Comment by bmn_ | Hacker News Reader

Comment by bmn_ | Hacker News Reader

LR parsers like yacc are obsoleted by Earley parsers, which Cox apparently didn't know about in 2010. Quoting <http://loup-vaillant.fr/tutorials/earley-parsing/what-and-wh...:

    The biggest advantage of Earley Parsing is its accessibility.
    Most other tools such as parser generators, parsing
    expression grammars, or combinator libraries feature
    restrictions that often make them hard to use. Use the wrong 
    kind of grammar, and your PEG will enter an infinite loop.
    Use another wrong kind of grammar, and most parser
    generators will fail. To a beginner, these restrictions feel 
    most arbitrary: it looks like it should work, but it doesn't.
    There are workarounds of course, but they make these tools 
    more complex.

    Earley parsing Just Works™.

    On the flip side, to get this generality we must sacrifice 
    some speed. Earley parsing cannot compete with speed demons 
    such as Flex/Bison in terms of raw speed. It's not that bad, 
    however:
    • Earley parsing is cubic in the worst cases, which is the 
    state of the art (and possibly the best we can do). The speed 
    demons often don't work at all for those worst cases. Other 
    parsers are prone to exponential combinatorial explosion.
    • Most simple grammars can be parsed in linear time.
    • Even the worst unambiguous grammars can be parsed in 
    quadratic time.

    My advice would be to use Earley parsing by default, and only 
    revert to more specific methods if performance is an issue…

In 2014, we now have Earley parsers in C, JavaScript, Lua, Perl and Python.

Further discussion on killing yacc:

http://jeffreykegler.github.io/Ocean-of-Awareness-blog/indiv... http://jeffreykegler.github.io/Ocean-of-Awareness-blog/indiv... http://jeffreykegler.github.io/Ocean-of-Awareness-blog/indiv... http://jeffreykegler.github.io/Ocean-of-Awareness-blog/indiv...

"Earley parsers, which Cox apparently didn't know about in 2010"

How do you draw that conclusion? I see nothing in the article which says that he did or didn't know about Earley parsers. A quick search finds this posting by Cox from 17 Apr 2006 at http://compilers.iecc.com/comparch/article/06-04-111 :

> Although few people do use Earley and Tomita parsers in practice now, I think general approaches, especially GLR, are gaining ground.

Furthermore, the Wikipedia page for GLR says:

> Recognition using the GLR algorithm has the same worst-case time complexity as the CYK algorithm and Earley algorithm: O(n^3). However, GLR carries two additional advantages:

> - The time required to run the algorithm is proportional to the degree of nondeterminism in the grammar: on deterministic grammars the GLR algorithm runs in O(n) time (this is not true of the Earley[citation needed] and CYK algorithms, but the original Earley algorithms can be modified to ensure it)

> - The GLR algorithm is "online" – that is, it consumes the input tokens in a specific order and performs as much work as possible after consuming each token.

> Compared to other algorithms capable of handling the full class of context-free grammars (such as Earley or CYK), the GLR algorithm gives better performance on these "nearly deterministic" grammars, because only a single stack will be active during the majority of the parsing process.

Perhaps Cox knew about and rejected bringing up Earley in favor of GLR, for several sound reasons that you didn't know about in 2014?

No need to get so agitated. Your reply comes across unnecessarily hostile for no good reason.

> How do you draw that conclusion? I see nothing in the article which says that he did or didn't know about Earley parsers.

Simple inference from it not being mentioned, even though I thought it deserved to be. Since that does not prove anything, I wrote "apparently" – I anticipated my assessment could be wrong, and indeed it was.

> the Wikipedia page for GLR says

I'm not happy with that article. It gives people the wrong ideas, it's not realistically useful to make comparisons with the decades-old original algorithm. Modern Earley parsers do contain optimisations that makes those distinctions mentioned there moot. And unless I completely misunderstand what the WP contributor aimed to express, the Earley algorithm is "online", too, and that is the case even for unmodified/unoptimised Earley parsing. See http://web.stanford.edu/class/archive/cs/cs143/cs143.1128/le... or just step through an implementation with a debugger. I think the reasons are not as "sound" as you concluded them to be.

To me it appears after all that GLR and Earley are equal in power, so Cox shouldn't simply reject, and implementations compete in areas other than the algorithm, e.g. sensible error reporting, simple interface for simple use cases, ability to consume grammars in standard formats, coverage by number of programming languages and such like.

Unnecessarily hostile? I even used "Perhaps" where you used "apparently", and quoted a block of third-party text like you did.

Cox wrote "These tools and many others all have the guarantee that if they tell you the grammar is unambiguous, they'll give you a linear-time parser, and if not, they'll give you at worst a cubic-time parser. Computer science theory doesn't know a better way. But any of these is better than an exponential time parser."

It's more generous to believe that Earley is simply one of the "many others" that were unenumerated, but equal in power to GLR.

