10 ms·

Challenging LR Parsing

• SloopJon 6y ago
  This post is a next-level look at parsing interactively in an editor or IDE. Not only do you want it to be fast, but you want it to be resilient enough to provide useful output given incomplete input. Most of my poor attempts at parsing have been in the context of an offline tool. I find error handling hard enough using Yacc or Bison. I missed the post that this one responds to, so I'm going back and reading that discussion too: https://news.ycombinator.com/item?id=24480504 https://news.ycombinator.com/item?id=24480504
• matthewaveryusa 6y ago
  Well said, there certainly is a chasm between the theory and what practically works As a sidenote, if like myself you like to dabble first before jumping in to theory, I would recommend starting with Jisson -- it's so much fun and you'll learn a lot quickly!: https://zaa.ch/jison/try/ https://zaa.ch/jison/try/ https://nolanlawson.github.io/jison-debugger/ https://nolanlawson.github.io/jison-debugger/
• sreekotay 6y ago
  You know --- more and more --- I kinda like the REBOL approach --- forget precedence. Rather, just go L to R except where superceded by parens. Explicit and easy to understand. (Though I say that not having lived it in production, lol - so take with a grain of salt) Over time, readability/grokability tends to trump other factors...
  • ChrisSD 6y ago
    That kind of thinking inevitably leads to S-expressions.
    • zokier 6y ago
      Or RPN
      • sreekotay 6y ago
        Maybe - REBOL was interesting in the functions are all RL to precedence and operators are all LR precedence. It's clean(-ish) without being too verbose/messy. It's not perfect - just thought it was an interesting direction to ponder...
    • sreekotay 6y ago
      You might be right about that. Maybe we need more bracket flexibility or something. Or something built for TOOLING - nobody just writes freeform anymore?
  • jcranmer 6y ago
    Because parsing "a += b * c" as "(a += b) * c" is a wonderful, intuitive program. (This does assume that op= is an expression in your language.)
    • phkahler 6y ago
      Including precedence rules will take care of that.
      • masklinn 6y ago
        That… would be jcranmer's point, they're replying to a toplevel comment advocating for not having precedence.
    • sreekotay 6y ago
      Edit: Realized what I was tring to say: Intuitive might be the wrong thing to chase. Intuitive to who? :) Trivial programs are trivial. We might be stuck in the BCPL derived syntax that trade a false notion of intuitive for complexity of implentation and maintenance. (And I do mean might --- it's not hard not to think incrementally from the languages we know) --- but "inituitive" is EXACTLY the problem: I guess I was arguing we might consider searching for simple and predictable (even if that comes at the cost of slightly higher initial learning?)
      • sreekotay 6y ago
        e.g. maybe write it as "b * c += a" ? ... lol full on LR - no ambiguiity.
  • tlb 6y ago
    Smalltalk too. I really liked it. In practice, C compilers warn you if you try to take advantage of && having higher precedence than ||, so other than a few very simple rules wise programmers use extra parens anyway.
  • mamcx 6y ago
    I suspect is not that bad to force to group things manually (1 + (....)) but is truly precedence the biggest issue? I think even without that hand-coded parsers are still superior for error reporting and other factors?
    • jraph 6y ago
      I just wrote a parser in TypeScript that parses itself (for fun, and not quite done yet). Precedence and associativity is a painful aspect of parsing JavaScript… and probably C, Java and many programming languages that look like C. It took me a non negligible part of the time to think about how to handle this. With the presence of infix, suffix and prefix operators, and weird operators like new…(), ? … : …, and a lot of rules and exceptions, it's a lot of fun (for instance, yield is an operator in JavaScript. It takes an optional expression. However, 1 + yield is forbidden…). The grammar of JavaScript expressions is… not simple. And then, you have Automatic Semicolon Insertion, which forces you to backtrack and snapshot / restore the lexer state when consuming white characters EVERYWHERE (well, not literally everywhere, but still) because things may behave differently if there is a new line. And of course, new lines cannot happen in some constructs (there are a few exceptions to the general behavior of ASI), so you need to take care of this too. And then you have the comma operator that is allowed in some places taking an expression (especially in older constructs, and disallowed in other places (especially in newer constructs). So you need to handle this too. Parsing the rest is easy. It's just that there are a lot of things in the language so it's a bit long to write.
      • mamcx 6y ago
        Even with pratt parsing? Or using other method?
        • masklinn 6y ago
          I can only expect an other method as both are trivial using a pratt parser (though right-associativity still feels a bit weird).
          • 6y ago
  • deleted 6y ago
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  • dTal 6y ago
    The APL approach, which is mostly just simple RL-associative + grouping parentheses, works surprisingly well in practice - although that probably owes something to the simplicity of the syntax and the way idiomatic APL uses mainly the built-in 1- and 2-argument functions.
    • sreekotay 6y ago
      That was the thought: is there some non-BCPL like grammar that makes sense. Like... (not fully formed thought here) JSON is easy but verbose - YAML is nightmare by cleaner. Maybe we're looking at it wrong....
      • dTal 6y ago
        As a nearby comment observes, this leads to s-expressions. The connection between APL syntax and s-expressions is surprisingly close. Start with prefix notation and right-associativity, add grouping parens, and you have something that looks like lisp. The Nial language is an outright fusion of the two that's fun and enlightening to play with. https://en.wikipedia.org/wiki/Nial https://en.wikipedia.org/wiki/Nial https://github.com/danlm/QNial7 https://github.com/danlm/QNial7
  • hinkley 6y ago
    The standing rule has always been 5th grade math comprehension. As with most things, switching from too much to none is generally not an improvement. Perhaps languages should have 8 levels of precedence instead of 18.
