Using a JSON TCP connection for what on Windows would be direct function calls (COM) or in Eclipse would be direct function calls between Java modules always felt a bit gross.

I agree, the only reason LSP exists as it does is for a world running on electron based applications. Plug-ins and application extensions are not new technology and they are nearly universally more efficient in the forms designed and used prior to 2005-ish. I understand why VSCode exists and why it is used so often by developers, but it is certainly a downgrade from more language specific options that could exist.

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.

In order to have direct function calls, you need to load the plugin's code in your own memory. Which means you expose your own memory to the plugin. Any malicious/buggy plugin could wreak havoc on your program - even managed code doesn't solve that problem in general.

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.

Fault isolation is good, but are the fault isolation benefits worth everything else though?

Windows COM does have a way to run the server out-of-process with a performance cost, and this fact is transparent to the client (if it only uses COM interfaces and not global variables or something). I assume you can even switch between the modes at runtime (with restarting the plugin).

I think this is a very reasonable question to ask. Language servers are certainly a significant source of slowdowns and editor unresponsiveness, and any time you're doing IPC, that's another source of latency and fragility.

My current Zed editor instance has 5 different language servers running, implemented in at least 3 different languages, some of which require a full VM runtime.

Could Zed (written in Rust) host a full .NET runtime to run the Roslyn language server for C#? Could an Electron-based editor? It feels a bit daunting. Plugins could be native shared libraries, but what happens when multiple language plugins all try to initialize huge process-wide runtimes like .NET or the JVM? Even multiple instances of the Python runtime are going to potentially be stepping on each others' toes.

IPC and separate processes is probably the pragmatic solution for now, even though I would love to live in a world where in-process was more of an option.

WASM could be the solution, but would lock language server authors into the subset of programming languages that can be compiled to WASM, and WASM still looks a lot like IPC in practice. (Zed already uses WASM components for its plugins.)

Enter hell: LSP assumes that it's the source of truth, but you still need to access the filesystem yourself, and do it in a synchronized way

LSP is an example of utterly horrid technical design.

Stop letting Microsoft design protocols and APIs. They are so. bad. at. it.

I've never looked into LSP, how would you design it?

Start with synchronous function calls instead of JSON. Microsoft knows how to do that - they invented COM and OLE. Function calls enable whatever data sharing is necessary to maintain a coherent view. Imagine trying to do OLE with JSON - just wouldn't work. (Does OLE still exist?)

Moreover, it's not just stupid, it also does not actually solve _anything_.

A synchronous function can also have obsolete indexing information if it races with the code updates.

That doesn't really fix anything. The filesystem is fundamentally a racy shared data structure. If you made the entrypoint API synchronous, any half-way decent editor would shove the LSP queries to the synchronous API into a different thread, because "Do Not Block the UI Thread" is a fundamental principle of good UI programming.

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.

Big assumption that the editor, which is required to make calls synchronously, would make them on a different thread. Isn't correctness more important than avoiding plugin calls in the UI thread? The good thing about having an actual function interface is that you can tailor each one to its actual needs, instead of insisting high-overhead async is okay for everything. It's like GDI vs X11.

Actually, COM is inspired by DCE/RPC and the initial versions had some similarities.

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.

I've never heard of anyone calling SOM 'better' at anything this century. I came across it when it was the foundation for OpenDoc at Apple, your comment brought back many bad memories

Compared with COM's design, it was much better.

Classes and metaclasses sound like architecture astronomy here. COM is fundamentally just a standard ABI and a way to look up DLL paths in the registry. There are more layers on top of that, but that's the basics and all you need for a plugin architecture.

That applies to SOM just as it does to COM.

Interesting to complain about one and then ignore exactly the same boilerplate for the other.

Leaving aside the fact that Microsoft's tooling for COM has already had multiple reboots between VB OCX, MFC, .NET Framework RCW/CCW, .NET ComWrappers, ATL, WRL, WIL, WinRT, each one with its share of astronomy.

No one uses the bare bones vtbl and nothing else.

Stop letting Microsoft design protocols and APIs.

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!

we'd still have editor that only support one language properly

Emacs supported zillions of languages before LSP.

Zoom out.

The particulars of Rust make this a little more difficult, I think. There’s a certain tension between making your language more concise and adding useful redundancies, and Rust has generally gone to the “concise” side, with some redundancies that can make the tooling a little more painful. Like with imports.

  impl std::fmt::Display for Blah {
  }

If your language makes you qualify your imports (like above) then your LSP can, delightfully, still reliably do certain ops like renaming, even when chunks of your project aren’t parsing. But if you glob import std::fmt, and glob import something else, you are fucked. Display could come from anywhere (maybe from a module that has a parse error at the moment). I really appreciate languages where glob imports (or their equivalent) are either disallowed entirely or where typical code doesn’t use it.

Meanwhile, if you add a new file, there’s this little dance where you say:

  mod mycoolmod;

And then you create mycoolmod.rs. Or you do it the other way around. A little redundancy (the file exists and it is declared), that seems to just create a little friction in the LSP because mycoolmod doesn’t get a working LSP until it’s declared in the parent (you have to create both, and then you get a transient diagnostic that your module is unused for a while yet). A small issue, just another little bit of friction in the tooling of Rust that has nothing to do with the type system.

A lot of things described in this article applicable not only for Rust, but for almost any language. Like it's obvious that requests should be handled asynchronously and that conversions from/to UTF-16 are needed. But it's actually not so hard.

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've not written a language server but have written a language plugin for IntelliJ.

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.

Reading this made me realize that I want two different things from an LSP that are sometimes at odds with each other:

1. Editing help

2. Reliable and comprehensive analysis

The editing help needs to deal with incomplete and inconsistent state and answers on a best effort basis. This is good when I'm writing code.

When I'm trying to understand code it usually is in a complete and compiling state but best effort is not enough. I expect complete and exhaustive answers.

Independently of that I'd love to read a similar analysis that compares the approaches of rust-anslyzer, rust-glancer and the JetBrains analysis engine in Rust Rover.