Announcing Basic WebAssembly support in Scryer Prolog

[rujialiu]

With PR #1986 merged, now Scryer Prolog has some basic WebAssembly support! Besides what README.md already says, here are some other points I would like to emphasize:

Pros:

• Rust wasm suppor is great, and everyone who has experience of wasm-bindgen should be able to write additional APIs quickly, usually more productive than writing wrappers for emscripten.
• The resulting wasm binary is quite small, still including all built-in prolog modules like clpz (except those only avaiable in optional features)

Cons:

• The current API is minimal. There is only one function to create a mock WAM, run a program from a string, then fetch the "standard output" of the program
• Only 6/12 crypto functions are supported
• No wasi support.

This is my first contribution to Scryer Prolog. I thought it would be easy (because my past experience of building my own rust projects to wasm), but it turns out to be much more complex and subtle.
Luckily, I was welcomed and helped by a lot of people @mthom @triska @aarroyoc @lucksus @fayeed @Skgland. Thank you all!

The work is split into several PRs (atom-64bit, optional-features, 32-bit-support and finally wasm32-support), and most of the changes are easy to follow, and I hope additionally maitaining 32-bit systems and wasm32 should not be a big burden in the future.

As I said, writing additional APIs should be technically not hard, but needs discussion and coordination to avoid becoming a big mess. That's why I'm keeping the API minimal now (in theory I can even remove that API but then there's no easy way to validate webassembly actually works).

When the support gets better and better, someday we should be consider native js ffi (just use wasm-bindgen inside current ffi module?).

There are still a loooot of things to do, but this is a good start :)

[triska]

Thank you so much @rujialiu and everyone who contributed, this is an extremely impressive contribution! Just the 32-bit support by itself would already have been an invaluable contribution, and the WASM support goes far beyond that!

Thanks to @infogulch, a pre-built WASM version is already available for download and ready for automatic generation in all future releases: #2006

The mentioned README description contains a usage example: https://github.com/mthom/scryer-prolog#building-webassembly

Thank you all!

[guregu]

Awesome! Thanks to everyone who contributed. Exciting times for Prolog.

I think the ideal API would use Wasm Components + WASI. Basically we would write wit definitions for the various basic Prolog types, and some functions for "query" and "redo" (and probably "consult string" as well). Then it would be possible to use wit-bindgen to generate the host APIs for JS and other languages. I'm currently working on this for Trealla, but I think it could work as a common API for all wasm-based Prologs. From there, it would be relatively straightforward to write a JS library like trealla-js that supports any compatible Prolog core (currently trealla-js follows a similar implementation to Scryer's, capturing stdout for everything, but I would like to replace it with Wasm components soon).
It would also be super cool to support calling into Wasm components from Prolog as well. This would let us take advantage of projects like Spin to do edge computing stuff. My Trealla implementation does this in C but I would love to see a more dynamic way to do this using Prolog, perhaps with library(ffi) or something similar.

If this sounds like a good idea, I would love to contribute. Perhaps we should make an issue for workshopping a wit definition?

[rujialiu]

Yes! This sounds cool! I don't have much experience about Wasm Components but I'm planning to dig into it in near future. It's also nice to have something similar with trealla-js because trealla is another interesting prolog on the web, and it'll be great to have one js codebase and able to switch between trealla and scryer if needed. If you go along the route I'd love to at least do some testing or debugging :)

[rujialiu]

It'll also be interesting to embed scryer prolog in go like you did in https://github.com/trealla-prolog/go and maybe do some comparison. I wonder whether it's easy once we have a good set of wit definitions @guregu . (Just curious. Personally I haven't coded in go for almost 10 years...)

[guregu]

@rujialiu agreed! I made some issues here for WASI (#2011) and WIT definitions (#2012). Let's work towards a nice embedding API for Prolog. Help is always appreciated :)

[aarroyoc]

Many thanks @rujialiu ! It's so amazing that I couldn't wait to try it! So, this afternoon I've been playing with a Scryer Prolog playground, that lets anyone try Scryer Prolog. Very basic, more like a proof of concept: https://play.scryer.pl/

[triska]

That's awesome, thank you a lot!

You can use call_residue_vars/2 (from library(atts)) and copy_term/3 to obtain the residual goals.

For instance:

?- call_residue_vars(dif(X, Y), Vs), copy_term(Vs, Vs, Gs).
   Vs = [X,Y], Gs = [dif:dif(X,Y)], ... .

[rujialiu]

The playground is niiiiice!!! @aarroyoc
An interesting way of pushing further development is to think about what features we'd like to add to this playground gradually and how we'd like to implement them, possibly in the same/similar way that trealla-js did, write some wit definitions and implement them.

[toblotron]

Hello! Nice work with the playground! After some help from triska I managed to understand the basics of how to run scryer through it.
A few questions:

• How can I get the next solution, after asking and receiving the respons to a query?
• Is it possible to make a (pausing) call from my scryer-in-WASM program, back to the surrounding javascript environment, and then from there decide to resume the execution of the scryer program?

I realize these things may not be within the intended scope of the playground, but if one never asks.. :)

[aarroyoc]

Hi @toblotron ! Welcome to Scryer Prolog!

Regarding your questions:

• Not possible yet. Neither the playground nor the WASM interface supports it. But the Programatic use of Machine / Scryer as library #1880 PR will introduce new functions to interact with Scryer Prolog as a library
• That would be like some kind of FFI for JS, right? It's not supported :(

As you can see WASM support is very recent, so there's a lot of things to polish

[rujialiu]

As @aarroyoc said, after #1880 is merged, we will start working towards some serious WASM support including what you asked. I guess the best way is to evolve playground together with the WASM support, otherwise we won't know whether the WASM support/API interface is comfortable/performant enough to develop a nice playground :)

[toblotron]

Great to see that things are happening on this front! 🙂

Right now I'm using tau-prolog, which is awesome, but scryer would probably be even better, (for example) since it has clpfd, which I'd love to work with

Cheers!