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.

[kinthaiofficial]

Configuring the formatting provider in HLS via eglot is a bit more involved than most LSP servers because HLS has multiple formatters and eglot needs to know which one to request.

eglot's formatting setup:
eglot uses lsp-format-document / eglot-format which sends textDocument/formatting to the server. HLS will use whichever formatter is configured in its settings.

Configuring HLS formatter via eglot:

(setq-default eglot-workspace-configuration
  '(:haskell
    (:formattingProvider "fourmolu"  ; or "ormolu", "stylish-haskell", "floskell", "brittany"
     :plugin
     (:fourmolu
      (:globalOn t)))))

Place this in your init.el before eglot connects, or use dir-locals.el per project.

Verifying the configuration reached HLS:
In HLS logs (M-x eglot-stderr-buffer), you should see the workspace configuration being applied on startup. If formatting still uses the wrong provider, the config may not be reaching HLS.

Common issue: fourmolu not found:
HLS looks for the fourmolu binary in PATH. If it's not installed, HLS silently falls back to another formatter. Install with:

cabal install fourmolu
# or
ghcup install fourmolu

Per-project override:
Use a .hlsfp.yaml or fourmolu.yaml in the project root for formatter-specific settings (line length, indentation style, etc.).

Format-on-save:

(add-hook 'haskell-mode-hook
  (lambda ()
    (add-hook 'before-save-hook #'eglot-format nil t)))

Is the issue that HLS isn't formatting at all, or that it's using the wrong formatter?