nix: add support for ghc 9.0.1

[berberman]

As discussed in #1871 (comment), this PR attempts to maintain a nix configuration that works for ghc 9.0.1. It's still work in progress, see the CI for results.

[anka-213]

It looks like it is trying to build ghc-lib-parser-8.10.5 when compiling for ghc-9. I'm guessing that is the default version in nixpkgs. Maybe overriding with something like

ghc-lib-parser = hsuper.ghc-lib-parser_9_0_1_20210324;

would work?

(Or hself instead of hsuper? I never learned when to use which.)

[maralorn]

(Or hself instead of hsuper? I never learned when to use which.)

My rule of thumb is this: Always use hself when you can, only use hsuper to break recursion cycles.

That mostly means use foo = f hsuper.foo but bar = f hself.foo.

Reasoning is that when you do bar = f hself.foo, there could be other overrides for foo in this or a subsequent overlay and you most likely want those overrides to apply to your new bar. You wouldn‘t get them from hsuper.foo.

[berberman]

Now both nix build .#haskell-language-server-901 and nix develop .#haskell-language-server-901-dev work; I'm able to use HLS binary created by nix build to load HLS project in nix develop shell!

[jneira]

I am not a nix user but lgtm