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Parser combinator library in C
- Jacobi 14y agoI don't know why it uses a garbage collector when simple reference counting would suffice. Also I hate libraries with global states/variables like this one ...
- hythloday 14y agoHow would you implement refcounting in the presence of longjmp? I don't see a graceful way to do it.
- simias 14y agoI don't like C libraries that use longjmp for error handling. It's like killing a fly with a bazooka. There are some cases when longjmp is useful of course, but If you want proper exceptions, use a language that supports them.
- deleted 14y ago[deleted]
- rcfox 14y agoThe OP is doing awesome stuff and sharing it with the world, and you want to criticize an implementation detail? Bikeshedding like this isn't useful to anyone.
- Jacobi 14y agoI appreciate the OP's efforts, certainly sharing code is a good thing. And one of the benefits of the sharing is to get a lot of feedback. Criticizing implementation details is important to spot, discuss and hopefully fix potential problems.
- hythloday 14y agoBut the only actionable advice you can take away from the criticism is "rewrite it in C++". I don't think that's in any way useful and I don't think it should be made.
- cantankerous 14y agoThis. The headline was a parser combinator in C. The author is working with what he's got. Can't argue with the hangups that come with it, unless he makes some claim that his language choice is somehow superior to another, which I don't see happening here.
- dllthomas 14y agoRight - "Don't use longjmp, do X in C instead" would have been a useful comment (particularly with example code). "Don't use longjmp, use C++ instead" is much less useful.
- simias 14y agoI agree that my comment was a bit harsh, but that's not what I said. I said that if he wanted exceptions he shouldn't have used C. As for the alternatives, well return values never killed anyone, did they? You can use gotos to simplify the control flow within the functions. Other than that, there really isn't much to explain. I don't like longjmp because I never expect them in C. I never think "hey, the control flow might jump to some point 20 stack frames above at any moment when I enter this library". And then I use mutexes. Cue the drama. You can't even protect yourself from the stack unwinding with handler-case or try ... catch/except.
- ori_b 14y agoIf I release code, having it criticized is a benefit. I want to learn from others, hear what they think of the design decisions, what they would have done differently, and what they think I did stupidly. If you criticize my work, you're doing me a favor.
- Someone 14y agoI am not sure I grok the code, but I think something along the following lines would work: - a 'recovery' list of (object pointer, old refcount) pairs - whenever you allocate something, add (object pointer, 0) to the list - when you update a reference count, check whether the object is in the list. If it is not, add it to the list, thus remembering the original refcount - whenever the code decreases reference count to zero, add the object to a 'to be deleted' list (this could possibly be a linked list chained through the 'refcount' fields; the old refcount will be in the recovery list) - when the parser reaches toplevel without longjmp, clear the first list, and delete any objects in the 'to be deleted' list. - when a longjmp occurs, walk the list and reset reference counts. For zero 'old refcounts', delete the object Elegant? Not really, but with proper macros/functions, it should not be much less elegant than reference counting on its own.
- silentbicycle 14y agoThat's pretty close to Deutsch-Bobrow deferred referenced counting. If you decide to implement it, you may want to read their paper about the technique.
- wbhart 14y agoI will likely remove the global state at some point, but the garbage collector is not something I plan to implement myself.
- bsaul 14y agocould anyone explain the use for this library ?
- wbhart 14y agoThe major parser libraries that I could find don't allow adding syntax at runtime or to add operators to the expression hierarchy at runtime. Many (though not all) of them also make it difficult to do custom error reporting. These were my major motivations.
- _delirium 14y agoAh right, interesting use-case. I usually think of combinator parsing's advantages versus table-driven parsing (e.g. LALR) being primarily its expressiveness (and amenability to a functional-programming style), but it has a nice runtime-modifiable aspect as well. I wonder how hard it'd be to build a runtime-modifiable table-driven parser, though. Assuming table compilation is fast enough, the most straightforward approach might be to just link in the grammar-compilation code so it's available at runtime, and rebuild the table each time a modification is made. I don't think you'd want to be doing complex incremental surgery on a table-driven parser, but you might not need to.
- alok-g 14y agoDoesn't Boost.Spirit [1] allow adding syntax and operators at runtime? (Seriously asking; have never used Spirit) [1] http://boost-spirit.com/home/ http://boost-spirit.com/home/
- _stephan 14y agoFParsec's OperatorPrecedenceParser component[1] does support adding (and removing) operators to the expression grammar at runtime. [1] http://www.quanttec.com/fparsec/reference/operatorprecedenceparser.html http://www.quanttec.com/fparsec/reference/operatorprecedence...
- beagle3 14y agoNot C .. but are you familiar with Ometa (and its variations, Ometa/JS, Ometa#, PyMeta, etc?) http://tinlizzie.org/ometa/ http://tinlizzie.org/ometa/ It's a turing-complete parser specification language with an interpreter that is very close to being a packrat parser, which embraces adding and extending syntax (in a more general way than your library does, if I understand correctly). Implementing the same idea in C should be possible. I have no idea why you want to add syntax at runtime, but the Ometa model might fit your plans better.
- jheriko 14y agonice, but would be better to have something simpler that doesn't rely on various libraries and features that are typically forbidden in high performance environments. incidentally almost every memory leak I deal with comes from not being allowed to allocate my own memory and having to fiddle around with gc and refcount rubbish. I have written whole non-trivial games with no detectable leaks.
- jheriko 14y ago(without stopping to check for leaks half way through)
- wbhart 14y agoCould you provide some more information about "features that are typically forbidden in high performance environments". Probably you want a parser expression grammar with a packrat parser if you really want speed anyway. At the moment I am not smart enough to see how to implement combinators in C in an elegant way without GC of some form. But I'll think about it. Maybe something will occur to me. Patches are welcome of course.