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Introduction to logic programming with Prolog

284 points| matchilling | 8 years ago |matchilling.com | reply

91 comments

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[+] barry0079|8 years ago|reply
When writing a Prolog program a while ago I stumbled across an old mailing list where someone posted the following joke. Q. "How many Prolog Programmers does it take to change a lightbulb?" A. No.
[+] tmaly|8 years ago|reply
LOL thank you
[+] zmonx|8 years ago|reply
I agree with what Sylvain Soliman said about this on Reddit: It's a nice tutorial for the 80's parts of the language:

https://www.reddit.com/r/programming/comments/7hp2xw/introdu...

In modern Prolog systems, you would use more declarative features such as constraints to model combinatorial tasks like map coloring.

Make sure to check out modern Prolog features if you are interested in learning the language seriously!

[+] kccqzy|8 years ago|reply
Are there any good materials (articles, books) for this kind of modern Prolog? I’m afraid my only experience with Prolog is those 80s features of Prolog and I’m not sure what you mean by constraints.
[+] agumonkey|8 years ago|reply
are modern prolog just a layer on top of the old semantics or somehow an incompatible paradigm ?
[+] virgilp|8 years ago|reply
I find the section 5.3 from the paper "Out of the tar pit"[1] describes it very well:

> It is for this reason that Prolog falls short of the ideals of logic programming. Specifically it is necessary to be concerned with the operational interpretation of the program whilst writing the axioms.

I haven't heard of any approach that generalizes the goals of logic programming in a far better way. Is there any? (on specific usecases, having a sort of rule engine for running your "business rules" can indeed be an excellent approach)

[1] https://github.com/papers-we-love/papers-we-love/blob/master...

[+] mathetic|8 years ago|reply
Ah, that's a problem with many facets.

It is not that we can't come up with a logic language that is more declarative, it is just that telling the program everything about the universe is so damn dull. For example, ordering of your body predicates will often be based on what you think intuitively their sizes of solutions are going to be e.g. `plus(A,B,C), solution(C,X).` You know that plus is injective, so you put it first but from a logical perspective putting solution predicate first is just as sensible. If the language allowed you to express relative sizes of the predicates, then this could have been done automatically, but it would involve coding the order of predicates somewhere else in the program!

You also need to get rid of cuts in Prolog, if you want to be closer to true declarative programming. You might like to then try Datalog (alas, not Turing complete).

Another approach is to separate concerns more clearly. The part of Prolog that encodes and solves a logical problem is often different than the one dealing with IO for example. Ideally, you want to be more declarative in the former domain and more imperative in the latter. This can (and probably have been done) with a monadic approach.

[+] tr352|8 years ago|reply
What you're looking for is called answer set programming. It has pretty much replaced logic programming as a research focus, and a number of implementations exist.
[+] fusiongyro|8 years ago|reply
Most programming languages fall short of some ideal. It doesn't mean you're not able to be productive or do interesting things.
[+] pgtan|8 years ago|reply
I try to introduce my boy (12) in programming trough Scratch but still no big success. However, we solved recently two math homework exercises with Prolog (logic) and MetaPost (geometry) which caused some effect, so I'm thinking to deepen in that languages or may be switch to Logo as intermediate lisp like solution.
[+] threatofrain|8 years ago|reply
I know a young child who is going through Scratch, but also no big success, but then the child was introduced to Racket and is now using it as a math companion tool, such as for visualizations and calculations.

The child was encouraged to build general solutions to homework, and suddenly a lot of schoolwork with tedious arithmetic seems boring to the child.

[+] legionth|8 years ago|reply
I hated it in university and hate it until this very moment. I get what this language is doing and why it is great for special usecases. But I can absolutly can not read code written in Prolog. Staring at 3 lines of code and not knowing what the hell it does, gives me nightmares till today.

You can write a quicksort in a few lines. You can traverse over a tree very easily. But understanding these few lines takes me more time than writing it in C, Java, PHP etc.. But I guess it is all training...

[+] pvinis|8 years ago|reply
I've had the opposite experience. In university and later, everytime I play with prolog, I love it. Some things make so much more sense when you write them this way, and it's not just the few very specific cases. When I learned about prolog and had the first couple of classes using it, I was delighted! Finally a language that works in a way my mind likes to think in.

I am not saying that one way or language is better than the other. But I'm saying that, like you are faster to rewrite in c or java, there are people that can rewrite or understand faster the other way. :)

Programming is fun!

[+] pjmlp|8 years ago|reply
I love Prolog, to the point of having taken part in the Portuguese university national logic championship back in the 90's.

On our degree, we had three logic programming classes, starting from logic programming as pure math all the way to deep Prolog exercises.

[+] charlysl|8 years ago|reply
I found that these kinds of languages make a LOT more sense after learning some logic [1], specially relational logic, to the extent that it's essentially the same thing.

[1] http://logic.stanford.edu/

[+] kbp|8 years ago|reply
> You can write a quicksort in a few lines. You can traverse over a tree very easily. But understanding these few lines takes me more time than writing it in C, Java, PHP etc

I'm still only a beginner at Prolog, but that's what I enjoy about it: it's an entirely different way of tackling problems that really stretches my brain, not just new syntax on the same old paradigm that I already know.

