"Could not find module" for module from dependency

[slondr]

Steps to reproduce:

1. Create a new project with cabal
2. Put this in Main.hs:

module Main where

import Brick

ui :: Widget ()
ui = str "Hello, world!"

main :: IO ()
main = simpleMain ui

3. Change build-depends in project.cabal to this:

    build-depends:    base ^>=4.18.3.0
                    , brick ^>=2.9

4. Open Main.hs and start lsp.

In this case, cabal build works just fine, as does eglot. But lsp reports the following error on line 3:

Could not find module 'Brick'.

I tried M-x cd to the project root directory, and also to the app/ directory where Main.hs is located, but that didn't seem to affect anything.

[dschrempf]

Could it be that you started HLS in the project (i.e., the file) already before changing the .cabal file? There is a known limitation that HLS does not pick up changes in .cabal files and has to be restarted (lsp-workspace-restart).

However, (and that's a big one), HLS recently learned to reload .cabal files automatically (haskell/haskell-language-server#4630). That is, the next release of HLS should not suffer from this limitation.

[slondr]

Thanks for the quick response. Sadly I still seem to get the error even after running lsp-workspace-restart.

[dschrempf]

We need more information to debug your session. But you can also get active, and have a look at *lsp- buffers.

I expect your workspace root to be off. When you start the lsp server, it reports the workspace root. Does this root coincide with your expected project root?