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The Simplicity of Prolog

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Re: The Simplicity of Prolog

#51

Post by tannhaeuser »

javcasas wrote: post #29
upghost wrote: post #23

Earlier quoted context omitted.

Respectfully have to disagree. Prolog has the best features for abstraction of any language I've used, I suspect it may be near an optimal fixed point for metaprogramming expression[3]. I will say the other side of that wall is breathtaking. Also not sure what you mean, CLP is a first class consideration in many Prologs, esp Scryer Prolog. Check these crazy demos out: [1] https://youtu.be/h5Xy4YjCZxM [2] https://yout…

My little pet peeve with prolog is the lack of context parameters (which can be thought as a type of abstraction). For example, imagine I'm writing a maze solver. The maze solver predicate receives obviously, but it has to pass as a parameter the maze again and again to all sub-predicates. There is no concept of "current maze", like in OO you would have with this.maze, or in Haskell you would do with a reader monad.…

Quantum Prolog has this feature where you can query against a dynamic database, not just the global default database; ie. where in regular Prolog (and in Quantum Prolog as well of course since it's full ISO) you query

    p(a).
    p(b).
    ?- p(X)
you can instead query

    KB = [ p(a), p(b) ], 
    KB ?- p(X)
introducing a clause-list as first parameter to "?-".

In the description [1], this is used to avoid destructive database manipulation via assertz/retract builtins, and thus to allow much more complex combinatorial planning problems and action post-conditions to be solved/optimized without resorting to ad-hoc hacks. But you can also use this for mere convenience in large knowledge graphs, and a technique very similar to it, albeit implemented in Prolog itself and not provided with native speed, has been used as a historic extension to Prolog DCG parsing (cf. definite-clause translation grammars by Dahl et al).

[1]: https://quantumprolog.sgml.net/container-planning-demo/part2...

Re: The Simplicity of Prolog

#52

Post by YeGoblynQueenne »

rscho wrote: post #46
billfruit wrote: post #40

Earlier quoted context omitted.

Not specifically aimed at this comment, but it looks like in most Prolog threads here, many commenters seems to plugging in Scryer Prolog, but perhaps, SWI is the most 'batteries included' and mature Prolog implementation for people not familiar with Prolog to try out .

There is a bit of... let's say friction between Markus Triska (Scryer) and Jan Wielemaker (SWI), I seem to remember. The SWI people are much less attached to ISO Prolog ( i.e. prone to experiment with non-ISO syntax), and some things that are fixed in the SWI implementation impeded the realization of M. Triska's projects. Generally, people tend to like the new and shiny but there's also a significant philosophical ga…

Wot. SLD Resolution is a "very dumb" theorem prover?

SLD Resolution is sound and refutation-complete and it is the basis not only of Prolog but also the most successful bunch of SAT solving algorithms in the last, dunno, several decades.

"Very dumb theorem prover"!

Re: The Simplicity of Prolog

#53

Post by YeGoblynQueenne »

jodrellblank wrote: post #50
billfruit wrote: post #40

Earlier quoted context omitted.

Not specifically aimed at this comment, but it looks like in most Prolog threads here, many commenters seems to plugging in Scryer Prolog, but perhaps, SWI is the most 'batteries included' and mature Prolog implementation for people not familiar with Prolog to try out .

SWI Prolog 7 added "X = Dict.key" syntax and that use of "." makes it fundamentally incompatible, ISO standard breaking, backwards incompatible to previous Prologs, sideways incompatible to other Prologs. This is a worse sin in Prolog than it seems at a glance, because one of the strengths Prolog has is code-is-data / data-is-code metaprogramming. That includes exporting code as Prolog terms (use cases you might use…

>> Code might say "connect_to_mongodb()" and you don't have mongodb in your system so you cannot run it, but you can read the code in as data and it will parse, just like reading in JSON which has a string mentioning some library you don't have; you can still introspect it and write reports like "what names does this data reference?", you can transform it and export it, or pass it through untouched. With SWI's new dot syntax the code might not parse at all, like an incompatible proprietary JSON syntax where you can't even import it. Code written 30 years ago which uses the dot in the old standard way might trigger SWI to try and read it in the Dict.key way and fail. Code exported from SWI 7 might include this syntax which other systems can't import.

