Let's create a Tree-sitter grammar
jonashietala.se
jonashietala.se
- Convert tree-sitter grammar.json to EBNF https://meimporta.eu/tree-sitter/json2ebnf.html
- An alternative syntax to write tree-sitter grammars https://mingodad.github.io/lua-wasm-playground/ (select "Tree-sitter-ebnf-generator" from "Examples" then click "Run" to see the tree-sitter grammar for the content in "Input Text (arg[1])")
- An Yacc/Lex online editor/tester with several example grammars https://mingodad.github.io/parsertl-playground/playground/
Tree-sitter is great until you inevitably run into a problem with it and find that it is impossible to fix properly if you can even figure out why the problem even exists.
The idea of a uniform api for querying syntax trees is a good one and tree-sitter deserves credit for popularizing it. It's unfortunately not a great implementation of the idea.
The wasm bindings are a necessity if you want to be anywhere near "stable", but they run 3x slower.
^ the JS/TS parser when invoked via the Node bindings, at least. Others are potentially better. The GH issues are rife with similar complaints.
Or is it an issue with poorly-written grammars? The second you need to write an external parser with tree sitter, you have to start writing C, so I can see how that could lead to segfaults very quickly.
I dunno, I guess I'm just a bit surprised to hear a bunch of negative feedback towards tree-sitter, because up until now most of what I've heard has been pretty positive.
^One example: https://github.com/wasp-lang/wasp/blob/main/waspc/data/Gener...
For syntax highlighting and code folding, what I said above is probably generally fine. Except for the fact that it will probably be wrong from time to time and then downstream users will waste hours of their lives trying to fix the broken highlighting instead of actually working on their real work.
On the implementation side, the whole thing has bizarre choices. It has created a culture in which checking in enormous generates files is common. It abdicates common and important features in popular languages like significant whitespace to error-prone external scanners. The GLR algorithm sometimes fails inexplicably and is essentially impossible to debug (not sure if the bugs are in the algorithm or the implementation). It bizarrely uses rust and node. The js dsl for grammars is needlessly verbose. The codegen is fairly slow in spite of using rust and the generated c files are insanely huge (probably a consequence of the GLR algorithm) which leads to horrific compile times which makes incremental development a nightmare. It also uses (unless it's been updated in the last six months) a terrible algorithm for unicode lexing.
In my opinion, the whole premise of tree-sitter is kind of wrong. What would be better would be a uniform dsl for generating an AST (which can easily be represented with c structs or classes in any oo language). Trying to make a universal glr parser for all languages is a bad idea. It would generally be much easier, more accurate and faster to write the AST in the dsl that external tools recognize and then handwrite the parser that generates the AST. In the best case, the compiler already exposes an api to get the ast for some source code and you just need to write a translation program from compiler format to dsl format. Worst case, you rip out the parser in the compiler and have it generate the dsl ast format. Most handwritten parsers are only a few thousand lines of codes (maybe 10k at the high end).
You're not required to do that, and never have been. We're going to move the grammars away from checking the generated `parser.c` in, but early in the project, it was a pretty pragmatic solution, given that many consumers of the grammars weren't using a particular package registry like NPM or crates.io.
> It abdicates common and important features in popular languages like significant whitespace to error-prone external scanners.
External scanners are the right tool for this. Nobody has proposed a better solution for this that actually works, and I don't think there is one, because every programming language with significant implements it differently.
> The GLR algorithm sometimes fails inexplicably and is essentially impossible to debug (not sure if the bugs are in the algorithm or the implementation)
It sounds like you personally have had trouble with debugging a grammar you've tried to develop, but it solves a problem.
> It bizarrely uses rust and node.
Tree-sitter isn't very coupled to node. We just shell out to a JS engine because we use JS (the most popular programming language, period) as a grammar DSL, rather than inventing our own language. Node is the most common one.
> the generated c files are insanely (probably a consequence of the GLR algorithm)
No, it's not because of GLR. A lot of the reason for the C code size is that we want to generate nice, readable, multi-line C expressions to represent data structures like parse tables, rather than encoding it in some cryptic, unreadable way. Also, incremental parsing requires that a bit more data be present in the parse table than would be required for a batch-only parser. What really matters is the code size of the final binaries. The generated `.so` or `.wasm` files for a Python parser are 503k, and 465k, respectively. The wasm gzips down to 69k. I would love to make it smaller, but there are pretty good reasons why it occupies the size that it does, and it's currently pretty manageable.
> It also uses (unless it's been updated in the last six months) a terrible algorithm for unicode lexing.
I'm not sure what you're referring to here. UTF8 and UTF16 are decoded into code points using standard routines from `libicu`.
I was too vague on unicode. I meant unicode character class matching. Last I checked, tree-sitter seemed to use a binary search on the code point to validate whether or not a character was in a unicode character class like \p{Ll} rather than a table lookup. This lead to a noticeable constant slowdown of like 3x at runtime if I recall correctly for the grammar I was working on compared to an ascii set like [a-z].
It's not surprising to me that there are issues with generated parsers, partly because many languages need external scanners to work, which have to be written in C and use a rather clunky API. tree-sitter-javascript also uses an external scanner so you might want to look there for whatever is causing the problem.
But in general, I'd have to argue that if your "very stable" software requires interfacing with very unstable external tools to work with a very popular language... eliminating that dependency is certainly an area for improvement!
