When main module is a symlink, HLS fails to load it

[chreekat]

Your environment

Which OS do you use?
NixOS 24.05.6966.190c31a89e5e (Uakari)

Which version of GHC do you use and how did you install it?
The Glorious Glasgow Haskell Compilation System, version 9.6.5 from OS package

How is your project built (alternative: link to the project)?
cabal build

Which LSP client (editor/plugin) do you use?
VIM - Vi IMproved 9.1 (2024 Jan 02, compiled Jan 01 1980 00:00:00)
+ coc.nvim

Which version of HLS do you use and how did you install it?
haskell-language-server-9.6.5 from OS package

Have you configured HLS in any way (especially: a hie.yaml file)?
No

Steps to reproduce

1. Use cabal init to create an executable package
2. mv Main.hs Real.hs
3. ln -s Real.hs Main.hs
4. Start vim

Expected behaviour

HLS should load the module and provide hints.

Actual behaviour

I get a popup on line 1 with text:

>>module Main where
   Loading the module '/home/b/tmp/symlinkmain/Main.hs' failed.

   It may not be listed in your .cabal file!
   Perhaps you need to add `Main` to other-modules or exposed-modules.

   For more information, visit: https://cabal.readthedocs.io/en/3.4/developing-
   packages.html#modules-included-in-the-package
    (cradle)

The error message is incorrect, at the very least.

Debug information

[fendor]

While the error message is bad and incorrect, cabal is the reason this fails:

> cabal repl Main.hs
Error: [Cabal-7121]
Failed extracting script block: `{- cabal:` start marker not found

Can you confirm this?

[chreekat]

@fendor yes, although a bare cabal repl works just fine!

[chreekat]

[b@kuusi:~/tmp/symlinkmain]$ cabal repl app/Main.hs
Error: cabal: Failed extracting script block: `{- cabal:` start marker not
found


[b@kuusi:~/tmp/symlinkmain]$ cabal repl
Build profile: -w ghc-9.6.5 -O1
In order, the following will be built (use -v for more details):
 - symlinkmain-0.1.0.0 (exe:symlinkmain) (first run)
Preprocessing executable 'symlinkmain' for symlinkmain-0.1.0.0..
GHCi, version 9.6.5: https://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/b/Projects/dotfiles/ghci
[1 of 2] Compiling Main             ( app/Main.hs, interpreted )
Ok, one module loaded.
|*Main|
λ

[fendor]

Yeah, but HLS uses cabal repl app/Main.hs by default, so that's a blocker for this issue.

There is a workaround, but it is not universally applicable. hie.yaml like this

cradle:
  cabal:
    component: all

and telling HLS to use multi component loading ala "haskell.sessionLoading": "multipleComponents". Requires cabal 3.12 and that all dependencies can be built using Cabal >= 3.12.

This setup implements an eager loading strategy where HLS loads all components in your project at once, which is likely the best user experience, as long as your RAM can take it. Don't forget to cabal configure --enable-tests --enable-benchmarks, though, see haskell/cabal#8605 (comment)

[chreekat]

Cool beans, I opened haskell/cabal#10627.