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Comment by patrickmn | Hacker News Reader

What's the best example of modern Datalog?

A good way to try Datalog is to use a state-of-the-art Prolog system that supports SLG resolution (tabling).

SLG resolution is an evaluation strategy of Prolog programs that provides favourable termination properties compared to SLDNF resolution which is Prolog's default execution strategy.

A great example for this is XSB Prolog, since it has pioneered and refined many of these techniques and serves as an example and benchmark for many other Prolog systems in this area:

https://xsb.com/xsb-prolog

The nice thing is that this approach allows you to use Datalog, and at the same time gives you a Turing-complete programming language that is comparatively easy to grasp once you understand Datalog, while sharing many of the same advantages and being syntactically only a slight extension of it.

A turning complete query language sounds like a recipe for DoS attacks.

It is only Turing complete if you allow new rules. Usually a client would only be allowed to formulate query goals.

In that sense, GraphQL is Turing complete as well, as the underlying implementation could just simulate a Turing machine.

What I read from your post here is that GraphQL is safe by default and would require effort to make it dangerous while Datalog is dangerous by default and would require configuration to make it safe. I know which one I'd rather put on a server.

You have read that incorrectly. A Datalog/Prolog query does not define new rules. That's like calling arbitrary code on remote machines. You would never allow that.

But IF you were allowed to define new rules, THEN you could simulate a turing machine or create infinite reductions.

> A great example for this is XSB Prolog,

I'm really sorry but it honestly cannot be. I was curious because Datalog/Prolog based technologies are intriguing so I clicked on that link.

https://xsb.com/xsb-prolog seems to have no download but links to

http://xsb.sourceforge.net/ which claims version 3.8 (October 2017) is current and links to

http://xsb.sourceforge.net/downloads/downloads.html which does not specify a version but offers a tar.gz that contains version 3.7 from July 2016.

Going back to sourceforge and heading to the SVN https://sourceforge.net/p/xsb/src/HEAD/tree/trunk/XSB/ states version 3.9, November 2019 and looks pretty active commit-wise.

Okay, let's do that. Checking out trunk, I see perl and C scrolling by. While I have no problem with Perl, I know quite a few developers - especially in the web context - that find it rather off-putting. I get a README that tells me to read the manual in order to install. Fair enough. Opening that ( http://xsb.sourceforge.net/manual1/manual1.pdf ) gives me a 622 page PDF from October 2017. Chapter 2:

"Make sure that after you have obtained XSB, you have uncompressed it by following the instructions found in the file README." There were no such instructions. But well, configure and make are in build, that's fine and the rest of the instructions are clear.

I was confused why configure looked for mysql and make did something with Java but whatever, both worked great out of the box. Let me check the binary. I get a REPL, claiming to be version 3.8 from October 2017 and despite having 7 lines of output I have no idea how to quit out of that process, so I turn to Google which gives me http://xsb.sourceforge.net/shadow_site/manual1/node14.html

Neither ctrl^c, nor ctrl^d work, and halt + return is ignored. I go ahead, kill the process and delete the directory.

Alternatives to the above? That original site tells me to contact some organisation, "xsb prolog" on Google turns up ancient results.

Now compare that with https://graphql.org/graphql-js/

Just like many of these rather research centric languages, it does not matter how cool their tech is if that's the first experience people get. In my view experiences like the above are why they fail to gain adoption in today's world, not some technical aspect or technique. At least unlike much of the linked data and SPARQL world this thing compiled out of the box and I would have been up and running in about 10 minutes if I had any intention to continue.

That sucks. XSB is old code and I believe it was meant mostly as a proof of concept even at the time it was written. I doubt many people use it, even in research, nowadays.

I believe what triska meant about XSB is that it demonstrated the benefits of SLG resolution, by being a proof of concept like I say. So yes, XSB would be more of academic interest.

If you want a modern Prolog to play around with you should try Swi-Prolog. From the conclusion of your comment I'm assuming you haven't tried it because you wouldn't have trouble "getting up and running" in 10 minutes. Or 5 or so. Seriously, Swi's maintainers (triska is a contributor) have gone out of their way to make it useable.

