Hello friends, I’m spending my christmas holiday trying to integrate tokio, a rust library for asynchronous code, with SWI-Prolog. Tokio allows one to write rust code where certain operations, like network communication, will suspend a task if the operation cannot be completed immediately, to be wo…

I don’t think the c stack has to be a problem. I can keep all required state around on the heap, as part of a continuation closure, and return from the foreign predicate. That means the c stack would be back into the state it was at the time the foreign predicate was called. The prolog stack however…

I see. I think that should be technically possible. I’m now doing holidays though :slight_smile: The rough idea is to allow foreign predicates to return some special code and make PL_next_solution() perform a yield. The special return could be a return code. Alternatively we could exploit t…

I would love to do a PR but studying up on the swipl internals is probably going to take longer than my holiday. But I’ll see what I can do :slight_smile: . I wonder if we can’t expand the way nondeterministic predicates work to also allow yielding, as nondets already have a way to pass in a bit of…

Let me try to get this straight. Just for myself, I tend to get confused about engines and yielding :slight_smile: We have a Prolog engine calling a predicate defined in Rust. In Rust you want to call Rust’s yield and when the Prolog engines is resumed using engine_next/2, you want the Rust f…

[image] jan: Let me try to get this straight. Just for myself, I tend to get confused about engines and yielding :slight_smile: We have a Prolog engine calling a predicate defined in Rust. In Rust you want to call Rust’s yield and when the Prolog engines is resumed using engine_next/2 , you want …

You want to suspend inside the foreign (Rust) function and resume? Roughly, yield stores all needed information in a foreign frame and the queryFrame struct and then returns from PL_next_solution(). Resuming calls PL_next_solution() with the QID that is a handle to the queryFrame. PL_next_solu…

[image] jan: That should be fairly close to what we want. I don’t have the Rust picture clear though. In my model, the stack is like this Rust ← PL_next_solution() ← C foreign function ← Rust ← Something blocking This thing needs to suspend. How does that happen? Notably, Rust seems to have so…

[image] jan: Rust ← PL_next_solution() ← C foreign function ← Rust ← Something blocking Sorry, I may have misunderstood. Is the scenario you describe here one where a c foreign function is being called from inside prolog, and this c foreign function in turn initiates a prolog call? In this …

[image] maren: It’s all a little bit tricky to describe You did a pretty good job :slight_smile: So, roughly we have Prolog calling an async Rust function (Future). Meaning after creating the future it calls poll() on it and may get Poll::NotReady as a result. Now it should return from …

[image] jan: Let me do a little research Ok. It seems this could at least theoretically fit in the API used for nondet foreign code. I’ll assume the foreign function can store the data required to resume (poll) in a pointer that is 4-byte aligned, not being the NULL pointer. I’m still not…

Yielding prolog engines from within a foreign predicate

Help!

EricGT January 1, 2022, 5:53pm 17

Maybe Matthijs could also try out PlantUML.

Note: If the text is hard to read, open image in a new tab should do the trick.

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Yielding prolog engines from within a foreign predicate

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