elm-language-server: Erratic/broken behavior with Neovim's integrated LSP client

Expected Behavior

Normal LSP operation.

Current Behavior

When using elm-language-server with Neovim’s built-in language client (configured via nvim-lspconfig), elm-language-server’s parser appears to be completely broken. I’ve recorded two screencasts demonstrating some of the erratic behavior. A summary follows:

On opening a new buffer, LSP behaves fairly normally (type checking works, diagnostics make sense, whatever). After a couple normal editing operations (delete some text, add some text), it appears elm-language-server’s internal model of the buffer contents becomes totally malformed, and elm-language-server starts complaining about random, bogus parsing errors & spitting out nonsense diagnostics. Here are some sample error messages it gives:

No definition found for 'mport '.
Parsing error. (on what should be totally valid Elm code)
Could not find a module to import named 'String ' in …
Unused pattern variable '='
A value named 'nt' is already defined (in code that does not define any nt, only uses Int in a type signature).

Possible Solution

Sorry, but I have no familiarity with the internal workings of elm-language-server or Neovim LSP, so I honestly have no idea where to start suggesting fixes. A few observations nonetheless:

This issue does not occur if I use coc.nvim instead of Neovim’s built-in LSP client, on the same elm-language-server installation.
My other language servers (e.g. TypeScript, Python, OCaml) have been working fine with Neovim’s built-in LSP client. My point is that it’s possible that the error is on Neovim’s side, seeing as their LSP implementation is still “nightly”, but since I’ve only been able to reproduce the problem in the unique combination of Neovim + ElmLS, I don’t know how to isolate the problem.

Steps to Reproduce (for bugs)

Set up elm-language-server with Neovim’s built-in LSP client. Follow instructions to install vim-plug, then contents of ~/.config/init.vim:
```
call plug#begin()
Plug 'neovim/nvim-lspconfig'
call plug#end()

lua << EOF 
require'lspconfig'.elmls.setup{}
EOF 
```
Edit literally any Elm file within a project.
Make some random changes.

I have made screencasts illustrating this process here and here, showing the erratic errors that pop up for me.

Context

How has this issue affected you? What are you trying to accomplish? Ha ha, you’re funny.

Your Environment

Version used: 2.0.2
Editor used: Neovim, with built-in LSP
Environment name and version (e.g. node.js 5.4): Node 14.15.4
Operating System and version: Void Linux

About this issue

Original URL
State: closed
Created 3 years ago
Reactions: 2
Comments: 18 (6 by maintainers)

Links to this issue

Buggy behavior with built-in lsp client and elm-language-server

Commits related to this issue

Add elm language support for neovim Add support for the elm language via Neovim's LSP. Additionally, add support for formatting elm code via `elm-format` through EFM. Note: A custom `on_attach` func... — committed to McAllaster/dotfiles by McAllaster 3 years ago

Most upvoted comments

I don’t believe this is solely related incremental synchronization, @razzeee. I’m using Helix and seeing this erratic behaviour fairly often. Not all the time, but it happens every time I sit down and code Elm. All changes are reported using incremental synchronization.

Video of me making some edits and saving the file, triggering a formatting request. https://asciinema.org/a/dexV5nWbBn5sQ9iDRD5X2r9qB and here’s the associated log.

Vide of me making some edits and saving the file. Then manually triggering formatting. https://asciinema.org/a/Bh47XkiludVBgxK6U8b5LuJal and here’s the associated log.

In both examples I get the same parsing error reported from the server. In both videos, it’s after formatting takes place.

hovsater on Jan 23, 2022

We do technically support incremental buffer updates, it’s just disabled by default. @kwshi you can try enabling it to see if it fixes your problem.

local custom_attach = function(client)
      if client.config.flags then
        client.config.flags.allow_incremental_sync = true
      end
    end
require('lspconfig').elmls.setup({
   on_attach = custom_attach;
})

mjlbach on Feb 5, 2021

I have this issue using coc.nvim, but it only happens when there are no real error messages.

gabriel-sevecek on Feb 5, 2021

I’ll just enable incremental sync by default for elmls client-side in nvim-lspconfig

@Razzeee Thanks for pointing this out

mjlbach on Feb 5, 2021