ghc 8.8.3 cannot find cabal 3.0.0.0 packages

I’ve been using cabal to manage my Haskell dependencies for years, but when I last updated my system it suddenly stopped working. I installed my dependencies with cabal install xmonad, and checked that it’s installed under ~/.cabal:

$ ls .cabal/bin/
xmonad@

But still when I go to compile my xmonad config file ghc says it cannot find it:

Could not find module ‘XMonad’

And indeed when I run ghc-pkg list it does not list xmonad. That’s weird.

$ cabal --version
cabal-install version 3.0.0.0

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.8.3

Apparently this is a known issue (including the “blame game” if it’s a cabal or a ghc issue, but that doesn’t make it any less annoying for us who just want it work). The ghc issue is still open.

Fortunately there’s a solution.

We can manually specify a package db for ghc-pkg so it’ll find our cabal modules:

$ ghc-pkg --package-db ~/.cabal/store/ghc-8.8.3/package.db list
...
xmonad-0.15
xmonad-contrib-0.16
...

And to make it permanent we can symlink the package.db file as a ghc config:

$ mkdir -p ~/.ghc/x86_64-linux-8.8.3/
$ ln -s ~/.cabal/store/ghc-8.8.3/package.db ~/.ghc/x86_64-linux-8.8.3/package.conf.d

The name of the folder x86_64-linux-8.8.3 may differ, this one worked for me.

And now ghc-pkg list should find xmonad and compiling with ghc should find our missing cabal modules.