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Comment by simion314 | Hacker News Reader

I am curious if you used a visual debugger for prolog? (or for a procedural language) printing things to the console is the poor man debugger.

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A big problem with visual debuggers is that they are too blunt of an instrument. Often I wish to examine specific things under specific conditions and thus use conditionals to narrow down what's being explored instead of bajillion breakpoints and/or log records. Debuggers could add conditional expressions, but then they'd be re-inventing WRITE statements anyhow.

As far as the nearby statement that maybe I didn't learn dissection correctly, that may indeed be the case, but was also my original point: perhaps the real problem is that nobody knows how to TEACH functional/logical programming right, and that's why they have not caught on in the mainstream despite being around several decades. I'd love to "get" the benefits, but it just won't click. Sometimes you have to teach programmers how to think, not just what to think.

> Debuggers could add conditional expressions, but then they'd be re-inventing WRITE statements anyhow.

SWI-Prolog's debugger can be invoked conditionally whenever you want from inside your running application. I've used it many times, and the code is simply:

    (Condition -> gtrace ; true)

It's true that you could stick a write there, but you would have to think about what pieces of data you will need. Your mileage may vary. I've found this style a lot more useful than "traditional" debuggers where you set breakpoints beforehand and hope you hit them, as you say.

> As far as the nearby statement that maybe I didn't learn dissection correctly, that may indeed be the case

The point was more specifically that you didn't learn Prolog properly (or at all) and don't realize that it forces you to dissect your code. As I said, in Prolog you cannot nest loops. You cannot nest loops. It's hard to write a big non-dissected piece of code if you cannot nest loops.

> perhaps the real problem is that nobody knows how to TEACH functional/logical programming right

I agree with regards to logic programming. We need better free tutorials. I don't particularly like the linked one, it feels like it doesn't start at a beginner's level. I would be interested if anyone who didn't know any Prolog before has managed to learn Prolog from it.

I learned logic programming before learning Prolog.

We got our introduction to it via the Tarkis' World book (90's edition).

https://ggweb.gradegrinder.net/tarskisworld

Re: "but you would have to think about what pieces of data you will need." -- But with procedural you don't have to think much; the parts are right there to selectively examine.

Re: "in Prolog you cannot nest loops...It's hard to write a big non-dissected piece of code if you cannot nest loops." -- But nesting is good for modularity. It's sometimes called "step-wise refinement", which is a brilliant abstraction that is usually easy and natural to comprehend. Nesting is how we have been splitting up society since the dawn of civilization: countries, states, counties, cities, etc. (or equiv.) Similar for big militaries. (Maybe this is a meta form of Conway's law? Either way Conway was on to something.)

Regarding your last sentence: Yes, I can confirm that this has happened, to a reasonable extent of "learned" in the sense that people went on to write programs using this material. However, I do not know how common this is, since I almost exclusively get feedback from those who found the material suitable. There were definitely complete beginners among them. For instance, here is a discussion that happened a few months ago, where it appears that also beginners comment positively:

https://www.reddit.com/r/programming/comments/7pbkfv/great_f...

More recently, two university instructors started to use this book in courses that assume no previous knowledge about the language:

Andrej Bauer in his specialist elective course Principles of Programming Languages at the University of Ljubljana:

https://ucilnica.fri.uni-lj.si/course/view.php?id=67

Norbert Zeh in his Principles of Programming Languages lectures at Dalhousie University in Canada:

https://web.cs.dal.ca/~nzeh/Teaching/3136/index.html

Much of my current work is aimed towards making the teleteaching environment GUPU available in a free Prolog system:

https://www.complang.tuwien.ac.at/ulrich/gupu/

This will help a lot to teach Prolog to beginners in a scalable way.

On a rather personal, and maybe controversial, note: I regard the self-taught Prolog master as prevalent as for example the self-taught piano, chess or Zen master. In my experience, learning Prolog properly requires dedicated guidance, and it would be highly unusual to achieve mastery without it.

> In my experience, learning Prolog properly requires dedicated guidance, and it would be highly unusual to achieve mastery without it.

Where would you find such a master, though... If you are not studying at a university where Prolog is taught, you're pretty much out of luck, I think. (I am assuming that asking questions in newsgroups/IRC/etc is not a substitute for it.)

Yes, I agree. I was lucky enough to study where the current convenor of the Prolog ISO working group teaches, using a Prolog- and Elisp-based teleteaching environment that he has built over several decades.

As in other disciplines, if you are elsewhere, you may have to move to get close to a master. This is not so unusual, is it? Vienna is a nice city for studying logic, and does attract students from all over the world!

Cool, good to see others are finding this useful.

> In my experience, learning Prolog properly requires dedicated guidance, and it would be highly unusual to achieve mastery without it.

There are two readings of this sentence. An unkind spirit might think you are saying that Prolog books cannot make you a master for some inherent reason related to Prolog, independent of any book. Whereas I would say that there is no inherent reason, it's just that currently existing Prolog books cannot make you a master because they are simply not very good. Or in other words, yes, guidance may be needed. But can it really not be guidance from a book?

