Multiple language servers on one language

[Markiewic]

I am working on an extension for Angular support. As you know, Angular does not use any of its own file extensions (like vue, for example), but uses files with the .ts and .html extensions.

So, now Zed correctly recognizes the language of the opened files (HTML and TypeScript).

We specified the following configuration in extension.toml:

[language_servers.angular]
name = "Angular Language Server"
languages = ["TypeScript", "HTML", "Angular"]

[language_servers.angular.language_ids]
"HTML" = "html"
"TypeScript" = "typescript"
"Angular" = "angular"

This means that if the file has the html language selected, then two language servers will work for it: vscode-html-language-server and our angular server. And indeed: functions such as hovering (the textDocument/hover method) work correctly, information is requested from both language servers and combined. Where there is something to tell the vscode-html-language-server server, information from it is displayed, and the same for the angular server.

But there is a problem with the "Go to definition" function. The angular server does not receive the textDocument/definition message at all. If you check what messages vscode-html-language-server receives, these messages are there (and it is logical that it responds with an empty array).

The same situation is with TS files. The vtsls language server receives textDocument/definition messages, but angular does not. Although such functions as auto-completion and hover work correctly.

It is possible to set in the user settings which language servers will be used for a particular language. And what is interesting is that if you set them like this:

{
  "languages": {
    "TypeScript": {
      "code_actions_on_format": {
        "source.fixAll.eslint": true
      },
      "language_servers": ["angular", "vtsls"]
    },
    "HTML": {
      "code_actions_on_format": {
        "source.fixAll.eslint": true
      },
      "language_servers": ["angular", "vscode-html-language-server"]
    }
  },
}

then everything will start working correctly. Yes, now it is the other way around, the vscode-html-language-server and vtsls servers stopped receiving textDocument/definition messages, but angular does.

The same applies to the textDocument/references and textDocument/rename methods. It turns out that these features only work for the language server that is listed first.

In this regard, I have two questions:

1. How can I set the angular language server as a priority using the extension configuration? It is probably incorrect to ask the user to manually set the necessary settings after installation.
2. Is it possible to make features such as defenition search and references search work for all related language servers?

[jpike88]

@maxdeviant now the angular extension is live, this issue sticks out as the final block to parity with VSCode's offering.

See link below, which contains a short video showing the issue in practice. Basically, 'go to definition' functionality seems to only stop at the first language server in whatever has been configured...

If I tagged the wrong person, could you point at to someone who can at least outline the limitation, or point us in the right direction to see if we can attempt a PR for this?

Thanks a lot

nathansbradshaw/zed-angular#38

[jpike88]

probably related links:

• Ruby: Go to definition stop working if ruby-lsp is enabled together with rubocop #15023
• Providing primary/secondary language servers in an extension #15279
• extensions: Allow defining language servers as secondary #15597 (review)
• extensions: Allow defining language servers as secondary #15597 (comment)

I think the solution involves using the first language server, then having an empty result fall through perhaps to the next one? Something like that. I see mentions of a primary and secondary language server, it seems that in this case, primary language servers catch all definition requests and if they come up with nothing, things just stop there.

I think the solution involves being able to have a primary language server, a secondary-main one (which acts as a fallback for definitions), and the rest act as a third level of importance so go to definitions requests which are maybe expensive, don't cascade all the way down the chain.

I'm talking a little bit out of my level... hope we can clear this up

[atscott]

Angular's language service extension for VSCode (and the language server) relies on the fact that the TypeScript and HTML language servers run alongside it. This is especially important for TypeScript and larger projects for performance reasons.

Angular has historically required global knowledge and whole-program compilation in order to provide complete information (generally due to the global nature of NgModules that roll up one AppModule). This is problematic for tasks that should be highly responsive, such as autocomplete. For this reason, we do not provide any autocomplete suggestions in TS files outside of inline templates. There's no Angular-specific information to add there, so we instead rely on the TypeScript language server to provide its completions, which are much faster.

[notpeter]

Wanted to link an additional related discussion:

• "Native" language support for R? #25758 (comment)

Currently the only way to by-default specify which extension provided LSP is "primary" for a given language (without requiring users to change their settings) is Zed shipping hard coded defaults in the global default.json which is pretty ugly:

• Add default settings for LaTeX #27286
• Better support for non-primary LSP extensions #27430

CC: @DavisVaughan, @lict99, @P1n3appl3

[jpike88]

I can think of two approaches (based on my knowledge of VSCode and VSCode angular extension):

• If calling the first LSP in the list for something like a code action or definition, and it returns a null/undefined value, simply move on to the next, and enumerate either what's in configuration or what's actually running in session until you get a value.
• allow LSP extensions to implement an (optional) method that lets them claim or skip on a provided filepath at the time of opening it (default is to allow the file to be processed, and stop there if there's no result).

[Askath]

Could this project be helpful for this? https://github.com/thefrontside/lspx
As eglot (Emacs lsp) has the same problem, a lsp multliplexer emerged which allows to run multiple Language Servers for a single language mode.

[jpike88]

Just in case people didn't get notified, a member of the Angular core team was nice enough to chip in here:

#24100 (reply in thread)

Does this allow us to focus a simpler initial implementation to accommodate this? To date, any angular developers using zed are limping along without the same functionality that can be enjoyed in VSCode. Maybe an interim solution can be deployed without having to do a big rewrite or extension of functionality.

[jpike88]

Just realized... instead of thinking of this on the language level, we can think of this instead on the glob level. Then, this whole 'multiple language server' multiplexing discussion can be defferred to another day.

Angular dev right now on Zed is a pain really due to the fact that the angular language service breaks on typescript files that aren't part of an angular project. Otherwise, I could have just chosen the angular language service across my monorepo, and get on with things.

Instead, I use vtsls for my whole project, and lose any syntax highlighting, diagnostics, and interactability with my internal templates. I still have VSCode on the side for this reason.

With a glob-based override, developers (whether using external or internal templating) can then add a simple rule that will direct all files or a folder containing their angular project directly to the angular language server. Zed would recognize the glob, and route all requests appropriately.

My config (now):

"languages": {
		"TypeScript": {
			"language_servers": [
				"!typescript-language-server",
				"!eslint",
				"vtsls",
				"angular",
				"..."
			]
		},
		"HTML": {
			"language_servers": ["angular", "..."]
		}
	},

My config (with new configurability)

"languages": {
		"TypeScript": {
			"language_servers": [
				"!typescript-language-server",
				"!eslint",
				"vtsls",
				"angular",
				"..."
			],
			"language_server_override": {
				"client/**/*.ts": "angular"
			}
		},
		"HTML": {
			"language_servers": ["angular", "..."],
			"language_server_override": {
				"client/**/*.html": "angular"
			}
		}
	},

This solves the problem quickly, while releasing pressure on the above discussion.

Can we move forward on this to begin?