How to configure formattingProvider in the Emacs eglot package

[wolverian]

I'm not sure how to configure the haskell.formattingProvider setting in Emacs eglot. I see HLS has documentation for lap-mode instead, which is an alternative LSP client for Emacs. It would be great to add eglot instructions as well, as it is now merged to Emacs upstream and will be in the next release as a built-in LSP client.

I've tried setting eglot-workspace-configuration so that calling eglot-show-workspace-configuration prints:

{
  "haskell.formattingProvider": "fourmolu"
}

If I run M-x eglot-signal-didChangeConfiguration I can see eglot sending a message to HLS:

(:jsonrpc "2.0" :method "workspace/didChangeConfiguration" :params
          (:settings
           (:haskell\.formattingProvider "fourmolu")))

but I'm not seeing any kind of reaction from the server. There's nothing in the server's stderr either.

[wolverian]

I just realised that 1.8.0.0 doesn't support formatting with the fourmolu with GHC 9.4.2, which I was using. This might have been my fault then, sorry!

[michaelpj]

Re: eglot in general, someone has to write the docs! We need an eglot user who actually understands how this works to step up and write some documentation on how to do it. Maybe that's you :)

[unhammer]

Reading https://www.gnu.org/software/emacs/manual/html_node/eglot/Project_002dspecific-configuration.html#Examples it seems the correct way is this:

(setq-default eglot-workspace-configuration '(:haskell (:formattingProvider "fourmolu")))

(or M-x add-dir-local-variable to make it project-specific). Then running M-x eglot-show-workspace-configuration I see

{
  "haskell": {
    "formattingProvider": "fourmolu"
  }
}

I'm not sure how to introspect what HLS actually parsed it as, but after setting that I get four spaces and leading commas in lists, whereas without it I got two spaces and trailing commas.