Add language server configuration

[kentookura]

Just a heads up that in the not too distant future, forester will ship with an integrated LSP server.

The code on the lsp branch is not yet ready for public consumption, but the basics do work, namely document sync and reporting parse errors. I think once this plugin has configured the client, it won't need a lot of updates as we keep adding features to the server, as the protocol itself is responsible for advertising which features are available, so they should just work.

[kentookura]

https://github.com/semanticart/minimum-viable-vscode-language-server-extension

[Trebor-Huang]

Now that I have time, I'll start looking into it

[Trebor-Huang]

A few comments:

• The VSCode language client expects the LSP executable to accept the argument --stdio when communication is through stdio, this seem hardcoded
• The client absolutely hates UTF-8 encoded positions and it will immediately stop running unless it sees UTF-16.
• The client requests textDocument/inlayHint very very frequently. It goes up to 10 times a second when I, for example, resize the document viewport. (I tested and it also happens with plugins of other languages like typescript, so it probably is intended and not my problem.) The current implementation doesn't seem to keep up. Is it recomputed each time?
• Some other stuff but I think it's because the implementation isn't complete yet? Like how it attempts to provide hover text over every word as if they are links, or that it doesn't support wiki links.

[kentookura]

• The LSP library has this module:
  https://ocaml.org/p/lsp/1.22.0/doc/Lsp/Cli/index.html#val-args
  https://github.com/ocaml/ocaml-lsp/blob/aae6986391a8519de3da6a7a341f2bd3376e0d2f/lsp/src/cli.ml#L14

  However, we use cmdliner, so I don't know if those two are interoperable. In any case, it won't be a problem to handle this flag.

• Maybe it is time to reevaluate this then:
  https://github.com/RedPRL/asai/blob/4720343d54625c2872d61ffeaf37e28a49c7aa4f/src-lsp/Asai_lsp.ml#L43
  I'll need to take a look at the spec, but I think a potential problem here is that we are using Asai to handle everything related to positions, so we might need to work on the library to support this.

• Caching the inlay hints should be feasible.

• Yes, many handlers are just stubs. In the past couple of months I have been working on improving the compiler architecture to better support incremental compilation for the LSP. I am nearing the end of that task, so I will be able to write proper implementations for these handlers soon.

Thanks a lot for working on this!

[kentookura]

From your comments it seems that you have managed to work around the UTF encoding issue? How else could you have verified the behavior of inline hints? Did you need to patch the OCaml source?

[Trebor-Huang]

I modified the source and just lied to vscode that we support UTF16

[kentookura]

Interesting. Can you confirm that diagnostics are reported to the correct location?

[Trebor-Huang]

They are only correct when the source code is pure ASCII, otherwise they get offset by a bit.