Why hasn't logic programming caught on?

Question

As time goes by, it appears more and more like functional programming is having more of an effect on other programming languages. We're starting on Prolog in my AI class, and it seems like there are some things there that would make programming in non-AI fields easier. My question is this: why hasn't logic programming caught on in the same way?

In this topic, it seems that a general consensus was reached that logic programming is useful but must be proven as such. Is there a reason why it's not seen as useful?

Update: Perhaps I should be a bit more clear. I'm not really asking about Prolog. I can see why it wouldn't be a good idea to choose Prolog for most real-world applications.

To give an example of more what I'm talking about, consider list comprehensions/map/filter in Python. These are clearly influenced by functional languages. Why is it that languages such as Python haven't also picked up on these kinds of things from logic programming languages like they have functional languages?

Answer 1

When you learn about logic-programming in Computer Science classes using Prolog, the main point is not to make you a proficient Prolog programmer, but rather to open up your mind to alternate forms of programming techniques (data-structures/algorithms) that you'd not have considered before.

To illustrate my point when I started studying Computer Science my engineering school used to require all students to write their software programs in Pascal, but since I graduated I've never used Pascal even once. But the skills I learned by picking the right data-structures and algorithms I am still using every single day.

Pascal is not showing up on my resume as a language I know, but it has been instrumental in my training as a software engineer. Its usefulness cannot only be measured by the number of line of Pascal code currently in production.

When you'll develop software you'll realize that, even though you are not writing a single line of Prolog code, you are at times re-using techniques that you may have first learned in these "useless" Prolog or AI classes you attended.

Evaluating the usefulness of a technology (specific programming language, specific software tool/application) is not simply a matter of evaluating its actual level of use, but rather its influence.

If you look at the influence logic-programming has had in the field of expert systems, computer games AI, air traffic control, and probably quite a number of other fields (suggestions anybody?) I don't think it can be said logic-programming has not caught on...

Answer 2

I spent about 4 years of my programming career working on a rule based "Expert System" for provisioning and configuring hardware for telephone exchanges based on customer requirements.

It was very successful, and as far as I know is still in daily use more than 10 years later. But finding programmers who could understand how it worked was a greater task than developing the system itself.

I think this is why the approach has not taken off, because few people have the necessary mindset for logic based programming compared to the number of people that can understand the concepts of procedural and functional programming.

Logic programming languages provide a mechanism for feeding facts and rules of inference into an "inference engine" which is then set in motion to apply the rules to the given facts in order to produce new facts. A particular logic language stands or falls on the strength of it's particular inference engine.

Prolog's inference engine has a very naive implementation and is very inefficient. The same problem could be solved more efficiently in most procedural languages just by writing a lot of if statements in a loop.

The language we chose for our "Configurator" was DEC's RuleWorks which is a refinement of the more widely known OPS5 language. This has an inference engine based on the Rete Algorithm which makes it far more efficient than the procedural approach.

Since DEC got swallowed by Compaq which got swallowed by HP, RuleWorks has become open source and be obtained from this web page.

It's a shame there is not more interest in such techniques because they can be very effective for solving a variety of otherwise intractable problems.

Answer 3

I recently used a pile of logic programming in a game design AI research project (teaser video!), but at least half of my logic-heavy project was functional or imperative Scala code implementing a basic game engine. The point of programming, if I may claim such a thing, is to bring the machine's understanding of what you want into sync with your own -- and most of syncing is requires giving the imperative details, at some level, of how you want something done. Those silly machine are always so literal...

Logic programming, whether it be the traditional, deductive, prolog-style stuff, or the more exotic inductive logic programming or answer set programming flavors, gives massive leverage for some flavors of problems at the cost of being able to easily communicate imperative knowledge (which is always needed somewhere in real-world apps). Sometimes the concerns of an interactive application make even the slightest hits to your productivity in expressing imperative knowledge unacceptable. Writing an entire game engine in a logic programming style will always be a bad idea (likewise for trying it using perl-compatible regular expressions which have the same computational power). In a hybrid language (or in one that lets you embed a logic interpreter easily) you can have the best of both worlds (I used jTrolog to embed Prolog in my Scala engine, forming a multi-paradigm voltron of sorts).

I think logic programming could certainly stand to be more popular and well understood, but, in some sense, pure-logic programming can't do much better than SQL or regexes in terms of "catching on" because its magic comes from taking away your imperative expressiveness (keeping you from getting lost in unimportant details, ideally). This explanation applies almost equally to functional programming. I love logic programming, but only because I have a choice about when to use it. The best way forward seems to be hybrid languages that present that choice in a consistent, well-designed manner.

Answer 4

My impression of plain Prolog is that it's a toy language. That's not to say that logic programming can't be useful. For example, in Twelf it's possible to declare semantics for a simple programming language quite easily and have the declarations act as an interpreter. I've also heard some good things about λProlog.

The problem I think in trying to use a logic programming language as a general purpose language is that some tasks just don't fit the concept very well. I think that logic programming features need to be incorporated into a language that also has imperative and functional constructs. There is at least one such language: Oz, but I've yet to try it out.

