Program error: Failed to load interface for ‘Main’ no unit id matching ‘main’ was found

[d-xo]

I have this error in the main executable file in a two projects (hevm, and act).

In both of these projects haskell-language-server works very well (type checking / doc lookup / jump to definition) in all files except the main executable (Main.hs in act and hevm-cli.hs in hevm).

cabal new-build works without errors in both projects.

I am using a globally installed version of hakell-language-server built against ghc8.6.5 using the nix deritvations here.

The full output from running haskell-language-server from the root of the act repo is here.

If you want to reproduce the issue locally, it's probably easiest to use the act project. The shell.nix in the repo root can be used to prepare a working build environment, but does not currently include an installation of haskell-language-server itself.

[d-xo]

The issue was missing module Main where declarations. Seems obvious in retrospect although it is quite confusing that cabal doesn't seem to need them.

[fendor]

Hey! I have encountered the same issue, the reason is GHC will auto-insert the module declaration (if it is missing) and add Main.hs to the unit main. Then it looks up the unit, and can't find it, hence the error.
I say we keep this issue open because it should not fail with this error message since cabal accepts it.

Earlier today, I actually looked into your repo's since I had the suspicion regarding module declarations but in the linked repo, the module declaration was present. Did you change that locally or something?

[d-xo]

I added the module Main where declarations yesterday

[jneira]

I cant reproduce with lastest hls and a Main module without the header, @xwvvvvwx could you confirm it it is fixed for you too?

[d-xo]

I can still reproduce with haskell-language-server 0.4.0.0 from nixpkgs.

[jneira]

Thanks for checking it again, it is possible that the os could be related (i am on windows) but i assumed that it should not be.
@fendor could that make sense?

[fendor]

No, it is close to impossible that the OS is related. I will check if I can still reproduce it and if it is actually what I think it is.

[jneira]

I've just double check it with ghc-8.10.1 (my previous test was with ghc-8.8.3), a main module without the header:

No error in the log.

[jneira]

@fendor did you have the opportunity of checking if it is reproduced?

[fendor]

I can't reproduce it anymore and furthermore, I would expect #1435 fixes it for good, too.

[jneira]

@fendor nice, thanks for confirming it
@xwvvvvwx feel freeto reopen if you continue experienced the issue