Write a parser with JavaScript

I love articles about writing parsers by hand - it's really fun and teaches you a lot.

Last year I decided to write a complete compiler in Javascript, completely by hand. The idea was to compile a vaguely C-like toy language to WASM without any external dependencies.

Part of the motivation was that while I've written many simple parsers, I mostly used parser generators with BNF grammars and I didn't feel like I had a good sense of how the code I ended up generating actually worked for something with complex syntax and semantics. I felt like I was writing specs for a parser, rather than writing a parser.

My toy language has vaguely C-like syntax with block scope and infix operators with various precedences, so it was a bit more complicated than JSON, but I ended up using something like Pratt parsing/Precedence Climbing (see https://www.oilshell.org/blog/2017/03/31.html) and wrote the whole thing in a way that's - hopefully - pretty easy to read for folks interested in wrapping their head around parsing complex syntax (e.g. with scope and name resolution). The lexer, parser and language definition ended up being about 1000 lines of JS (counting some pretty extensive comments).

Any JS programmers that are interested in really getting into the nitty-gritty of writing your own parser/compiler should check it out. The source is here: https://github.com/j-s-n/WebBS (relevant files for parsing are in /compiler - start with lexer.js, parser.js and syntax.js).

If you want to play around with the language and inspect the generated ASTs and WASM bytecode, there's an interactive IDE with example code here: https://j-s-n.github.io/WebBS/index.html#splash

My intuition as someone who writes compilers, when handed a BNF grammar, is to reach for a parser-generator like yacc, so that I can keep the BNF grammar as the canonical representation of the grammar, and have everything else derived automatically and kept in sync. (Even though, yes, in this case, JSON is a very static spec that hasn’t changed since it was introduced.)

Presuming for a moment that there isn’t a parser-generator utility that emits JavaScript, though, I’d be led to wonder whether it wouldn’t still be a more efficient use of my time (compared to hand-rolling a parser, usually a pretty finicky task) to just use yacc itself to emit C, and then use Emscripten to get WASM, and then call that from JavaScript. I’d probably do that first—it’d take about an hour—and then profile the resulting parser (including the JS-WASM overhead.) Hopefully, it’d fall within the tolerances for parsing time. Only if it didn’t would I sigh and begin to hand-roll a parser.

That's a heck of an interview question. I like what he was tempted to write better. I really don't want engineers toiling for hours over something a language or framework already provides.

I get the purpose, of course, but geez.

For those interested, there is also jison, a flex+bison ~1:1 reimplementation in js. I used it for a domain-specific format, it is really nice, easy and go-to, compared to traditional alternatives.

https://zaa.ch/jison/

Very nice. Here's a similar post but for Python.

http://notes.eatonphil.com/writing-a-simple-json-parser.html

I wonder what happened to that C++ grandmaster course http://cppgm.org

A great parser framework is Chevrotain by SAP

Wait this is a JSON parser, not a JavaScript parser! Still nice work! There is something "raw" about writing parsers by hand (even if they are recursive descent). Teaches one a lot about clean design and modularity etc!

I'm surprised Ohm isn't mentioned, it is spectacular.

Anybody interested in parsers is gonna love it:

https://github.com/harc/ohm