I have written a language server for my language too. The hardest thing was to find a way allowing providing useful autocompletion for a document in edited state, when it's not syntactically-correct. This is the trickiest part how to deal with such incorrectness without missing all the context necessary.
I started with writing a correct recursive descent parser. I then extended it to detect, report, and recover from common syntax errors as I encountered them so that the parser is robust. And adding a parser test case for each of these (e.g. one test for each branch through an EBNF construction).
Some examples are:
1. missing keywords when the keyword can be detected from the current context (e.g. missing semicolon at the end of a statement);
2. using the wrong token (e.g. `:` instead of `::` in a C++ namespace qualified name);
3. detecting and ignoring whitespace in a whitespace-sensitive qualification (e.g. in XML QNames);
4. keeping in the prolog state (where functions are defined) when there are errors so that functions after the error don't get lost;
5. lexing incomplete literals like `10e` so they can be handled as integers in the parser and emitting an error for them.
It's a dead-end. Sure, it can work in simple cases, but there will be always a case where such syntax recovery isn't possible. That's why relying only on syntax recovery isn't an option.
Because of that I use a different approach. I do parse on each document editing, but such parsing is guaranteed to produce valid results only up to the point with broken syntax, where editing usually takes place. Such parsing is enough to reconstruct location of the point where editing takes place (namespace/class/function) and to reconstruct local context (local variables declared prior to editing place). This allows to perform almost perfect autocompletion by suggesting global and local names available at the editing point. In order to provide proper suggestion of non-local names declared after the editing point, I do keep a structure for the most recent document state with valid syntax.
With features like "go to definition" I do the same. I store a hash-table with location to definition point mapping, but it's updated only from time to time and only if document syntax is valid. In order to be usable for cases with edits made after building such hash-table I just perform text-based position mapping using accumulated edit events.
IPC is the natural solution to that - run a process in its own address space and communicate with it via I/O. TCP is not most optimal, but it is uniquous. Same thing with JSON.
In LSP, most of the processing happens in the LSP server anyway - communication overhead between client and server is negligible in comparison to it.
And overall overhead for running a language server in a separate process isn't that big. LSP is designed in such a way that only minimal amount of information is needed to be passed, like edits or short responses. Packing/unpacking JSONs isn't a bottleneck, the heaviest job like program analysis is done in the language server itself without interprocess communication overhead involved.
There are some arguments that do carry water in favor of using a client/server protocol transmitting JSON, in particular, the ability for a nearly complete decoupling of analysis of code and the displaying and editing of that code. Also, LSP was (to my knowledge) the first language/platform/usecase agnostic protocol intended for use in code editors.
I get why this is where a big chunk of developers have ended up, but I do bemoan the lost potential for a language to grab mindshare and popularity on the usability and performance of its tooling and developer experience via-a-vis a custom designed and hyper specific code editor. I mean, Rust and Elm received endless praise for their error messages as a massive boon to developer experience, so it is a facet of language design and implementation that can act as great advertisement. I just hate that the prevalence of LSP at this point precludes custom editors as a first choice in the current zeitgeist.
This isn’t for Electron app only, but is also helpful for Emacs, vim or helix (especially when they lack said language plugins).
Now, some IDEs provide capabilities for a language far exceeding what can even be implemented in a LSP.
However, to come back to your point, there are much better high performance OS IPC mechanisms for inter process communication than sending JSON down through a TCP wire.
As others point out, Electron.
impl std::fmt::Display for Blah {
}
Meanwhile, if you add a new file, there’s this little dance where you say:
mod mycoolmod;
LSP is an example of utterly horrid technical design.
Stop letting Microsoft design protocols and APIs. They are so. bad. at. it.
Unfortunately nobody else stepped up to do it.
I'm glad that the code editors out there didn't wait for your theoretical better designed protocol and decided to adopt LSP. Otherwise we'd still have editor that only support one language properly, and the rest is treated like text. If the price to pay is that it sucks for the handful of people who have to work with it, so be it. For every LSP developer that suffers there are tens of thousands of downstream users who benefit from better language support in their favorite editor!
Emacs supported zillions of languages before LSP.
Zoom out.
Resolving file paths is not trivial because it's language specific.
Syncing source changes on the LSP server side is necessary to keep packet side small and patching is trivial (span, new text).
(I made an LSP-powered text editor)
Imagine if all the editing tools in microsoft word were specific to the language you used, and if you mixed German and English, each had different tooling.
Now if you mix Rust, HTML, JS, and CSS, they'll all have separate tooling, seperate "go to definition", and separate refactoring. Worst of all, none of them can see the definitions of the other ones.
So what you'd actually want to do is parse each language into your AST, with proper annotations as to what is what, and have all the "go to definition", the UI rendering, highlighting, refactoring, etc all done generically by the IDE ontop.
Which is much much closer to how Jetbrains IntelliJ does it, and why their tooling can handle "find usages" on an HTML element to find matching querySelector in .js files and matching selectors in .css
And if you subscribe to the AI stuff, you'd also want your AI to operate on this AST so it can learn skills that generalize across all languages.
Once you get past that, JSON-over-TCP is just another kind of asynchronous RPC mechanism, one that has the advantage that you can build it in just about any language with out-of-the-box tools. Trying to make a plugin system or a COM or CORBA or OLE based system really cuts out the ability to build language servers in most languages, because you have to be able to build the code in just the right way.
In no way does this mean LSP is a perfect solution, but anything synchronous would be a step backwards.
LSP is a function call protocol. So that is already done?
Parallel to that, IBM had SOM on OS/2, which was even better allowing for metaclasses and proper class inheritance, it was the key mechanism between Smalltalk and C++ on OS/2, where Smalltalk enjoyed a role similar to .NET on Windows nowadays.
OLE naturally still exists when using Office natively on Windows, other vendors seem to have forgotten about it.
COM's role on Windows has grown since Vista, and the Windows team redid many of the Longhorn ideas originally implemented in .NET into COM/C++, with WinRT being an evolution of COM.
A synchronous function can also have obsolete indexing information if it races with the code updates.
So the LSP crashes and/or goes into a runaway memory consumption loop. And your main application dies with it.
Or what if you want, you know, to be able to use the same LSP from TWO different applications at the same time?
Never mind issues with other managed runtimes not expecting to deal with something else in their address space.