Anyway, if you put in all this work to get XSB working I think you should definitely give Swi-Prolog a try. Sunk cost fallacy, innit.

Here's the link:

https://www.swi-prolog.org/Download.html

On windows, there's an installer. On Linux you can yum-install or apt-get etc an earlier version and try it out, then if you like it you can follow the Download page's links to figure out how to install a newer version (that does take a bit more work).

If you're stuck with Prolog, there's a discourse group, here:

https://swi-prolog.discourse.group/

People there are always happy to help newcomers (and will never send you to RTFM).

But be warned that learning Prolog is not easy. I help with markings and labs for Prolog courses at my university and students always have a hard time with it, until some things start to click.

EDIT: "halt." should work at any Prolog prompt. The "." is a statement terminator. You probably didn't get anything after "halt return" because you missed the dot.

EDIT II: Oh, I forget. You can try Swi-Prolog on Swish (Prolog notebooks):

https://swish.swi-prolog.org

So no need to install anything but keep in mind that some stuff is limited for security reasons (you can do a lot of damage with a language that lets you rewrite it on the fly).

Much appreciated, I kind of got what triska meant (didn't realize they were a contributor to that project), I just wanted to highlight that this is a real barrier for adoption of these alternative stacks. I find this interesting enough to spend more than those 10 minutes when I have time, sure. It's just that a random dev on the search for some component to solve something in their stack likely wont.

Just gave the SWI implementation a try and that was honestly a much better experience and I could immediately jump on their getting started guides to get a feel. Thanks for the pointers as well, I've got a few colleagues that actively use prolog but good to see that there's an active community out there.

> EDIT: "halt." should work at any Prolog prompt. The "." is a statement terminator. You probably didn't get anything after "halt return" because you missed the dot.

Whoops. Yup, I think I might have, I was under the impression that dot in the documentation was the sentence delimiter.

(And btw, my bad about the Perl comment, I just realized when setting up the SWI one that they seem to share a file extension with Perl.)

I agree, it's very bad to have to go through so much hassle when you just want to have a quick look to evaluate a possible tool for a problem you have right now. This is a difficult situation to resolve: usabiity won't improve until there's more users and until usability improves there won't be more users.

I'm glad you like Swi. It's not the fastest implementation but it's certainly the one with the largest community and the most quality-of-life features, documentation server, unit tests library, package manager etc. I love it :)

>> (And btw, my bad about the Perl comment, I just realized when setting up the SWI one that they seem to share a file extension with Perl.)

Oh yes, I totally forgot about that (can't say I use much perl!). It can look funny if you don't expect it :)

Anyway I hope you have time to look into the language more in the future. Like I say, it's hard to learn but it's worth the pain.

>Just like many of these rather research centric languages, it does not matter how cool their tech is if that's the first experience people get. In my view experiences like the above are why they fail to gain adoption in today's world, not some technical aspect or technique. At least unlike much of the linked data and SPARQL world this thing compiled out of the box and I would have been up and running in about 10 minutes if I had any intention to continue.

Just what I was thinking as I read the parent. This is the real answer to the OP's question.

Datomic. Also interesting:

https://github.com/sixthnormal/clj-3df

Datomic provides a fantastic range of query features, but the underlying Datalog engine does have drawbacks in how it evaluates results eagerly and requires intermediate result sets to fit in memory [0].

For comparison, I work on https://opencrux.com which uses lazy Datalog evaluation and can spill to disk when necessary. Naturally there are downsides to this approach also, including a dependence on seek-heavy use of local KV indexes.

clj-3df is definitely interesting although not strictly comparable :)

[0] https://docs.datomic.com/on-prem/query.html#memory-usage

Oh hello! I hadn't realised JUXT had their own products now, will definitely check this out.

You might want to check out Grakn (https://github.com/graknlabs/grakn/) - its query language is an implementation of recursive datalog with negation and comes with lazy execution. I am one of its developers.

Certainly not the best, but worth mentioning the Python implementation pydatalog.

https://sites.google.com/site/pydatalog/home

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