chi
**Last Updated:** Mon Nov 28, 2022 01:00 PM UTC
**Owner:** nobody


Hi,

I find the following program, which has a recursive query, makes the Datalog engine hang. 

:- table sxud/1.

kuts(-28).

sxud(A) :- kuts(A).
huvd(B) :- kuts(B).
kuts(B) :- huvd(B).




I run it with the command: 

xsb --noprompt --quietload --nobanner --nofeedback -e "example.P, sxud(A), writeln([A]), fail. halt."





But if I query the result of kuts, I can get the result, as follow:

:- table kuts/1.

kuts(-28).

sxud(A) :- kuts(A).
huvd(B) :- kuts(B).
kuts(B) :- huvd(B).



I run it with command:

xsb --noprompt --quietload --nobanner --nofeedback -e "example.P, kuts(A), writeln([A]), fail. halt."



the result is -28.

Did I do something wrong?



---

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[chi]

Hi,

Thank you very much for your quick answer. I still have a very small question, even if I am tabling sxud/1, shouldn't the fix point be reached when the facts of the relation in the recursive query are unchanged?

[Michael Kifer]

Without tabling recursive preds it won't.

Nov 28, 2022 20:34:12 chi cztest@users.sourceforge.net:

Hi,

Thank you very much for your quick answer. I still have a very small question, even if I am tabling sxud/1, shouldn't the fix point be reached when the facts of the relation in the recursive query are unchanged?

---

** [bugs:#255] The Datalog engine hang on recursive query**

Status: open
Group: wrong answer
Created: Mon Nov 28, 2022 01:00 PM UTC by chi
Last Updated: Mon Nov 28, 2022 01:00 PM UTC
Owner: nobody

Hi,

I find the following program, which has a recursive query, makes the Datalog engine hang.
~~~
:- table sxud/1.

kuts(-28).

sxud(A) :- kuts(A).
huvd(B) :- kuts(B).
kuts(B) :- huvd(B).
~~~

I run it with the command:
~~~
xsb --noprompt --quietload --nobanner --nofeedback -e "example.P, sxud(A), writeln([A]), fail. halt."
~~~

But if I query the result of kuts, I can get the result, as follow:
~~~
:- table kuts/1.

kuts(-28).

sxud(A) :- kuts(A).
huvd(B) :- kuts(B).
kuts(B) :- huvd(B).
~~~
I run it with command:
~~~
xsb --noprompt --quietload --nobanner --nofeedback -e "example.P, kuts(A), writeln([A]), fail. halt."
~~~
the result is -28.

Did I do something wrong?

---

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[chi]

Thank you very much for your patient answer! I got it! We can close this ticket now.