Hacker News Reader

Comment by wazoox | Hacker News Reader

  > I think it would be best to have a Prolog-like language 
  > as a library (like Lua or SQLite), to embed it for rules 
  > & scripting.

You may be interested to know that there are various prolog Perl modules on CPAN, particularly AI::Prolog and Language::Prolog::Yaswi .

View on HN

Perl isn't really my thing* , but I'll take a look at how they're implemented. I was thinking a C library with a C API that will make it easy to use from Lua, Python, Ruby, etc. Yaswi is probably just a wrapper to SWI-Prolog. There are a few of those, and I'm tempted to write one for Lua.

I think designing a Prolog-like language primarily for embedding, rather than wrapping a standalone Prolog, will have an interesting effect on the design though - Lua, for example, benefits tremendously from being able to leave certain issues to C. An embedded logic language could likewise focus on what it does best.

* Somebody is probably going to pop out of the woodwork and say that my impression of Perl is based on Perl 10 years ago. They're right, as that's roughly when I switched to Python. It'd be tough to convince me to switch back, though. I'm fine with awk for "tiny Perl script" hacks, and prefer other languages for larger stuff.

Check out js-prolog: http://ioctl.org/logic/jsprolog-history

It's a quick-and-dirty implementation of a Prolog interpreter, but it's a great resource for prototyping. JavaScript ports very well to Lua, so you should be able to port the code in a busy afternoon.

I made a couple of suggestions to increase the speed of the interpretation, but I'm not sure Jan ever integrated them into the code. The big one was using a trace to destructively modify environments rather than recreating them on choice points; WAM and tabling would be great, but it's a non-trivial piece of work following Warren's tech report, even with the help of the (very good) tutorial.

Thanks! That looks fun.

I'm planning on doing a C VM-based implementation with tabling* . I'm not sure about coroutines, and the VM will not necessarily follow the WAM design verbatim - Peter Van Roy has several suggestions for improvements in his thesis.

* Prolog-ese for memoization

I saw this one pass by a few weeks ago, it may interest you:

http://yieldprolog.sourceforge.net/

"Yield Prolog lets you embed Prolog programs directly in Python, C# [1] or Javascript [2] by using the yield keyword."

If it's implemented in C with a simple API, it should be easy to use from anything that has a C FFI. I'd rather not make it too language-specific - there's a huge duplication of effort as new languages appear.

Lua and SQLite are both great examples of libraries whose APIs make them straightforward to embed in other languages.

The nicest embeddable Prologs I've found are, for whatever reason, in Java. I use Tuprolog (http://alice.unibo.it/xwiki/bin/view/Tuprolog/) a bit, which embeds a Prolog interpreter pretty cleanly. There's also a compiler, PrologCafe (http://kaminari.scitec.kobe-u.ac.jp/PrologCafe/), though I find it a little harder to use seamlessly.

I agree that something as easy to embed from C or languages with C-embedding hooks would be nice.

It's odd that there isn't a C one, really. I've done very little with Java and the JVM, but hopefully those will prove helpful to someone else.

I don't have a time frame for a C-embedded Prolog (I've already got several other projects I'm trying to finish), but it's high on my list.

Reply on news.ycombinator.com