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>> Just try writing (without hacks/cuts) a run-length encoder that produces only one answer ([9,9,9,8,8,7] -> [[9,3], [8,2], [7,1]]) and works forwards and back

>> Just try writing (without hacks/cuts) a run-length encoder that produces only one answer ([9,9,9,8,8,7] -> [[9,3], [8,2], [7,1]]) and works forwards and backwards and then tell me the execution model is not a limitation. But, why not use the cut? It's a feature of the language and it's there exactly so you don't have to bust your head trying to find a way to do things the hard way. I honestly don't understand what's the fuss with the poor cut. If you want an efficient language that runs on a computer then you have to be pragmatic and recognise that concessions have to be made to accommodate the limitations of the hardware. If you want something pure and ideal, then you should probably not be trying to do that on a computer. You should probably not be trying to do it at all. There are no pure formal systems, or if there are, we can't explain or understand them because they can't communicate with the external world (hey, no side effects, OK?). See Haskell, for instance. The article above says something really strange about Haskell, that it: >> ... finally broke through the wall of pure-functional programs being seen as ivory-tower languages that were impractical, or at least really awkward, for any system that needed to do I/O or interact with users Which to me is insane. I think it's talking about monads. Monads, as an escape from the ivory tower, impractical, or at least awkward formalisms of... what, LISP? I mean for the love of dogs. Prolog is a balanced language. People complain about it because it's not unbalanced, that's the problem. If it were 100% declarative and nobody could program anything in it, it would have legions of dotting admirers praising it for its perfection, and whining because they have to do all their work in languages that are hardly 10% declarative, like Rust, or Python or Ruby (and I'm being generous). People have to get their head around this: our computer science is primitive and it is full of holes, dark pits of despair and undecidability, intractability, and terminal inefficiency. To be practical, a language has to give up a little part of its true, pure self, that it could achieve in a perfect world. So Prolog has the cut, and you can use it to control its backtracking, which you need because it's implemented as a depth-first search, because that's efficient and convenient. So use the cut, it's a feature.

