Astro Hacker News - Parsing Advances

smj-edison |next [-]

Huh, that's a really interesting approach. I just wrote my first Pratt parser a month ago, and one of the most annoying things was debugging infinite loops in various places (I had both tokenizer bugs where no characters were consumed and parser bugs where a token was emitted but not advanced). It's doubly annoying in Zig, because the default test runner won't print out stdout at all, and won't print stderr unless the program terminates by itself (Ctrl + C doesn't print). I resorted to building the test and running it manually, or jumping into a debugger to figure out recursion issues. It's working now, but if (really when) I run into issues in the future I'll definitely add some helper functions to check emitting invariants.

someone_jain_ |root |parent [-]

its also very annoying that one can't have two test names where one is substring of other

sureglymop |next |previous [-]

In rust I really like the grmtools set of tools: https://github.com/softdevteam/grmtools.

It is lexx/yacc style lexer and parser generation and generates an LR1 parser but using the CPCT+ algorithm for error recovery. Iirc the way it works is that when an error occurs, the nearest likely valid token is inserted, the error is recorded and parsing continues.

I would use this for anything that is simple enough and recursive descent for anything more complicated and where even more context is needed for errors.

ratmice |root |parent [-]

I always feel that when saying lex/yacc style tools, it comes with a lot of preconceived notions that using the tools involves a slow development cycle with code gen + compilation steps.

What drew me to the grmtools (eventually contributing to it) was that you can evaluate grammars basically like an interpreter without going through that compilation process. Leading to a fairly quick turnaround times during language development process.

I hope this year I can work on porting my grmtools based LSP to browser/wasm.

sureglymop |root |parent [-]

I've seen your commits, thank you sincerely for your work!

dcrazy |next |previous [-]

I’m curious why the author chose to model this as an assertion stack. The developer must still remember to consume the assertion within the loop. Could the original example not be rewritten more simply as:

    const result: ast.Expression[] = [];
    p.expect("(");
    while (!p.eof() && !p.at(")")) {
     subexpr = expression(p);
     assert(p !== undefined); // << here
     result.push(subexpr);
     if (!p.at(")")) p.expect(",");
    }
    p.expect(")");
    return result;

matklad |root |parent [-]

I assume you ment to write `assert(subexpression != undefined)`?

This is resilient parsing --- we are parsing source code with syntax errors, but still want to produce a best-effort syntax tree. Although expression is required by the grammar, the `expression` function might still return nothing if the user typed some garbage there instead of a valid expression.

However, even if we return nothing due to garbage, there are two possible behaviors:

* We can consume no tokens, making a guess that what looks like "garbage" from the perspective of expression parser is actually a start of next larger syntax construct:

``` function f() { let x = foo(1, let not_garbage = 92; } ```

In this example, it would be smart to _not_ consume `let` when parsing `foo(`'s arglist.

* Alternatively, we can consume some tokens, guessing that the user _meant_ to write an expression there

``` function f() { let x = foo(1, /); } ```

In the above example, it would be smart to skip over `/`.

kccqzy |next |previous [-]

How about another way, which is memoization: at each position in the source code we never attempt to parse the same production more than once. This solves infinite looping as discussed by the author because the “loop” will be downgraded by the memoization to execute once. Of course I wouldn't literally use a while loop in code to represent the production. I would use a higher-level abstraction to indicate one-or-more or zero-or-more in the production; indeed I would represent productions as data not code.

This also has another benefit of work sharing. A production like `A B | C B` will ensure that in case parsing A or C consumes the same number of characters, the work to parse B will be shared, despite not literally factoring the production into `(A | C) B`.

luizfelberti |root |parent |next [-]

I also find this to be an elegant way of doing this, and it is also how the Thompson VM style of regex engines work [0]

It's a bit harder to adapt the technique to parsers because the Thompson NFA always increments the sequence pointer by the same amount, while a parser's production usually has a variable size, making it harder to run several parsing heads in lockstep.

[0] https://swtch.com/~rsc/regexp/regexp2.html

Porygon |root |parent |next |previous [-]

Memoization to limit left-recursive recursion is nicely described in Guido van Rossums' article here: https://medium.com/@gvanrossum_83706/left-recursive-peg-gram...

I recently tried that approach while simultaneously building an abstract syntax tree, but I dropped it in favor of a right-recursive grammar for now, since restoring the AST when backtracking got a bit complex.

kccqzy |root |parent [-]

You can look at the Earley parser. It handles left recursion well well using a method that’s basically memoization.

smj-edison |root |parent |previous [-]

That's a slick way, would you essentially have a second counter that you'd set to the current cursor whenever you use `.currentToken()` or something like that?

eru |next |previous [-]

Writing parsers by hand this way can be fun (and might be required for the highest performance ones, maybe?), but for robustness and ease of development you are generally better off using a parser combinator library.

tubs |root |parent [-]

Are you?

The majority of production compilers use hand rolled parsers, ostensibly for better error reporting and panic synch.

cipherself |root |parent [-]

One anecdote in the same vein, a couple of months ago, I wanted to parse systemd-networkd INI files in Python and the python built-in ConfigParser [0] and pytest's iniconfig parser [1] couldn't handle multiple sections with the same name so I ended up writing 2 parsers, one using a ParserCombinator library and one by hand and ended up using the latter given it was much simpler to understand and I didn't have to introduce an extra dependency.

Admittedly, INI is quite a simple format, hence I mention this as an anecdote.

[0] https://docs.python.org/3/library/configparser.html

[1] https://github.com/pytest-dev/iniconfig

thechao |root |parent |next [-]

As a project gets larger the cost of owning a dependency directly begins to outweigh the impedance mismatch between 3rd party software & software customized to your project.

I've got 10 full time senior engineers on a project heading in to its 15th year. We rewrite even extremely low level code like std::vector or malloc to make sure it matches our requirements.

UNIX was written by a couple of dudes.

kccqzy |root |parent |previous [-]

That’s because Python is a bad language for writing parser combinators and parsers based on them. Try Haskell.

cipherself |root |parent [-]

I have written parsers using parser combinators in Haskell and Clojure. I find that ML-like (Haskell, OCaml, StandardML) languages generally are great at writing parsers, even hand-written ones in it is a superior experience.

In this case, this was a project at $EMPLOYER in an existing codebase with colleagues who have never seen Haskell code, using Haskell would've been a major error in judgement.

eru |root |parent [-]

I agree!

Haskell is a great language. It can even be a great language for beginners, especially if there's some senior help on hand.

But it's a terrible language to foist upon an unsuspecting and even unwilling victim.

tgv |previous [-]

So ... someone calls their parsing strategy "resilient LL parsing" without actually implementing LL parsing, a technique known since the 1970s, and then has an infinite recursion bug? Probably skipped Parsing 101.