In my experience, a dedicated instructor, an automated teleteaching environment (GUPU), and a lot of textual notes (either as a separate book, or integrated in the teleteaching environment, or both) is the most effective combination to teach Prolog, and removing one of the three either creates frustration for students or fails to impose the necessary direction.

In addition, for social and other reasons, you cannot write down in a book everything that advanced Prolog students should know. As a consequence, some things are better said than written, and I therefore think that a book alone is unlikely to be sufficient.

>> As I said, in Prolog you cannot nest loops. You cannot nest loops.

What do you mean here? My earlier comment was jokular, but you can definitely nest loops in Prolog:

  ...
  deconstruct([L|Ls],Acc1,Bind):-
      deconstruct(L,Acc1,Acc2)
     ,deconstruct(LS,Acc2,Bind).
  ...

etc. That's a nested loop, right there.

Do you mean it in the sense that it's still the same predicate rather than a consecutive change of scopes?

Most concretely I mean it in the context of the common auxiliary-predicate-with-an-accumulator pattern:

    foo_bar(Foo, Bar) :-
        foo_bar_aux(Foo, Bar, 0).

    foo_bar_aux(Foo, Bar, N) :-
        ...

Which in a language like OCaml would be hiding the loop in a recursive function defined inside the main one:

    let foo_bar foo =
        let rec foo_bar_aux n =
            ...
        in
        foo_bar_aux 0

So yes, I think this is what you mean by being the same predicate rather than a "change of scopes".

Of course there are many loop-like things that you can nest. If the arguments align just right, you can also use `maplist(maplist(maplist(baz)), ...)` instead of three auxiliary predicates, but I don't encounter many cases where this is possible or desirable.

I see what you mean, thanks. I think we're talking about the same thing then.

Your Console.log will not help you in a recursive function

If you have the right tracking variables, it's great for the job: "console.write(currentNodeID, parentNodeID, depthLevel, nodeVisitCount);" And one can wrap that into a conditional if they don't want to echo the entire journey. It's usually good to create such tracking variables in non-trivial systems anyhow. Bleep happens.

Correction: should be "nodesVisitedCount". Also note that output formatting was sacrificed for brevity of the example.

> printing things to the console is the poor man debugger.

You may want to try working with network before making this statement, when you cannot step through a function because of timeouts. Or maybe even looking at tracing debuggers, like strace, ltrace, or Erlang's gdb, which all do little more than "printing things to the console".

Logging (and logging-like tools) is a very important way of debugging services, you know.

Sure, if you do not have the debugger you use the logging, but you will miss the main features of a debugger like see the entire call stack, all the variables and parameters at each call stack level, you can evaluate expressions in that context.

I use logging to log problems that would happen on a user machine or server where I can't open the debugger and see it. When an error happens on a server or a customer machine and I can't use the debugger I have to guess the problematic section and put lots of logging and try to reproduce, then add more logging and so on until I find the problem.

Debugger are much better but I notice my other colleagues do not use them and prefer the console.log or similar

Each do better under certain situations. One is a hammer and another a tweezer. Different debugging jobs/steps need different tools. But the real question is whether the hammer/tweezer kit that comes with Prolog is better than the hammer/tweezer kit of other paradigms/languages.

In my personal opinion and observation, the hierarchical step-wise refinement approach is quicker for most developers to grasp, learn, and debug. Maybe if those of us who don't see the grokkability benefits try it for 50 years, it will finally click. But you better hope to be in management by then because the programming field doesn't like "old people". I'm just the messenger.

Prolog is ideal for some kind of problems, good programmers should know what logic programming is and what problems are perfect for logic programming, then if in real life you hit such a problem you can chose prolog or a library that does logic programming.

I'd like to see a comparison to the alternatives to illustrate objective benefits. I've been down that road before in my-language-can-beat-up-your-language debates, and the end result is usually, "Well, my approach looks easier and simpler TO ME; you just think wrong." Other times they do queries in the app language when they should be using a database.

I don't think I understand your point, prolog is a logic programming language, some problems are easy to express like facts and rules so Prolog is a good solution, you can see examples in this HN thread of real usage.

It is like recursion, some problems can be solved nice with recursion but you do not use recursion everywhere. Also you can use a library in your favorite programming language and not use prolog, examples were presented in this thread.

My point is that a good programmer should know about the concept not to use it everywhere, I never had the chance to use prolog so far for my work.

Btw I am also past the stage where I have a favorite language and OS, every piece of software has it's issues.

The examples I see are what I'd call "lab puzzles" and not practical real-world enough. I invite you to point out a specific example that's highly practical/realistic.

Did you see the real world uses? like prolog was and may still be used in Windows? The puzzle examples are for educational purpose.

Again I am just a noob, I learned Prolog and did some basic things years ago, a cool page I used to learn was this tutorial that implements a text based game , more refreshing then the tutorials with puzzles or the familiry

http://www.amzi.com/AdventureInProlog/a1start.php

The situation is maybe similar with GA(genetic algorithms) when you learn it you have some simple examples, when you want to solve a real problem things get complex fast(how to encode the problem,what fitness function should I use ...)

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