superdisk 4y ago
The thing about the cut is that it makes debugging extremely difficult because it (nearly always) completely nullifies the usual semantics of how you expect predicates to work. Stuff will seem to work and then break in mysterious ways. Also I've personally never found a valid use for them, there always seems to be a better, more logical alternative for any potential use case I've found. It's kind of unfortunate IMO that most oldskool Prolog code out there is peppered with cuts and (->)/2 since it propagates a ton of bad habits. Regarding the RLE problem, I posted a logical solution without cuts: https://news.ycombinator.com/item?id=35632344 https://news.ycombinator.com/item?id=35632344
- YeGoblynQueenne 4y ago
  I don't remember having any problems with debugging programs with cuts. But it should be said that I restrict myself to green cuts [1]. There are usual ways to avoid those, but not without making the code more complicated than it needs to be. Debugging soft cuts is more of a problem. That's because soft cuts don't appear in the tracer and it's difficult to follow program flow when it doesn't directly correspond with the source code as-written. Or at least that's the way the tracer behaves in SWI-Prolog which I use almost exclusively these days (plus Visual Prolog, but let's not talk about that). There may be a setting to configure it. I generally don't use soft cuts, I find them a bit clutter-y anyway. There's a run-length encoding program in the "99 Prolog problems" and it also doesn't use cuts: https://www.ic.unicamp.br/~meidanis/courses/mc336/2009s2/prolog/problemas/ https://www.ic.unicamp.br/~meidanis/courses/mc336/2009s2/pro... _______________ [1] A "green cut" is one that does not change the program's Least Herbrand Model (LHM). Or, in plain English, a green cut does not change the set of queries to the program, that succeed. A "red cut" is one that does change the program's LHM. A green cut only stops unproductive backtracking, or backtracking that diverts a proof down infinite branches. A red cut is one that cuts finite success branches. You should never use red cuts, and you should never run on a barge :P
  - ghusbands 4y ago
    > There's a run-length encoding program in the "99 Prolog problems" and it also doesn't use cuts I tested them. p10, p11 and p13 all infloop when you ask what encodes to [[3,7], [2,8], {what 9 encoded to}]. p12 claims that [7,7,7,8,8,9] and [7,7,7,8,8,[9,1]] are valid encodings for [7,7,7,8,8,9] and then gives "Arguments are not sufficiently instantiated". As I implied in my original comment, there are plenty of examples out there, none of which actually work in both directions, without metaprogramming. Using clpfd seems to work well in all directions, which is neat, but that's a solver built atop prolog, rather than prolog itself.
    - YeGoblynQueenne 4y ago
      Right, that's interesting. Did you try tabling the looping program? That's one way to avoid (some) looping behaviour. Note that p10 - p13 are meant to be called in mode (+,?) according to their comments, so (-,+) (or (-,?)!) is asking for trouble. I can see you were looking for a program that would run in arbitrary mode that is also non- or semi-deterministic ("gives one answer". Did you mean more like functional?) but I replied to another comment just to point to existing RLE programs, not to give examples of what you were asking. The instantiation error is the kind of thing that you'd use a "green" cut to avoid. The cut in that case would avoid unnecessary backtracking and not change the results of the program. I don't know about the malformed result. I'd have to pick at it a bit and I don't want to do this now. I also don't want to try and write a "pure" version, first because I don't think it satisfies any real-world need, and second because I already have a version that seems to work OK and uses cuts. I wrote it several years ago (possibly around 2010 ish). I'm copying it here keeping the idiosyncracies of my coding and commenting style at the time. %! run_length_encoding(+List, -Run_length_encoding) is det. % % Run_length_encoding is a list of all key-value pairs where each % key is an element in List and value the number of consecutive % repetitions of that element up to the first differing element. % % For example: % == % ?