[+] simias|8 years ago|reply
Something tells me that it would probably be easier if this was your first programming language. Actually I expect that some would find this declarative style a lot more intuitive and natural.

But when you've spent many years writing imperative code it feels so alien. It reminds me of the first time I attempted to write Verilog, it's hard to break the mold and change the way you think about the flow of a program and how you construct algorithms. Eventually it clicked for me and Verilog though, so I expect with enough dedication one could do the same with Prolog.

[+] yummy|8 years ago|reply
I don't think working with numbers is a strong side of Prolog. Quicksort in Haskell is also 3 lines, but it's totally readable. I can make use of functional programming in my daily life, but logic? Never. But think of the Zebra puzzle. In Prolog the solution is 100% declarative.

BTW, I hate Prolog too.

[+] simion314|8 years ago|reply
I agree that is hard to think in prolog, formulating the problem as logic rules is not natural for us after we done imperative programming. I imagine this is true on small problems that are popular in education but in a real world scenario a hard problem(that is suitable for prolog) would be much harder to do it in C or other imperative language, and probably adding new rules would be easier in prolog.
[+] flavio81|8 years ago|reply
Some problems are harder to express in Prolog (than in conventional imperative logic), but then there are some problems that are far, far easier to solve in Prolog.
[+] vanderZwan|8 years ago|reply
I've seen a few contalks about logic programming, read a few articles, and aside from the intellectual curiosity in itself I can see that it is very elegant in certain circumstances (although I lack the hands-on experience to feel what those circumstances are).

Since doing a whole program in prolog isn't really practical for me, is there any way to use the logic programming paradigm in mainstream programming languages to deal with specific problems that fit it?

[+] 1001101|8 years ago|reply
Another excellent resource for learning Prolog is the book "The Art of Prolog." One of the finest language texts you will ever read.
[+] sizeofchar|8 years ago|reply
I read it recently and I can't recommend it enough. Even if you aren't really interested in Prolog, it is a must read.
[+] dmitriid|8 years ago|reply
> Prolog has a straightforward syntax. You will pick up the few rules very quickly. Let us transform the Socrates example from earlier into an actual Prolog program and analyse what is happing inside the compiler.

Yeah, and no. This skips over the fact that this will not work in swipl, and you need to write those rules in a separate file, and compile/load them.

This is mentioned way after the entire introduction, after the reader has already given up on trying anything with prolog

[+] sgt101|8 years ago|reply
Is anyone reading this aware of any work on logic programming in the large as in efforts that are harnessing the fact that answerset programming avoids committed choice and so enables us to write reliable tractable programs, but aims to make that possible in teams or in distributed settings?
[+] ninjamayo|8 years ago|reply
I haven't encountered anything across answer set programming in the large as you put it. It is still pretty academic and mainly used for solving puzzle and optimization problems.
[+] sgt101|8 years ago|reply
Also work that tackles industry things like testing your logic programs?
[+] chkte|8 years ago|reply
I know how to code in Prolog. How can I take profit of it, apart from my academic (undergraduate) interests?

Can I build something in particular, can I look for specific jobs?

[+] gota|8 years ago|reply
There are not that many companies using prolog code, but specialized AI companies still do. I've worked as a consultant programmer developing large decision-support systems for engineering projects and the bulk of the domain logic was written in prolog. Most operational stuff was in C++ or Python, still, so even in that project there were as many jobs for Python programmers as there were prolog programmers

You don't truly appreciate the declarative paradigm until you are able to describe (and find the best among) all billions of complex possibilities of configurations of nuts and bolts following all these engineering laws, practices and rules of thumb... in a hundred lines or less

I love Python and I'll introduce myself as a Python programmer, but part of me will always have a deep respect for prolog and how utterly awesome it is in the right context

[+] maxxxxx|8 years ago|reply
How does Prolog relate to ML systems? It seems they work in a similar way. Input some learning data and then the system produces output based on these.
[+] lou1306|8 years ago|reply
Eh, they are quite different under the hood. To put it real simple (and probably quite sloppy): Prolog determines the output by applying logic. ML uses mathematics to analyze the input and "guesses" the output, based on the training set.

As a corollary: Prolog will negate anything that can't be proven true from the rules you give it. A ML algorithm will always give you some answer, though the quality depends on the learning data.

[+] guruparan18|8 years ago|reply
Back in 2006, I studied Prolong in my college as a pre-up course for A.I. From my experience then, Prolog was easy for programming small tasks. I ended up building a card game with some intelligence as a final project. Even then, I spent most of the time designing cards to show up in console than time on actual coding for game play/intelligence.

It could be fun to retry Prolog now.

[+] ptrkrlsrd|8 years ago|reply
Great tutorial! Made me want to go back to Prolog again. A little sidenote for those who run MacOS with brew - you can install Prolog with "brew install swi-prolog" instead of using the binary version.