I think you may misunderstand parser generation and what tree sitter does. You can't eliminate external scanners, and there isn't another choice for them than C.
If you were just using an existing parser than you never touched any program written in Rust.
And the crash in this case does appear to be a result of a broken Rust-generated parser.c file, not some external tool, but I don't know enough about the project to say for sure.
It's not really accurate to say that Tree-sitter segfaults constantly. The Tree-sitter CI performs randomized fuzz testing based on a bunch of popular grammars, randomly mutating entries from their test corpus. If you have a reproduction case for the segfault, it'd be useful to report.
Since you mention NPM, it sounds like you may be talking about a segfault that's specific to the Tree-sitter Node.js bindings, but it's hard to be sure.
Anyways, since you're here, I dug a bit more into this to make a more useful report. Starting with v0.20.2, the following file will cause a segfault when parsed using tree-sitter-javascript: https://github.com/tursodatabase/libsql/blob/main/libsql-sql... . It worked fine in v0.20.1, and it's still broken with the latest v0.20.4. Based on the diff here: https://github.com/tree-sitter/tree-sitter-javascript/compar..., I don't on the surface see a way to dig deeper into this than trying to read through a 170,000 line (!!!) diff to parser.c.
And looking at that diff raises another complaint: the names of parser nodes must be considered part of the public API, as they are exposed in descendantsOfType, .type, etc., but they are 100% not documented anywhere, and are liable to change without notice in patch version bumps. This makes developing against it a massive pain, as any version increase is liable to break any code expecting a particular nomenclature.
I don't mean to dunk on your project, I'm sure it solves some problems very well. But it is remarkably difficult to confidently depend on in a production environment.
in parser.c
Announcement: https://marijnhaverbeke.nl/blog/lezer.html
Homepage: https://lezer.codemirror.net/
Link in the post is currently broken, but here’s the author’s grammar on GitHub: https://github.com/treeman/tree-sitter-djot
> But, you might wonder how does a Tree-sitter grammar look like,
How does $thing look?
or
What does $thing look like?
Similarly, one often hears "Where are we at?" It is common enough to be seen as a regionalism.
-- Your friendly tree-sitting grammarian
I wonder if it's one of those little things that's surprisingly rare, like the th sound.
I’m a native English speaker who says “Where are we at”. I think it’s informal, but I never considered it to be wrong or awkward like the first example. Perhaps I’m in the region where it’s common.
Often elided to "Where we at?".
It's dialect, I wouldn't use either in formal/technical writing, but I would unselfconsciously say the latter, as in "ok, moving down the agenda, where we at with item 3?".
1. But you might wonder: what does a Tree-sitter grammar look like, and how do you create one for a language?
2. But you might wonder what a Tree-sitter looks like and how to create one for a language.
In either case, I agree the right word is "what" not "how". The comma after "but" bugs me tremendously. The colon can be replaced by an em dash (—) if the writer prefers it—but I would not use a question mark for the second clause without separating it from the first "but you might wonder" in some way.
The comma left in alternative #1 is to needed (recommended?) because the "and" is joining two clauses with different subjects. I think it's not strictly needed in alternative #2 because it's really "(but you might wonder (what a tree sitter looks like and how to create one for a language))" where the inner term is a list of things the reader might wonder, and lists with only two items do not take a comma before "and"
I would also prefer "Where are we?" over "Where are we at?"
Not an English speaker, so just my $0.02.
One issue that I remember was the web application to test the grammar needed an exact version of the emscripten stuff. And I had to sort out what that version was myself.
In the footer there’s a link to OPs source code. The whole site gets compiled with Rust. Interesting gem to find.
Also, the bold version of iosevka is an order of magnitud heavier than the other fonts :-).
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1: https://docs.aws.amazon.com/AmazonCloudFront/latest/Develope...
I'm wondering why isn't there any language agnostic AST, commonly referred to as intermediate representation, and higher level logic such as data flow graph built on Tree-sitter. I mean writing the Grammer is of course something that is mandatory but it seems like programming languages are too much different in the architectural level of Tree-sitter
Where we've found success is in stepping back and creating formalisms that are language-agnostic to begin with, and then using tree-sitter to manage the language-specific translations into that formalism. A good example is how we use stack graphs for precise code navigation [1], which uses a graph structure to encode a language's name binding semantics, and which we build up for several languages from tree-sitter parse trees [2].
[1] https://dcreager.net/talks/stack-graphs/
[2] https://github.com/github/stack-graphs/tree/main/languages
For your data flow graph, are you going to handle copy constructors, assignment operator overloads, implicit conversions etc. like you see in C++? Or how about overload resolution: figuring out which method wins out over all candidates requires details about the convertibility of types, and how the language ranks candidates based on conversions required. And let's not forget Koenig lookup.
Things like Tree-sitter can discover definitions, declarations and invocations, but matching the right declaration for an invocation isn't trivial in the presence of overloads.
Consider three languages, one with binary operators but no function application, and another with only function application, and a third with both. What would the "agnostic" AST be, except a union of the AST of the first two?
Is the plan to at some point deprecate the existing vim-style syntax highlighting (syn match/region etc.) in favor of treesitter-based syntax highlighting? Where can I find this discussion if it is happening?
:TSUdate sdjot