Edit: There's one idea I've wanted to try for some time: Feed a relational database to Prolog as atoms and use it to do queries instead of SQL. I've a feeling it would be a great improvement over SQL.

Answer 5

The first problem I had with Prolog is that it isn't a logic programming language. It lacks the three-valued standard logic of "true", "false", and "don't know", conflating the two latter ones. In other words, the two truth values are actually "can be shown" and "cannot be shown". This gives Prolog real problems with the idea of "not", which is pretty basic to logical reasoning.

In normal logic, it's perfectly reasonable to prove a proposition by disproving its negation, this being called "reductio ad absurdum". (Yes, there are people who have tried to reconstruct mathematics without using it, but that's getting a bit esoteric.) This simply doesn't work in Prolog, since there's no distinction between proved false and not proved anything.

Therefore, when I did a class project in Prolog, I got into trouble whenever I thought of it as programming logic. I'd always wind up doing something that required actual negation. Perhaps other people don't do that, but I wound up thinking of it as a pattern-matching language, and then had little difficulty finishing the project.

It's not possible to have a true logic-based language where the programmer can write things and really rely on the results. First-order predicate calculus (i.e., logic with variables, true-or-false functions, "and", "or", "not", "for all", and "there exists") is positively undecidable. (There are reasons why we keep pouring coffee into mathematicians rather than generate all possible theorems mechanically, after all.) There is no way for a programmer to know a priori whether a given proposition will be proved or not, even if the programmer already knows it to be true or false.

Edit: I also forgot the critical necessity of ordering clauses properly. In logic, it doesn't matter in what order you write things down. In Prolog, I kept getting into infinite loops, until I stopped treating it as a logic-based language. Again, it has some nice features as a pattern-matching language, but it isn't logic, and it seemed to me like a one-trick pony of a language. YMMV, but some other people seem to agree with me.

Answer 6

When we learned prolog (just a couple of weeks in a programming languages class, so I'm hardly an expert), the professor also pointed out that depending on your definitions, logic programming may not actually be programming at all.

When we say programming, we usually refer to something like 'issuing a series of tasks or instructions to the computer'. That's what imperative or functional programming languages do. That's what every "real" programming language does. Prolog, or logic programming, doesn't really do that. It's more like SQL. You get to ask the computer a number of questions, pretty much. It'll answer, to the best of its ability, based on the data you fed it previously, but unlike other programming paradigms, you're not really telling the computer what to do.

It's just very specialized, and not suited for general purpose programming. And the things it is specialized for are not often needed.

Functional programming, on the other hand, is definitely general purpose programming, and can be used for anything, with no major problems. Which is why the latter is catching on, and logic programming isn't. I think... :)

Answer 7

Functional programming is becoming more popular because it has some important benefits when applied to multi-threaded programming, and as we move more and more toward multi-core processors multi-theaded programming will become more and more important.

Answer 8

From my own experience with Prolog it is very nice for specific tasks, but as a general purpose programming language it is simply not as flexible as a lot of other languages in my opinion.

Answer 9

It is a fact that Prolog is very nice for specific tasks but in my 11 years of work experience I have never had a problem where Prolog would be the best solution. It is just a matter that the kind of problems where Prolog would be ideal, is very rare.

Answer 10

I found Prolog to be absolutely fascinating when I took it in my AI classes in college, but I can only think of a handful of situations where I'd use it outside of AI today. And even in those situations, I'd rather not use it. It took me a long time to finally "get" Prolog, and when I did, I thought it was great, but I immediately saw it's limited range of usefulness.

Still, I highly recommend learning it, if for no other reason than to learn to approach programming in another different way. Being able to look at a problem from many different vantage points absolutely makes you a better programmer.

Answer 11

This is the standard case of the using the right tool for the job. You do see logic programming in certain situations: they're usually called something like rule-based or expert systems.

In your computability theory course, you'll discuss the fact that all of these general purpose languages are effectively equivalent (from a mathematical standpoint). However, prototyping and long-term development are very different domains. So, it's entirely possible that, in a rule-based system, you might work out your ruleset using something like Prolog (if that works well for you) and then later implement the final system on your delivery platform (e.g., Java).

BTW, I always loved the way Prolog's answer to almost everything is "no."

Answer 12

There are calls in this discussion thread for a logic programming language that is truly declarative (statements can be made in any order), and that could be used for querying database as a replacement of SQL.

Datalog is what you are looking for. It's used in data integration, security applications and program analysis, for example.

Answer 13

The fact that you're starting on Prolog in your AI class should be a hint. AI hasn't caught on too well, either.

I remember back in '80 challenging a professor to demonstrate significant uses for AI (ok, "scoffing' would be a more accurate term, but I was younger then). He couldn't do it then, and today, I suspect he's teaching about 1/10th the applications for AI that he was raving about back then.

Maybe the same applies to Prolog. I don't remember the last time I've seen a company looking for Prolog experience. Maybe never, or maybe I saw it and ignored it.