| I guess my use of the term "stupid" made my post sound more pejorative than I meant it to be. I have enjoyed writing Prolog programs on occasion. Some of my best friends are Prolog programmers! But I was trying to explain why Prolog is not the declarative language that we all hope for (well, a lot of us do, anyway) and that it initially appeared, to some, to be. And I think the answer to that is exactly what I said. That doesn't mean it's not a useful tool! > [The cut is] not there to "exploit" anything. The fact that it can be exploited is troublesome, but so is all sorts of stuff in programming, like buffer overruns or sql injection. Whoa! "Exploit" is a general term; it doesn't just refer to security vulnerabilities. Dictionary.com offers this definition for the verb: "to utilize, especially for profit; turn to practical account: to exploit a business opportunity". (The definition I would offer is "to take advantage of", which can mean to make use of a vulnerability, but doesn't have to: you can take advantage of a convenient feature of a program.) So when I say people "exploit" the search strategy by using the cut, I mean only that they gain benefit from it; if the search strategy were not so well-defined, the cut would be unusable. > Why does your algorithm have to be exponential if it's in Prolog? What the hell are you on about? If you try to write a significant Prolog program without understanding the search strategy, as if it really were a declarative language, your program will wind up being exponential or worse. Merely throwing hardware at this problem won't fix it. That's all I'm saying. I agree I didn't say it well the first time. |
:P
>> Whoa! "Exploit" is a general term; it doesn't just refer to security vulnerabilities.
Of course. I didn't mean it in that sense. Buffer overruns can be exploited for non-malicious purposes also. Basically, any quirk of any language or architecture, or really any system, can be exploited for whatever purpose. Let us pause for a second and reflect on The Story of Mel [1] as handed down to us by our forefathers and mothers of old.
So what I'm saying is that the cut is there to stop unnecessary backtracking and not to change the behaviour of your program, but of course there's nothing stopping some poor fool from using it in the latter manner, as indeed I've done too many times in the past myself, being the particularly foolish fool that I am. Prolog programmers even have a term for the two types of cut: "green" and "red", respectively. Any Prolog tutorial or bit of documentation will tell you that the cut is to be used sparingly and only to stop unnecessary backtracking, not to make your program change behaviour. The good ones go to some lengths to give you examples of what happens if you don't stick to the advice. If you screw up after that you have nobody to blame but your inexperience, and that gets better with time.
So the cut is dangerous, but that's all it is.
I learned to program with C# oddly, at the University of Hull in the UK. There, our programming 101 tutor explained why C#, or, hell, Java- and not C or C++. His point was that those other languages are more powerful but because of their power they give you the tools to cut yourself to pieces. He used a metaphor involving a chainsaw, that I don't remember very well (that was back in 2005, 11 years now). But basically his point was that a chainsaw is powerful but you can really hurt yourself with it if you don't know how to use it, so you don't want to start learning ... er, forestry? with one.
Well, think of Prolog as a particularly dangerous language. I will admit this: Prolog is not your friend. It doesn't even try to make things easy for you. It's not an "easy language to pick up and start hacking in immediately". It takes a long time to learn it well and it keeps surprising you in very nasty ways for a very long time.
But- once you've learned it pays back in spades and you can do stuff in it that you can really not do in any other language, like declare grammars and then execute them, and use them to parse strings and generate strings. Or run your code backwards. Or do list appends in single-steps and so on.
Well. Apologies, that got a bit out of hand.
[1] http://www.catb.org/jargon/html/story-of-mel.html