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> over the past 2 years of heavy Prolog use Oh, cool. Mind if I pick your brain a bit? Recently, there was an HN post[0] of a paper that makes a case against

> over the past 2 years of heavy Prolog use Oh, cool. Mind if I pick your brain a bit? Recently, there was an HN post[0] of a paper that makes a case against pure logic languages in favor of "functional logic" ones, which they exhibit with Curry[1]. The setup argument is that Prolog's specs backtracking, which strongly downlimits it from full SLD resolution, causing fatally sharp edges in real world usage. Being fairly naive to the paradigm, my interpretation is that writing real Prolog programs involves carefully thinking about and controlling the resolution algorithm, which feels very different than straight knowledge declaration. I believe cut/0 is the go-to example. Is that your experience with Prolog in practice? The real meat of the paper, however, is in its case that functional logic languages fully embed Prolog with almost 1-to-1 expressivity, while also providing more refined tools for externalizing knowledge about the intended search space of solutions. Thoughts? How are you using Prolog, logic, or constraint programming? What languages and tooling in this arena do you reach for? What is some of your most hard-earned knowledge? Any lesser-known, but golden, websites, books, or materials you'd like to share? Cheers! [0]:https://news.ycombinator.com/item?id=41816545 https://news.ycombinator.com/item?id=41816545 [1]:https://www.curry-language.org/ https://www.curry-language.org/

z5h 2y ago
So first, let's keep in mind that with no execution model, Prolog is still a "syntax" for Horn clauses. It's still a way to document knowledge. Add SLD resolution and we can compute. The paper (intentionally I presume) orders clauses of a simple predicate to illustrate (cause) a problem in Prolog. But what I actually find is the more time spent in Prolog, the more natural it is to express things in a way that is clear, logical and performant. As with any language/paradigm, there are a few gotchas to be experienced. But generally speaking, SLD resolution has never once been an obstacle (in the past 2 years) of coding. The general execution model of Prolog is pretty simple. The lack of functions actually makes meta-programming much clearer and simpler. A term is just data, unless it's stated as a goal. It's only a valid goal if you've already defined its meaning. So I'd be concerned that Curry gives up the simplicity of Prolog's execution model, and ease of meta-programming. I struggle with the lack of types in Prolog, but also know I can (at least in theory) use Prolog to solve correctness problems in Prolog code. I'm currently using SWI-Prolog. Performance is excellent, it has excellent high-level concurrency primitives[0] (when was the last time you pegged all your cores solving a problem?), and many libraries. I might be one of the few people who has committed to using the integrated editor (PceEmacs) despite being a Vim person. PceEmacs is just too good at syntax highlighting and error detection. At the same time, I'm a huge fan of Markus Triska. His Youtube[1] stuff is mind-expanding (watch all of it, even if you never write Prolog). He has an excellent book online[2]. I admire the way he explains and advances pure monotonic Prolog, and I appreciate the push for ISO conformance and his support for Prologs that that do the same (SWI is not on that list). If you want to learn Prolog, watch all of Markus Triska's videos, read his book, and learn what Prolog could be in a perfect world. Then download SWI-Prolog, and maybe break some rules while getting things done at a blazing speed. Eventually you'll gravitate to what makes sense for you. The Art of Prolog is a classic "must have". Clause and Effect is a good "hit the ground running" (on page 70 you're into symbolic differentiation via term rewriting). 0 https://www.swi-prolog.org/pldoc/man?section=thread https://www.swi-prolog.org/pldoc/man?section=thread 1 https://www.youtube.com/@ThePowerOfProlog https://www.youtube.com/@ThePowerOfProlog 2 https://www.metalevel.at/prolog https://www.metalevel.at/prolog
- microtonal 2y ago
  I'm currently using SWI-Prolog. Still checking every now and then if SICStus has open sourced. I used Prolog daily during my PhD and SICStus had such nice features. E.g. it could raise an exception when no more heap space can be allocated or when a 'call' would not finish within a given time. These features made it much easier to use Prolog in real-world systems (this was a parsing system and when parsing a very large corpus, this was highly preferable over simply crashing the interpreter). Maybe things have changed, but this wasn't possible with SWI at the time. Even worse, most C extensions would use malloc directly, making it impossible to track allocations done by extensions.
  - YeGoblynQueenne 2y ago
    >> Still checking every now and then if SICStus has open sourced. There used to be a free (though proprietary) version of Quintus, the predecessor of SICStus. It might still be around somewhere if you look hard enough.
- fao_ 2y ago
  > The Art of Prolog is a classic "must have". I figured it would be a good introduction to prolog, but to date there doesn't seem to be any prolog interpreter that lets me copy the things in the book to play with them?
- YeGoblynQueenne 2y ago
  >> So I'd be concerned that Curry gives up the simplicity of Prolog's execution model, and ease of meta-programming. That was my concern with the paper listed above also. Functional syntax is to my mind needlessly over-complicated. Types are useful but you can roll your own if you need them, and Prolog makes that easy enough. >> I'm currently using SWI-Prolog. Performance is excellent, it has excellent high-level concurrency primitives[0] (when was the last time you pegged all your cores solving a problem?), and many libraries. I might be one of the few people who has committed to using the integrated editor (PceEmacs) despite being a Vim person. PceEmacs is just too good at syntax highlighting and error detection. Hah! Hello fellow PCEmacs >> vim user :D Markus Triska's stuff is good and he's undeniably an expert in Prolog programming, I mean duh, but he and a couple of others have needlessly caused a rift in the Prolog community (mainly between themselves and everyone else) by being so stroppy about the ISO non-conformance of SWI-Prolog. I hope Markus is reading this. The Prolog community is very small and dwindling and we can't afford such drama. We need more Prolog compilers, yes, ISO conformance is good, yes, but SWI-Prolog is a robust and battle-tested implementation and it is the ISO standard that should be leaning on its experience of being a real-world Prolog used by real programmers and not just the other way around.
z5h 2y ago
> What is some of your most hard-earned knowledge? 1. If you find yourself straying too often from coding in relations, and instead coding in instructive steps, you're going to end up with problems. 2. Use DCGs to create a DSL for any high level operations performed on data structures. The bi-directionality of Prolog's clauses means you can use this DSL to generate an audit trail of "commands executed" when Prolog solves a problem for you, but you can also use the audit trail and modify it to execute those commands on other data.
- javcasas 2y ago
  How do you debug DCGs? I get "false." instead of "syntax error at line 23", which is unacceptable for bigger inputs. Also DCGs for high level operations? Do you mean "use DCGs to parse strings that contain instructions" or do you parse things other than strings with DCGs? I'm assuming you take the parsed instructions and run them through some kind of interpreter that does the execution and audit trail.
  - YeGoblynQueenne 2y ago
    (Not OP) >> How do you debug DCGs? I get "false." instead of "syntax error at line 23", which is unacceptable for bigger inputs. You need to include exception handling in your DCG rules. For example, in Prolog-like pseudocode: pink_apples([A|As]) --> [A], { red_apple(A), throw(error(type_error(pink_apple,red_apple),_)) }. % Raises type error: ERROR: Type error: `pink_apple' expected, found `red_apple' (an atom) ERROR: In: ERROR: [12] throw(error(type_error(pink_apple,red_apple),_10070)) Called from a source file the error output will list the line in the source file where the exception was raised. There are more tools to debug the error: https://www.swi-prolog.org/pldoc/man?section=exception https://www.swi-prolog.org/pldoc/man?section=exception DCGs parse lists, not strings, as such. So the input can be anything you can put in the form of a list.
    - upghost 2y ago
      And this is the person who saved me from nearly dropping Prolog!!

