# Querying Prolog from Foreign Predicates

**URL:** <https://swi-prolog.discourse.group/t/querying-prolog-from-foreign-predicates/570>\
**Category:** Help!\
**Created:** [April 22, 2019, 8:04pm UTC](https://swi-prolog.discourse.group/t/querying-prolog-from-foreign-predicates/570 "2019-04-22T20:04:28Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![AdamSobieski](https://avatars.discourse-cdn.com/v4/letter/a/da6949/32.png) [@AdamSobieski](https://swi-prolog.discourse.group/u/AdamSobieski)\
**Post date:** [April 22, 2019, 8:04pm UTC](https://swi-prolog.discourse.group/t/querying-prolog-from-foreign-predicates/570/1 "2019-04-22T20:04:28Z")

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I would like to ask about querying SWI-Prolog from foreign predicates and non-deterministic foreign predicates. I was looking at unifying parameter variables with values returned from a _nested query_ iterated across invocations of a non-deterministic foreign predicate. I was looking at calling `PL_next_solution` for a _nested query_ opened in a foreign non-deterministic predicate each time that the non-deterministic foreign predicate was invoked. I’m wondering whether such _nested queries_ are currently possible with SWI-Prolog?

I reviewed the documentation. I wonder why it is the case that “a foreign context can have at most one active query. This implies that it is allowed to make strictly nested calls between C and Prolog (Prolog calls C, calls Prolog, calls C, etc.), but it is not allowed to open multiple queries and start generating solutions for each of them by calling `PL_next_solution()`. Be sure to call `PL_cut_query()` or `PL_close_query()` on any query you opened before opening the next or returning control back to Prolog.” ([http://www.swi-prolog.org/pldoc/man?section=foreign-create-query](http://www.swi-prolog.org/pldoc/man?section=foreign-create-query))

Are there any examples of querying SWI-Prolog from deterministic and non-deterministic foreign predicates?

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**Author:** ![abaljeu](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/abaljeu/32/136_2.png) [@abaljeu](https://swi-prolog.discourse.group/u/abaljeu)\
**Post date:** [April 22, 2019, 10:45pm UTC](https://swi-prolog.discourse.group/t/querying-prolog-from-foreign-predicates/570/2 "2019-04-22T22:45:29Z")

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The reason for this limitation is bound to be the nature of the SWI engine. It has a single call stack and a single trail stack, and having two open queries would require two stacks. I bet you could get there by having two threads or two engines running, bu I have not tried this.

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**Author:** ![AdamSobieski](https://avatars.discourse-cdn.com/v4/letter/a/da6949/32.png) [@AdamSobieski](https://swi-prolog.discourse.group/u/AdamSobieski)\
**Post date:** [April 23, 2019, 2:30am UTC](https://swi-prolog.discourse.group/t/querying-prolog-from-foreign-predicates/570/3 "2019-04-23T02:30:02Z")

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I found this: “The SWI-Prolog engine uses three _stacks_. The _local stack_ (also called _environment stack_) stores the environment frames used to call predicates as well as choice points. The _global stack_ (also called _heap_) contains terms, floats, strings and large integers. Finally, the _trail stack_ records variable bindings and assignments to support _backtracking_.” ([http://www.swi-prolog.org/pldoc/man?section=limits](http://www.swi-prolog.org/pldoc/man?section=limits))

I would be interested in some new functionality to save and restore state from non-deterministic foreign predicates. In theory, one could open a foreign frame during an invocation of a non-deterministic foreign predicate and then, just prior to returning, clone or copy the contents of the stack(s) atop or after that mark. In theory, a non-deterministic foreign predicate could, if/when subsequently invoked, open a foreign frame and restore state to the stack(s) atop or after it.

Also, the `PL_open_foreign_frame()` documentation says that the SWI-Prolog kernel creates a foreign frame prior to calling a foreign predicate and the `PL_close_foreign_frame` documentation states that the kernel closes the foreign frame after returning from the foreign predicate.

Does it make sense to save and restore portions of the stack(s) for non-deterministic foreign predicate invocations? Any other ideas regarding whether or how it might be possible to achieve _nested queries_?

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<div class="post-metadata">

**Author:** ![abaljeu](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/abaljeu/32/136_2.png) [@abaljeu](https://swi-prolog.discourse.group/u/abaljeu)\
**Post date:** [April 26, 2019, 1:26pm UTC](https://swi-prolog.discourse.group/t/querying-prolog-from-foreign-predicates/570/4 "2019-04-26T13:26:46Z")

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Using the C++ wrapper to the C api I would write

```
PlEngine a;
PlEngine b;

start query on a
start query on b

while (neither query is done)
{
    next the query on a
    next the query on b
}

```

I have not tested this.

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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:** [April 28, 2019, 9:06am UTC](https://swi-prolog.discourse.group/t/querying-prolog-from-foreign-predicates/570/5 "2019-04-28T09:06:36Z")

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Roughly that can work. You do need to ensure the right engine is active at the right time in the computation. Notably not only queries are engine specific, but also `term_t` handles. Trying to access any of these with the wrong engine being active will cause a crash.