Re: Multiple instances for embedded prolog interpreter

On 07/23/2013 02:22 PM, Stefan Radomski wrote:
> Hey there,
>
> we embedded SWI prolog as a logic engine via its C++ interface and
> it's working great. But we do need more than one instance of the
> PLEngine or at least distinct states. Seeing that PL_initialise will
> just initialize a global prolog interpreter, and the foreign
> predicates will not receive any kind of callbacks, we figured that
> there can only ever be one instance per runtime.

Yes.  There is no plan to change that either, although it wouldn't
be terribly complicated because there are only a few global variables
that contain structures holding the whole lot together.

> If that's the case, can we "simulate" multiple instances i.e. by
> swapping the interpreters state before calling any of the PL*
> functions? We do not necessarily need concurrency, just distinct
> states. We pondered about using several PLFrames to achieve something
> like that but it seems that they will just "rollback" any term
> references when closing.

Frames merely control the scope of term_t (PlTerm) handles.  What
you need are engines.  Actually, the terminology is a bit misleading
because the one and only Prolog instance in the C++ interface is
called engine.  It predates the introduction of multiple engines in
SWI-Prolog ...

I think what you want is what is described in

	http://www.swi-prolog.org/pldoc/man?section=threadmanymany

Don't be misguided by the threading; you can use this stuff just as
well in a single thread.  Be careful that you do not pass term_t
(PlTerm) handles created on one engine to another one.  It will
crash.  Note that the engines have the same sharing behavior as
multiple Prolog threads, so while writing your Prolog code you
must program as if you are running in a thread.  This mainly
refers to dynamic code: :- dynamic is shared, :- thread_local
is not.

	Enjoy --- Jan

>
> If this issue has been discussed previously, please be so kind as to
> provide a link.
>
> Best regards Stefan Radomski
>
>
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