• thesz 6y ago
  The chart parsing (CYK and friends) will produce exactly what is needed - as little contiguous completely parsed parts of program as it is possible. The syntax tree from example above will be different, but not much - parts "fn f(" and "struct { ... }" will be adjacent, not included in partial tree. There's a paper [1] about efficient parallel implementation possible. [1] https://jyp.github.io/pdf/PP.pdf https://jyp.github.io/pdf/PP.pdf Enjoy. ;)
• cwzwarich 6y ago
  > There’s a lot of literature about error recovery for LR parsers, how come academics haven’t figured this out already? I have a bold claim to make: error-recovery research in academia is focusing on a problem irrelevant for IDEs. Specifically, the research is focused on finding “minimal cost repair sequence” The parsing literature can be a bit difficult to navigate because the field is effectively dead for research purposes. However, other models of errors have been considered. Tim Wagner's PhD thesis is the best discussion of the application of LR parsing techniques in IDEs, and this paper extracted from it discusses error recovery in particular: http://harmonia.cs.berkeley.edu/papers/twagner-er.pdf http://harmonia.cs.berkeley.edu/papers/twagner-er.pdf In the non-interactive context, Richter's work on error recovery by substring parsing is also very interesting, in that it doesn't cause spurious errors when attempting to recover from errors, and it only depends on the language and not the grammar or parsing technique used: https://dl.acm.org/doi/10.1145/3916.4019 https://dl.acm.org/doi/10.1145/3916.4019 For LR grammars (and many other grammars), the necessary substring parsing can be done in linear time using variations of the GLR algorithm.
  • tlb 6y ago
    For error recovery, do any parsers have a notion of which tokens are more likely to be wrong? For example, in an actively edited C++ file, braces are more likely correct than angle brackets which are overloaded. So it's reasonable to auto-insert closing > if you see a } with an open <, but not reasonable to insert a } if you see a > with an open <.
    • ltratt 6y ago
      > For error recovery, do any parsers have a notion of which tokens are more likely to be wrong? lrpar has a %avoid_insert declaration which allows users to say "these token types are better avoided if there are other equivalently good repair sequences". It's simple, but it works well https://softdevteam.github.io/grmtools/master/book/errorrecovery.html#biasing-repair-sequences https://softdevteam.github.io/grmtools/master/book/errorreco...
      • maxbrunsfeld 6y ago
• Nelkins 6y ago
  Somehow F# manages to do a pretty good job (although maybe I've just gotten used to any quirks re: error recovery and messages). Anybody know if they're doing anything differently? https://github.com/fsprojects/FsLexYacc https://github.com/fsprojects/FsLexYacc https://github.com/dotnet/fsharp/blob/main/src/fsharp/lex.fsl https://github.com/dotnet/fsharp/blob/main/src/fsharp/lex.fs...
• leeoniya 6y ago
  relevant: parser recovery in codemirror6 (inspired by tree-sitter but adapted for IDEs): https://marijnhaverbeke.nl/blog/lezer.html https://marijnhaverbeke.nl/blog/lezer.html a standalone incremental JS parser with error recovery: https://github.com/escaya/escaya https://github.com/escaya/escaya
• deleted 6y ago
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• zamalek 6y ago
  He suggests pratt parsing as a production parser which, while unbelievably elegant, is recursive. Correctly recovering from a stack overflow (without leaking memory or resources) is, at best, a very hard problem worth researching in academia. Recursion is generally not production code, its usefulness does not extend much further than confusing university students. That's the hidden benefit of parsers that use DFAs: they generally don't use recursion. They will parse something until you run out of memory, not stack space (which is at least 3 orders of magnitude smaller than the heap).
  • ghusbands 6y ago
    It's trivial to replace the stack usage with a data structure. I've written a number of extended-Pratt parsers that don't use the stack. Also, a significant number of successful parsers have used recursion. As long as you successfully handle stack overflow and only hit it on pathological inputs, it works reasonably well.
    • zamalek 6y ago
      > It's trivial to replace the stack usage with a data structure. Interesting. Do you have any examples of that, reading the article actually had me thinking how that would be achieved.
      • flashmozzg 6y ago
        You could trivially (or rather, mechanically) emulate HW bounded stack with a heap-based stack collection (like std::stack).
• rstuart4133 6y ago
  Oh ... so that is what a Pratt parser is. Thanks for the nice clear writeup. But methinks it should be called a Wirth parser. Nicholas Wirth invented the same thing in the 1970's. He initially used it to parse his first language, Pascal. He drew them as Syntax diagrams: https://en.wikipedia.org/wiki/Syntax_diagram https://en.wikipedia.org/wiki/Syntax_diagram The loops that characterise Pratt are evident in those diagrams. You can read his book on Pascal here: https://www.research-collection.ethz.ch/bitstream/handle/20.500.11850/68910/eth-3059-01.pdf https://www.research-collection.ethz.ch/bitstream/handle/20.... At the end you will find the syntax diagrams for the entire language. The compiler was derived from those diagrams in the straight forward way described in the article. And everything the article says is true, if you are going to manually write a parser, they are by far the easiest starting point. And I don't doubt error messages and error recovery are much easier too. Parser generators don't give you the same flexibility to play with a parsers guts. But you do lose some things. A parser generator checks for ambiguities, something you can easily miss in a hand written parser. A parser generator also enforces a clean split between the parser and whatever you are doing with the syntax tree, whereas hand written parsers lead you toward a ball of mud where one lump of code has all the tasks intermixed. And there are LR parsers out there that support the loops and priorities that raw BNF productions make so hard. Like mine :D http://lrparsing.sourceforge.net/doc/html/ http://lrparsing.sourceforge.net/doc/html/