z5h 2y ago

So first, let's keep in mind that with no execution model, Prolog is still a "syntax" for Horn clauses. It's still a way to document knowledge. Add SLD resolution and we can compute. The paper (intentionally I presume) orders clauses of a simple predicate to illustrate (cause) a problem in Prolog. But what I actually find is the more time spent in Prolog, the more natural it is to express things in a way that is clear, logical and performant. As with any language/paradigm, there are a few gotchas to be experienced. But generally speaking, SLD resolution has never once been an obstacle (in the past 2 years) of coding. The general execution model of Prolog is pretty simple. The lack of functions actually makes meta-programming much clearer and simpler. A term is just data, unless it's stated as a goal. It's only a valid goal if you've already defined its meaning. So I'd be concerned that Curry gives up the simplicity of Prolog's execution model, and ease of meta-programming. I struggle with the lack of types in Prolog, but also know I can (at least in theory) use Prolog to solve correctness problems in Prolog code. I'm currently using SWI-Prolog. Performance is excellent, it has excellent high-level concurrency primitives[0] (when was the last time you pegged all your cores solving a problem?), and many libraries. I might be one of the few people who has committed to using the integrated editor (PceEmacs) despite being a Vim person. PceEmacs is just too good at syntax highlighting and error detection. At the same time, I'm a huge fan of Markus Triska. His Youtube[1] stuff is mind-expanding (watch all of it, even if you never write Prolog). He has an excellent book online[2]. I admire the way he explains and advances pure monotonic Prolog, and I appreciate the push for ISO conformance and his support for Prologs that that do the same (SWI is not on that list). If you want to learn Prolog, watch all of Markus Triska's videos, read his book, and learn what Prolog could be in a perfect world. Then download SWI-Prolog, and maybe break some rules while getting things done at a blazing speed. Eventually you'll gravitate to what makes sense for you. The Art of Prolog is a classic "must have". Clause and Effect is a good "hit the ground running" (on page 70 you're into symbolic differentiation via term rewriting). 0 https://www.swi-prolog.org/pldoc/man?section=thread https://www.swi-prolog.org/pldoc/man?section=thread 1 https://www.youtube.com/@ThePowerOfProlog https://www.youtube.com/@ThePowerOfProlog 2 https://www.metalevel.at/prolog https://www.metalevel.at/prolog

