cabal install --lib doesn't work any more with GHC 9.2

[wz1000]

As mentioned by the release notes of GHC 9.2.1, GHC now respects the XDG specification for its user data directory (which includes package environments as well as GHCi configuration files). Howver, cabal hasn't been updated to reflect these changes yet, which means it still writes package environments to ~/.ghc, instead of $XDG_DATA_HOME.

This is the function in cabal that needs to be updated:

cabal/Cabal/src/Distribution/Simple/GHC.hs

Line 373 in 3bab1db

getGhcAppDir :: IO FilePath

The change to GHC was introduced in https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4567

[Mikolaj]

We are working on it for a decade now: #680 (three digits!), but I guess it's time to speed it up.

[phadej]

This is not about #680, but about fixing

cabal/Cabal/src/Distribution/Simple/GHC.hs

Lines 370 to 374 in 5b4258c

	-- | Return the 'FilePath' to the GHC application data directory.
	--
	-- @since 3.4.0.0
	getGhcAppDir :: IO FilePath
	getGhcAppDir = getAppUserDataDirectory "ghc"

which would need to take the GHC version now.

[Mikolaj]

Right, so one option is to use XDG when working with [edit: new versions of] GHC, but not use it when doing our own cabal things. And it seems we have good support for this distinction?

Edit: and we have no choice other than mixing XDG and no-XDG, because old GHCs don't use XDG, right? :(

[phadej]

The release notes for 9.2.1 say:

GHC now follows by default the XDG Base Directory Specification. If $HOME/.ghc is found it will fallback to the old paths to give you time to migrate. This fallback will be removed in three releases.

So why 9.2.1 doesn't work?

[phadej]

This is bug in GHC, I opened an upstream issue https://gitlab.haskell.org/ghc/ghc/-/issues/20684

[Mikolaj]

This seems to fix the problem, but I'm not 100% sure it's going to be backported to 9.2: https://gitlab.haskell.org/ghc/ghc/-/merge_requests/7010

[Mikolaj]

Good news, the GHC fix has just been backported and merged to GHC 9.2.2 (to be released).

[Mikolaj]

It seems we don't need to do anything, but let's keep this issue open until GHC 9.2.2 and possibly until somebody verifies it works.

[wz1000]

It would still be good to move towards respecting the XDG spec, but it is not a pressing concern as of now.

[Mikolaj]

It would still be good to move towards respecting the XDG spec, but it is not a pressing concern as of now.

Yes, if anybody would like to help, that'd be awesome. We track that work in another ticket mentioned above.

[gbaz]

The fix for this is in the next ghc -- 9.2.2.