So just write a translation layer, or add some flags to your Prolog so it can parse SWI's syntax. SWI does that (it has flags to adjust itself to other Prologs' syntax). Do other Prologs do that? Not to my knowledge, but why not? It's no big deal and certainly not a big enough deal to cleave a rift in the Prolog community, as if it wasn't small enough and dwindling already. I think some people have convinced themselves it's "better to be first in the village than second in the city" and they just don't want to work with others.

And as it sounds like you probably know, SWI is by far not the only Prolog to commit that cardinal sin of breaking portability. Basically every Prolog ever does that. Every single one. The ISO standard is just as opinionated as everybody else about what Prolog should be like (and btw ISO is not Edinburgh, let's not forget- and who came first, huh?) except it has delusions of grandeur because ISO.

I will take batteries included over nose-in-the-air "we have strict adherence to standards" any day.

Re: The Simplicity of Prolog

#54

Post by rscho »

YeGoblynQueenne wrote: post #52
rscho wrote: post #46

Earlier quoted context omitted.

There is a bit of... let's say friction between Markus Triska (Scryer) and Jan Wielemaker (SWI), I seem to remember. The SWI people are much less attached to ISO Prolog ( i.e. prone to experiment with non-ISO syntax), and some things that are fixed in the SWI implementation impeded the realization of M. Triska's projects. Generally, people tend to like the new and shiny but there's also a significant philosophical ga…

Wot. SLD Resolution is a "very dumb" theorem prover? SLD Resolution is sound and refutation-complete and it is the basis not only of Prolog but also the most successful bunch of SAT solving algorithms in the last, dunno, several decades. "Very dumb theorem prover"!

"Prolog is an efficient programming language because it is a very stupid theorem prover."

-- Richard O'Keefe

Re: The Simplicity of Prolog

#55

Post by derdi »

jodrellblank wrote: post #50
billfruit wrote: post #40

Earlier quoted context omitted.

Not specifically aimed at this comment, but it looks like in most Prolog threads here, many commenters seems to plugging in Scryer Prolog, but perhaps, SWI is the most 'batteries included' and mature Prolog implementation for people not familiar with Prolog to try out .

SWI Prolog 7 added "X = Dict.key" syntax and that use of "." makes it fundamentally incompatible, ISO standard breaking, backwards incompatible to previous Prologs, sideways incompatible to other Prologs. This is a worse sin in Prolog than it seems at a glance, because one of the strengths Prolog has is code-is-data / data-is-code metaprogramming. That includes exporting code as Prolog terms (use cases you might use…

> [...] might [...] might [...] might [...] might [...] might [...]

All of what you say is true, and yet practical applications that did break are somehow not talked about quite as much as hypothetical applications that might have broken. SWI could reuse infix dot precisely because it was universally considered bad style to use it in the old style, and hence was not used in the old style.

Which is not to say that I think the record syntax is particularly useful. But I wish not every Prolog discussion devolved into "Markus Triska fanpersons regurgitate walls of text about infix dot".

Re: The Simplicity of Prolog

#56

Post by [deleted] »

rscho wrote: post #54
YeGoblynQueenne wrote: post #52

Earlier quoted context omitted.

Wot. SLD Resolution is a "very dumb" theorem prover? SLD Resolution is sound and refutation-complete and it is the basis not only of Prolog but also the most successful bunch of SAT solving algorithms in the last, dunno, several decades. "Very dumb theorem prover"!

" Prolog is an efficient programming language because it is a very stupid theorem prover. " -- Richard O'Keefe

[deleted]

Re: The Simplicity of Prolog

#57

Post by YeGoblynQueenne »

rscho wrote: post #54
YeGoblynQueenne wrote: post #52

Earlier quoted context omitted.

Wot. SLD Resolution is a "very dumb" theorem prover? SLD Resolution is sound and refutation-complete and it is the basis not only of Prolog but also the most successful bunch of SAT solving algorithms in the last, dunno, several decades. "Very dumb theorem prover"!

" Prolog is an efficient programming language because it is a very stupid theorem prover. " -- Richard O'Keefe

So what's that now? An appeal to authority?

SLD-Resolution is sound and refutation-complete (and also complete with subsumption). If you think that O'Keefe is right to say that it's a "very dumb theorem prover" then explain to me why _you_ think so, not who said it.

Because I, too, can quote you authorities- and probably bigger than O'Keefe.

Re: The Simplicity of Prolog

#58

Post by rscho »

YeGoblynQueenne wrote: post #57
rscho wrote: post #54

Earlier quoted context omitted.

" Prolog is an efficient programming language because it is a very stupid theorem prover. " -- Richard O'Keefe

So what's that now? An appeal to authority? SLD-Resolution is sound and refutation-complete (and also complete with subsumption). If you think that O'Keefe is right to say that it's a "very dumb theorem prover" then explain to me why _you_ think so, not who said it. Because I, too, can quote you authorities- and probably bigger than O'Keefe.

Not an appeal to authority. Just showing that quote wasn't of my invention. I don't really get why you're so riled up. I'm not saying SLD resolution is dumb, just that Prolog stays simple by not including all the clever heuristics and circumstantial techniques of other solvers. Which is a big part of what makes Prolog a nice programming language. It's more a compliment than a critic, really!

Re: The Simplicity of Prolog

#59

Post by 2-3-7-43-1807 »

rscho wrote: post #47
2-3-7-43-1807 wrote: post #43

Earlier quoted context omitted.

The declarative expression of the problem is elegant but even if this toy example is successfully resolved, make it a little more complicated and you'll have the interpreter ping-pong between two states infinitely. As I see it. Prolog as a language and idea is great but the existing solvers are useless for any real problem ... or you'll have to resort to cuts and memorizing states and at least partially implement an…

Well, I mean... Prolog is an implementation of 1st order logic with syntax sugar. So basically, Prolog is incredible if your problem can be expressed within the framework, but less so if it can't. That's pretty much why there were extension attempts such as lambda Prolog.

you're simplifying the practicalities of prolog. there are many problems that you can express declaratively to the t. and yet many (all?) solvers aren't able to reduce it to a solution. the search isn't even successfully brute forcing - it will get stuck in some branch and switch endlessly between its leafs. and then it's up to you to figure that out and help the solver. even then prolog has its value but it fails at delivering the primary promise: you describe the problem, it finds a solution.

Re: The Simplicity of Prolog

#60

Post by YeGoblynQueenne »

rscho wrote: post #58
YeGoblynQueenne wrote: post #57

Earlier quoted context omitted.

So what's that now? An appeal to authority? SLD-Resolution is sound and refutation-complete (and also complete with subsumption). If you think that O'Keefe is right to say that it's a "very dumb theorem prover" then explain to me why _you_ think so, not who said it. Because I, too, can quote you authorities- and probably bigger than O'Keefe.

Not an appeal to authority. Just showing that quote wasn't of my invention. I don't really get why you're so riled up. I'm not saying SLD resolution is dumb, just that Prolog stays simple by not including all the clever heuristics and circumstantial techniques of other solvers. Which is a big part of what makes Prolog a nice programming language. It's more a compliment than a critic, really!

I'm not riled up, I even upvoted your OP, but it's uncouth to drop a quote without any context as a reply to a comment. Of course there's going to be misunderstandings.

I didn't remember that quote from O'Keefe. Prolog is indeed not trying to be smart and SLD-Resolution is dead simple - it's a sound and complete deductive inference system with a single rule. The reason Prolog is in turn so simple is because, thanks to the refutation-completeness of SLD-Resolution, you can implement it as a Depth-First Search for resolvents and then spam it until you get a result (or until you hit an infinite branch... oops). That's certainly orders of magnitude more simple than every other solver or automated theorem prover out there, like you say.

If that's what O'Keefe means, that Proolog is not trying to be smart, then OK, but that's not dumb. Every other solver tries to be smart and ends up having to solve an unsolvable problem. Who's dumb now then?

But maybe that's the compliment, I don't remember the context of O'Keefe's comment. Was it in the Craft of Prolog?

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