microtonal 2y ago
I'm currently using SWI-Prolog. Still checking every now and then if SICStus has open sourced. I used Prolog daily during my PhD and SICStus had such nice features. E.g. it could raise an exception when no more heap space can be allocated or when a 'call' would not finish within a given time. These features made it much easier to use Prolog in real-world systems (this was a parsing system and when parsing a very large corpus, this was highly preferable over simply crashing the interpreter). Maybe things have changed, but this wasn't possible with SWI at the time. Even worse, most C extensions would use malloc directly, making it impossible to track allocations done by extensions.
- YeGoblynQueenne 2y ago
  >> Still checking every now and then if SICStus has open sourced. There used to be a free (though proprietary) version of Quintus, the predecessor of SICStus. It might still be around somewhere if you look hard enough.
fao_ 2y ago
> The Art of Prolog is a classic "must have". I figured it would be a good introduction to prolog, but to date there doesn't seem to be any prolog interpreter that lets me copy the things in the book to play with them?
YeGoblynQueenne 2y ago
>> So I'd be concerned that Curry gives up the simplicity of Prolog's execution model, and ease of meta-programming. That was my concern with the paper listed above also. Functional syntax is to my mind needlessly over-complicated. Types are useful but you can roll your own if you need them, and Prolog makes that easy enough. >> I'm currently using SWI-Prolog. Performance is excellent, it has excellent high-level concurrency primitives[0] (when was the last time you pegged all your cores solving a problem?), and many libraries. I might be one of the few people who has committed to using the integrated editor (PceEmacs) despite being a Vim person. PceEmacs is just too good at syntax highlighting and error detection. Hah! Hello fellow PCEmacs >> vim user :D Markus Triska's stuff is good and he's undeniably an expert in Prolog programming, I mean duh, but he and a couple of others have needlessly caused a rift in the Prolog community (mainly between themselves and everyone else) by being so stroppy about the ISO non-conformance of SWI-Prolog. I hope Markus is reading this. The Prolog community is very small and dwindling and we can't afford such drama. We need more Prolog compilers, yes, ISO conformance is good, yes, but SWI-Prolog is a robust and battle-tested implementation and it is the ISO standard that should be leaning on its experience of being a real-world Prolog used by real programmers and not just the other way around.

z5h 2y ago

> What is some of your most hard-earned knowledge? 1. If you find yourself straying too often from coding in relations, and instead coding in instructive steps, you're going to end up with problems. 2. Use DCGs to create a DSL for any high level operations performed on data structures. The bi-directionality of Prolog's clauses means you can use this DSL to generate an audit trail of "commands executed" when Prolog solves a problem for you, but you can also use the audit trail and modify it to execute those commands on other data.

javcasas 2y ago
How do you debug DCGs? I get "false." instead of "syntax error at line 23", which is unacceptable for bigger inputs. Also DCGs for high level operations? Do you mean "use DCGs to parse strings that contain instructions" or do you parse things other than strings with DCGs? I'm assuming you take the parsed instructions and run them through some kind of interpreter that does the execution and audit trail.
- YeGoblynQueenne 2y ago
  (Not OP) >> How do you debug DCGs? I get "false." instead of "syntax error at line 23", which is unacceptable for bigger inputs. You need to include exception handling in your DCG rules. For example, in Prolog-like pseudocode: pink_apples([A|As]) --> [A], { red_apple(A), throw(error(type_error(pink_apple,red_apple),_)) }. % Raises type error: ERROR: Type error: `pink_apple' expected, found `red_apple' (an atom) ERROR: In: ERROR: [12] throw(error(type_error(pink_apple,red_apple),_10070)) Called from a source file the error output will list the line in the source file where the exception was raised. There are more tools to debug the error: https://www.swi-prolog.org/pldoc/man?section=exception https://www.swi-prolog.org/pldoc/man?section=exception DCGs parse lists, not strings, as such. So the input can be anything you can put in the form of a list.
  - upghost 2y ago
    And this is the person who saved me from nearly dropping Prolog!!