- run_length_encoding([a,a,a,b,b,b,b,b,b,c,c,c,a,a,f], RLE). % RLE = [a-3, b-6, c-3, a-2, f-1]. % == % run_length_encoding([], []-0):- !. run_length_encoding([H|List], Run_length_encoding):- run_length_encoding(List, [H-1], Run_length_encoding). % Last element in input list or single-element input list. run_length_encoding([], [C-F|Fs], Run_length_encoding):- reverse([C-F|Fs], Run_length_encoding). % Run of N consecutive identical elements run_length_encoding([C|Cs],[C-F|Fs], Acc):- ! % Orange ish; backtracking will produce successive % counts of repetitions of C from different indices in the list % I think. ,F_ is F + 1 ,run_length_encoding(Cs, [C-F_| Fs], Acc). % End of run of N consecutive identical elements. run_length_encoding([C|Cs], Fs, Acc):- run_length_encoding(Cs,[C-1|Fs], Acc). I give this as an example of a simple, short program you can write in Prolog to accomplish a simple, useful task, while using the cut to make your life easier, which is what I'm arguing about here. Note the uncertainty I had at the time about the use of the cut in the second auxiliary clause. I've left that comment in, in the interest of being honest about the difficulties in learning how to use the cut correctly. As I say, that was written many years ago. I'm not arguing that it's easy to learn to use the cut, I'm just saying that it makes your life easier once you've learned how to use it. I should probably have made that more clear in my comment. Please let me know if my code above breaks. I haven't tested it in ages. But please respect the documented call modes and determinism :) Edit: if you're wondering about the use of reverse/2, the goal is to simplify debugging; if you put accumulators in the head, you don't see their instantiations until recursion unrolls, so you don't know what's going on. Edit 2: To further clarify, as documented, the program only runs in one mode, (+,-). It was an auxiliary for another program that calculates the Shannon entropy of a string (tokenised as a list of characters) so there was no need for other modes. Not every Prolog program needs to run in all possible modes. And even when one does, it's often simpler to write multiple auxiliaries for additional modes. And why not do the simpler thing, when you can? The point is to write programs that have the desired behaviour. The constant complaint about Prolog (in this discussion also) is that it makes it hard to do simple things, not that it doesn't look pretty. So we should talk about how easy it is to do simple things, not how hard it is to write pretty programs.
ghusbands 4y ago
The sibling comment has a good general answer. In this particular case, if you use cut, it seems like it works, but then you try to solve rle([9,9,A,B,8,7], [[9,X], [8,Y], [7,1]]) or other bidirectional things and you find the cuts prevent you from finding some answers. Generally, I find that cuts (and implication and such) are prone to only work in the specific cases the programmer considered and to cause missed solutions, elsewhere.
- YeGoblynQueenne 4y ago
  Like I say above, that's the difference between using a green cut vs. a red cut. See also my link to the RLE version in "99 Prolog Problems". So far it's been my experience that it's very hard to find a Prolog program that can't be written without a cut.

