GHCup discontinued (or why you should use Nix) (2025)

3 months ago 2

April 1, 2025, Posted by Julian Ospald

After working on GHCup for 6 years, I’ve realized that it’s obsolete. Everyone should just use nix.

Nix is a purely functional package manager with declarative configuration, reproducible builds and high quality packages. In fact, it is so simple that you just have to understand:

the nix package manager: https://nix.dev/manual/nix/2.26/store/
the nix language: https://nix.dev/tutorials/nix-language.html
how nixpkgs work: https://nixos.org/manual/nixpkgs/stable/
- the straight forward contribution guide: https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md
nix-shell: https://nix.dev/manual/nix/2.24/command-ref/nix-shell
nix flakes
or an easier alternative to nixpkgs, haskell.nix: https://input-output-hk.github.io/haskell.nix/tutorials/getting-started
you can also start purchasing templates: https://template.cs-syd.eu/

Here’s an example of a nix flake:

{ nixConfig.allow-import-from-derivation = true; description = "cabal-audit's flake"; inputs = { # flake inputs nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable"; # flake parts parts.url = "github:hercules-ci/flake-parts"; pre-commit-hooks.url = "github:cachix/git-hooks.nix"; devshell.url = "github:numtide/devshell"; # end flake parts # end flake inputs }; outputs = inputs: inputs.parts.lib.mkFlake {inherit inputs;} { systems = ["x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin"]; imports = [inputs.pre-commit-hooks.flakeModule inputs.devshell.flakeModule]; perSystem = { config, pkgs, lib, ... }: let hlib = pkgs.haskell.lib.compose; hspkgs = pkgs.haskell.packages.ghc98.override { overrides = import ./nix/haskell-overlay.nix {inherit hlib;}; }; in { # this flake module adds two things # 1. the pre-commit script which is automatically run when committing # which checks formatting and lints of both Haskell and nix files # the automatically run check can be bypassed with -n or --no-verify # 2. an attribute in the checks.<system> attrset which can be run with # nix flake check which checks the same lints as the pre-commit hook pre-commit = { check.enable = true; settings.hooks = { cabal-fmt.enable = true; fourmolu.enable = true; hlint.enable = true; alejandra.enable = true; statix.enable = true; deadnix.enable = true; }; }; devShells.plain-haskell = hspkgs.callPackage ./nix/haskell-shell.nix { inherit (pkgs.haskell.packages.ghc98) haskell-language-server fourmolu; }; # https://flake.parts/options/devshell for more information; one of the advantages is # the beautiful menu this provides where one can add commands that are offered and loaded # as part of the devShell devshells.default = { commands = [ { name = "lint"; help = "run formatting and linting of haskell and nix files in the entire repository"; command = "pre-commit run --all"; } { name = "regen-nix"; help = "regenerate nix derivations for haskell packages"; command = builtins.readFile (lib.getExe config.packages.regen-nix); } ]; devshell = { name = "cabal-audit"; packagesFrom = [config.devShells.plain-haskell]; packages = [pkgs.cabal2nix pkgs.alejandra]; startup.pre-commit.text = config.pre-commit.installationScript; }; }; packages = { inherit (hspkgs) cabal-audit; inherit (pkgs) groff; default = config.packages.cabal-audit; cabal-audit-static = pkgs.pkgsStatic.callPackage ./nix/static.nix {}; regen-nix = pkgs.writeShellApplication { name = "regen-cabal-audit-nix"; runtimeInputs = [pkgs.cabal2nix pkgs.alejandra]; text = let v = "ef73a3748f31d8df1557546b26d2d587cdacf459"; cmd = pkg: '' cabal2nix https://github.com/haskell/security-advisories.git \ --revision ${v} \ --subpath code/${pkg}/ > ./${pkg}.nix ''; in '' pushd "$PRJ_ROOT"/nix ${lib.concatStrings (map cmd ["osv" "cvss" "hsec-core" "hsec-tools"])} cabal2nix ../. > ./cabal-audit.nix alejandra ./. popd ''; }; }; }; }; }

I’m sure you’ll get it after going through the documentation above.

To summarize, the benefits are: