[z5h]

> What is some of your most hard-earned knowledge?

1. If you find yourself straying too often from coding in relations, and instead coding in instructive steps, you're going to end up with problems.

2. Use DCGs to create a DSL for any high level operations performed on data structures. The bi-directionality of Prolog's clauses means you can use this DSL to generate an audit trail of "commands executed" when Prolog solves a problem for you, but you can also use the audit trail and modify it to execute those commands on other data.

[javcasas]

How do you debug DCGs? I get "false." instead of "syntax error at line 23", which is unacceptable for bigger inputs.

Also DCGs for high level operations? Do you mean "use DCGs to parse strings that contain instructions" or do you parse things other than strings with DCGs? I'm assuming you take the parsed instructions and run them through some kind of interpreter that does the execution and audit trail.

[YeGoblynQueenne]

(Not OP)

>> How do you debug DCGs? I get "false." instead of "syntax error at line 23", which is unacceptable for bigger inputs.

You need to include exception handling in your DCG rules. For example, in Prolog-like pseudocode:

  pink_apples([A|As]) --> [A], { red_apple(A), throw(error(type_error(pink_apple,red_apple),_)) }.

  % Raises type error:
  ERROR: Type error: `pink_apple' expected, found `red_apple' (an atom)
  ERROR: In:
  ERROR:   [12] throw(error(type_error(pink_apple,red_apple),_10070))

Called from a source file the error output will list the line in the source file where the exception was raised. There are more tools to debug the error:

https://www.swi-prolog.org/pldoc/man?section=exception

DCGs parse lists, not strings, as such. So the input can be anything you can put in the form of a list.

[upghost]

And this is the person who saved me from nearly dropping Prolog!!

[YeGoblynQueenne]

Hope you don't end up cursing me down the line :P

[z5h]

> How do you debug DCGs? I get "false." instead of "syntax error at line 23", which is unacceptable for bigger inputs.

I also sympathize. "false" as the default failure mode is a challenge with Prolog. Most Prologs I've used have good debugging/stepping features (see spy and trace predicates), logical debugging of pure monotonic Prolog can often help (explained by Markus Triska), you can easily write (use existing) meta predicates that assert a called predicate must not fail otherwise throw an exception. For example: here the ./ is supposed to look like a checkmark. So `./ true.` is true. `./ false` throws an exception.

  :- op(920, fy, './').
  :- meta_predicate './'(0).
  './'(X) :- call(X) *-> true ; prolog_debug:assertion_failed(fail, X).

[upghost]

Ahh nice, I like this one!

[upghost]

I sympathize. I nearly dropped Prolog for this reason until I learned about term_expansion/2 and goal_expansion/2.

If what Prolog is doing you consider incorrect, _make it_ incorrect.

DCGs can be used to convert any data structure to a sequence. Actually, they are capable of any graph to graph , so they could produce a sequence of commands.

The oft-cited Markus Triska has some great work on this:

  https://youtu.be/vdabv9EkYrY?feature=shared

  https://www.metalevel.at/zurg/

You can also use Prolog as a coordinator on external systems.

See https://github.com/mthom/scryer-prolog/blob/master/src/lib/p...

[rdtsc]

I was watching some of the videos and in the intro Markus mentioned some of the most popular Prolog systems https://www.youtube.com/watch?v=8XUutFBbUrg

SICStus, GNU Prolog, XSB, Ciao, Scryer, Traella, Tau but does not mention at all of SWI-Prolog. I remember using SWI-Prolog back in the day, did it somehow fall out favor, or is there some animosity between implementation and Markus is just not in the SWI-Prolog camp?

[upghost]

My 2 cents, it's hard for me to believe there is any animosity there.

You don't have to take my word for it, you can just see what he says and does. But I've seen nearly every one of his videos and blog posts, read through many previous comments on HN, there's not a single disparaging remark in any of them. In fact he even makes his philosophy clear in this post [1]:

  Things that I, at least, always do without even thinking about it. For instance, when I mention Scryer Prolog in Internet discussions, I always try to mention at least one other Prolog system too in a flattering way, out of respect and admiration for other systems and also to encourage more cooperation between systems.

What I gather is Markus strongly advocates for ISO compliant Prolog implementations and especially open source ones, because that's where his heart is right now. But one thing to remember is that Markus has also contributed 10s of thousands of lines of code to SWI (check out the author tag in many of the SWI libraries), has co-authored authored many papers with Jan Wielemaker, and there is plenty of professional respect there. This is like trying to understand the nuance reasons why Steven Hawking disagrees with Einstein (or whoever) on something. Probably in agreement about 99.999% of most things but strong disagreement on a 0.001% that probably doesn't matter much to you and I about whether or not black holes are Humperdink-Blazensort compliant or... waves hands stuff.

As you've seen from the comments though, SWI is an extremely successful, established system. Nearly every book and example you will read uses SWI. It has great libraries, great IDEs, FFI, embedding, support, documentation, all kinds of great stuff -- SWI is already REALLY successful, and for very good reasons!

So my guess is just trying to give a voice to the little guys who are up and coming and are in line philosophically with his beliefs about the language and open source. I personally would describe that as "support", I don't think I would use the word "animosity".

[1]: https://github.com/mthom/scryer-prolog/discussions/2605

[rdtsc]

> This is like trying to understand the nuance reasons why Steven Hawking disagrees with Einstein (or whoever) on something.

I like the analogy!

> So my guess is just trying to give a voice to the little guys who are up and coming and are in line philosophically with his beliefs about the language and open source.

That makes perfect sense. Thanks for answering! It's an interesting insight into the world of "Prologs" to a complete outsider. I had only used SWI-Prolog in the university and remember it having a variety of modules, web development libraries and other such things, so at least superficially to a novice, that was pretty impressive.