You can certainly argue that there are pluses and minus to all of them, but they are irrelevant for the context of the essay. That section is very short and can't be seen as being a complete summary of alternatives, but rather observation that "newer tools that provide compelling alternatives still embody [the spirit of yacc]", including bison.

The lack of a reference to Earley is not indicative that the author does not know it. Consider that ANLR uses adaptive LL( * ) because:

> The biggest problem for the average practitioner is that most parser generators do not produce code you can load into a debugger and step through. This immediately removes bottom-up parser generators and the really powerful GLR parser generators from consideration by the average programmer. There are a few other tools that generate source code like ANTLR does, but they don't have v4's adaptive LL( * ) parsers. You will be stuck with contorting your grammar to fit the needs of the tool's weaker, say, LL(k) parsing strategy. PEG-based tools have a number of weaknesses, but to mention one, they have essentially no error recovery because they cannot report an error and until they have parsed the entire input.

That's from https://theantlrguy.atlassian.net/wiki/pages/viewpage.action... . The page doesn't mention Earley parsers either. I don't think that Terence Parr, author of ANTL and that quote, is ignorant of Earley parsers in 2013. (Especially as Parr mentions Earley in 2007 in http://blog.athico.com/2007/06/interview-with-antlr-30-autho... . Note also the issues with GLR in https://qconsf.com/system/files/presentation-slides/quest-fo... and compare to the lone reference in that presentation to Earley).

FWIW, I was using an Earley-based parser for Python as part of the SPARK package back in 2000, and I'm by far an expert in the field, so I think it's unreasonable to assume, as you did, that a practitioner in the field wouldn't know about it and have other reasons for not enumerating it specifically.

"Reject" is my word, not Cox's.

Nor did I mean to imply that the reasons on Wikipedia were the same as the ones the Cox used when deciding to not mention Earley, only that there could be reasons. Quoting Parr at http://blog.athico.com/2007/06/interview-with-antlr-30-autho... " GLR and Earley and CYK can deal with the same class of grammars (all context-free grammars), but GLR is more efficient." That one reason alone might be enough for Cox to have decided to mention GLR and leave Earley in the category "and many other[ tools]".

Earley parsers give no guarantee that the grammar is not ambiguous. This is very important, since you don't want your parser return some arbitrary parse tree, but you want guarantees that the parse tree that you intended is returned. yacc does this IMHO in the correct way: give warnings (shift/reduce or reduce/reduce conflict in case of a possible ambiguity) and let the user resolve these manually. Since these conflicts are a strong sign of a bad specification of the language, the parser generator should be very cautious in resolving these ambiguities - doing this automatically probably leads to strange consequences the user did not intend.

> you don't want your parser return some arbitrary parse tree

You have a mistake in your assessment of Earley parsers. For ambiguous grammars and input, they do not return an arbitrary tree, but all possible trees. In this regard they are no worse than yacc: "the user resolve[s] these manually" by picking the correct result(s). To a human, it's visible at a glance which is the correct result(s), entirely without needing to learn how to decipher these bizarre warning messages you mentioned.

> conflicts are a strong sign of a bad specification of the language

So what? That's not pragmatic thinking. We cannot go back in time and influence the design of ambiguous computer languages so they are not ambiguous. Natural languages never are without ambiguities!

The parsing job needs to be done, no matter the complexity of the language. Instead of wishing it weren't so, just use a tool that can deal with it. Earley parsers can, yacc cannot.

> You have a mistake in your assessment of Earley parsers. For ambiguous grammars and input, they do not return an arbitrary tree, but all possible trees.

There can easily be an exponential number of parse trees for an ambiguous grammar, which is a contradiction to the runtime guarantee of the Earley algorithm.

> To a human, it's visible at a glance which is the correct result(s), entirely without needing to learn how to decipher these bizarre warning messages you mentioned.

The bad warning messages are a problem of yacc and not of the LALR(1) algorithm that yacc uses by default.

> So what? That's not pragmatic thinking. We cannot go back in time and influence the design of ambiguous computer languages so they are not ambiguous.

The languages are typically not ambiguous, but their reference grammar is (most famous problem is the dangling else (http://en.wikipedia.org/wiki/Dangling_else)).

The problem is that the ambiguity only gets detected at runtime, when the parser returns 2 trees instead of a single one.

Its very useful to be able to identify ambiguities statically, during the language design stage.

The problem is this is optimising entirely for the wrong thing.

And "killing Yacc" is a pointless exercise if the point is to get traction. What it needs to compete with is hand-written recursive descent parsers.

Please explain how you arrive at the conclusion that an Earley parser optimises entirely for the wrong thing. What is the wrong thing?

Competing with a manually written parser is easy: Earley parsers are derived from a CFG in standard format, such as ISO EBNF or IETF ABNF. Such a grammar is much easier to reason about than parser in a general programming language such as C.

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