superdisk 4y ago

The thing about the cut is that it makes debugging extremely difficult because it (nearly always) completely nullifies the usual semantics of how you expect predicates to work. Stuff will seem to work and then break in mysterious ways. Also I've personally never found a valid use for them, there always seems to be a better, more logical alternative for any potential use case I've found. It's kind of unfortunate IMO that most oldskool Prolog code out there is peppered with cuts and (->)/2 since it propagates a ton of bad habits. Regarding the RLE problem, I posted a logical solution without cuts: https://news.ycombinator.com/item?id=35632344 https://news.ycombinator.com/item?id=35632344

YeGoblynQueenne 4y ago
I don't remember having any problems with debugging programs with cuts. But it should be said that I restrict myself to green cuts [1]. There are usual ways to avoid those, but not without making the code more complicated than it needs to be. Debugging soft cuts is more of a problem. That's because soft cuts don't appear in the tracer and it's difficult to follow program flow when it doesn't directly correspond with the source code as-written. Or at least that's the way the tracer behaves in SWI-Prolog which I use almost exclusively these days (plus Visual Prolog, but let's not talk about that). There may be a setting to configure it. I generally don't use soft cuts, I find them a bit clutter-y anyway. There's a run-length encoding program in the "99 Prolog problems" and it also doesn't use cuts: https://www.ic.unicamp.br/~meidanis/courses/mc336/2009s2/prolog/problemas/ https://www.ic.unicamp.br/~meidanis/courses/mc336/2009s2/pro... _______________ [1] A "green cut" is one that does not change the program's Least Herbrand Model (LHM). Or, in plain English, a green cut does not change the set of queries to the program, that succeed. A "red cut" is one that does change the program's LHM. A green cut only stops unproductive backtracking, or backtracking that diverts a proof down infinite branches. A red cut is one that cuts finite success branches. You should never use red cuts, and you should never run on a barge :P
- ghusbands 4y ago
  > There's a run-length encoding program in the "99 Prolog problems" and it also doesn't use cuts I tested them. p10, p11 and p13 all infloop when you ask what encodes to [[3,7], [2,8], {what 9 encoded to}]. p12 claims that [7,7,7,8,8,9] and [7,7,7,8,8,[9,1]] are valid encodings for [7,7,7,8,8,9] and then gives "Arguments are not sufficiently instantiated". As I implied in my original comment, there are plenty of examples out there, none of which actually work in both directions, without metaprogramming. Using clpfd seems to work well in all directions, which is neat, but that's a solver built atop prolog, rather than prolog itself.
  - YeGoblynQueenne 4y ago
    Right, that's interesting. Did you try tabling the looping program? That's one way to avoid (some) looping behaviour. Note that p10 - p13 are meant to be called in mode (+,?) according to their comments, so (-,+) (or (-,?)!) is asking for trouble. I can see you were looking for a program that would run in arbitrary mode that is also non- or semi-deterministic ("gives one answer". Did you mean more like functional?) but I replied to another comment just to point to existing RLE programs, not to give examples of what you were asking. The instantiation error is the kind of thing that you'd use a "green" cut to avoid. The cut in that case would avoid unnecessary backtracking and not change the results of the program. I don't know about the malformed result. I'd have to pick at it a bit and I don't want to do this now. I also don't want to try and write a "pure" version, first because I don't think it satisfies any real-world need, and second because I already have a version that seems to work OK and uses cuts. I wrote it several years ago (possibly around 2010 ish). I'm copying it here keeping the idiosyncracies of my coding and commenting style at the time. %! run_length_encoding(+List, -Run_length_encoding) is det. % % Run_length_encoding is a list of all key-value pairs where each % key is an element in List and value the number of consecutive % repetitions of that element up to the first differing element. % % For example: % == % ?- run_length_encoding([a,a,a,b,b,b,b,b,b,c,c,c,a,a,f], RLE). % RLE = [a-3, b-6, c-3, a-2, f-1]. % == % run_length_encoding([], []-0):- !. run_length_encoding([H|List], Run_length_encoding):- run_length_encoding(List, [H-1], Run_length_encoding). % Last element in input list or single-element input list. run_length_encoding([], [C-F|Fs], Run_length_encoding):- reverse([C-F|Fs], Run_length_encoding). % Run of N consecutive identical elements run_length_encoding([C|Cs],[C-F|Fs], Acc):- ! % Orange ish; backtracking will produce successive % counts of repetitions of C from different indices in the list % I think. ,F_ is F + 1 ,run_length_encoding(Cs, [C-F_| Fs], Acc). % End of run of N consecutive identical elements. run_length_encoding([C|Cs], Fs, Acc):- run_length_encoding(Cs,[C-1|Fs], Acc). I give this as an example of a simple, short program you can write in Prolog to accomplish a simple, useful task, while using the cut to make your life easier, which is what I'm arguing about here. Note the uncertainty I had at the time about the use of the cut in the second auxiliary clause. I've left that comment in, in the interest of being honest about the difficulties in learning how to use the cut correctly. As I say, that was written many years ago. I'm not arguing that it's easy to learn to use the cut, I'm just saying that it makes your life easier once you've learned how to use it. I should probably have made that more clear in my comment. Please let me know if my code above breaks. I haven't tested it in ages. But please respect the documented call modes and determinism :) Edit: if you're wondering about the use of reverse/2, the goal is to simplify debugging; if you put accumulators in the head, you don't see their instantiations until recursion unrolls, so you don't know what's going on. Edit 2: To further clarify, as documented, the program only runs in one mode, (+,-). It was an auxiliary for another program that calculates the Shannon entropy of a string (tokenised as a list of characters) so there was no need for other modes. Not every Prolog program needs to run in all possible modes. And even when one does, it's often simpler to write multiple auxiliaries for additional modes. And why not do the simpler thing, when you can? The point is to write programs that have the desired behaviour. The constant complaint about Prolog (in this discussion also) is that it makes it hard to do simple things, not that it doesn't look pretty. So we should talk about how easy it is to do simple things, not how hard it is to write pretty programs.

ghusbands 4y ago

The sibling comment has a good general answer. In this particular case, if you use cut, it seems like it works, but then you try to solve rle([9,9,A,B,8,7], [[9,X], [8,Y], [7,1]]) or other bidirectional things and you find the cuts prevent you from finding some answers. Generally, I find that cuts (and implication and such) are prone to only work in the specific cases the programmer considered and to cause missed solutions, elsewhere.

YeGoblynQueenne 4y ago
Like I say above, that's the difference between using a green cut vs. a red cut. See also my link to the RLE version in "99 Prolog Problems". So far it's been my experience that it's very hard to find a Prolog program that can't be written without a cut.