flake.nix

{
  description = "Category-parametric programming";

  nixConfig = {
    ## https://github.com/NixOS/rfcs/blob/master/rfcs/0045-deprecate-url-syntax.md
    extra-experimental-features = ["no-url-literals"];
    extra-substituters = ["https://cache.garnix.io"];
    extra-trusted-public-keys = [
      "cache.garnix.io:CTFPyKSLcx5RMJKfLo5EEPUObbA78b0YQ2DTCJXqr9g="
    ];
    ## Isolate the build.
    registries = false;
    sandbox = "relaxed";
  };

  ### This is a complicated flake. Here’s the rundown:
  ###
  ### overlays.default – includes all of the packages from cabal.project
  ### packages = {
  ###   default = points to `packages.${defaultGhcVersion}`
  ###   <ghcVersion>-<cabal-package> = an individual package compiled for one
  ###                                  GHC version
  ###   <ghcVersion>-all = all of the packages in cabal.project compiled for one
  ###                      GHC version
  ### };
  ### devShells = {
  ###   default = points to `devShells.${defaultGhcVersion}`
  ###   <ghcVersion> = a shell providing all of the dependencies for all
  ###                  packages in cabal.project compiled for one GHC version
  ### };
  ### checks.format = verify that code matches Ormolu expectations
  outputs = {
    flake-utils,
    flaky,
    flaky-haskell,
    nixpkgs,
    self,
    systems,
  }: let
    pname = "haskerwaul";

    supportedSystems = import systems;

    cabalPackages = pkgs: hpkgs:
      flaky-haskell.lib.cabalProject2nix
      ./cabal.project
      pkgs
      hpkgs
      (old: {
        configureFlags = old.configureFlags ++ ["--ghc-options=-Werror"];
      });
  in
    {
      schemas = {
        inherit
          (flaky.schemas)
          overlays
          homeConfigurations
          packages
          devShells
          projectConfigurations
          checks
          formatter
          ;
      };

      # see these issues and discussions:
      # - NixOS/nixpkgs#16394
      # - NixOS/nixpkgs#25887
      # - NixOS/nixpkgs#26561
      # - https://discourse.nixos.org/t/nix-haskell-development-2020/6170
      overlays = {
        default = final: prev:
          flaky-haskell.lib.overlayHaskellPackages
          (map self.lib.nixifyGhcVersion
            (self.lib.supportedGhcVersions final.system))
          (final: prev:
            nixpkgs.lib.composeManyExtensions [
              ## TODO: I think this overlay is only needed by formatters,
              ##       devShells, etc., so it shouldn’t be included in the
              ##       standard overlay.
              (flaky.overlays.haskellDependencies final prev)
              (self.overlays.haskell final prev)
              (self.overlays.haskellDependencies final prev)
            ])
          final
          prev;

        haskell = flaky-haskell.lib.haskellOverlay cabalPackages;

        ## NB: Dependencies that are overridden because they are broken in
        ##     Nixpkgs should be pushed upstream to Flaky. This is for
        ##     dependencies that we override for reasons local to the project.
        haskellDependencies = final: prev: hfinal: hprev: {};
      };

      homeConfigurations =
        builtins.listToAttrs
        (builtins.map
          (flaky.lib.homeConfigurations.example self [
            ({pkgs, ...}: {
              home.packages = [
                (pkgs.haskellPackages.ghcWithPackages (hpkgs: [hpkgs.${pname}]))
              ];
            })
          ])
          supportedSystems);

      lib = {
        nixifyGhcVersion = version:
          "ghc" + nixpkgs.lib.replaceStrings ["."] [""] version;

        ## TODO: Extract this automatically from `pkgs.haskellPackages`.
        defaultGhcVersion = "9.6.5";

        ## Test the oldest revision possible for each minor release. If it’s not
        ## available in nixpkgs, test the oldest available, then try an older
        ## one via GitHub workflow. Additionally, check any revisions that have
        ## explicit conditionalization. And check whatever version `pkgs.ghc`
        ## maps to in the nixpkgs we depend on.
        testedGhcVersions = system: [
          self.lib.defaultGhcVersion
          "8.10.7"
          "9.0.2"
          "9.2.5"
          "9.4.5"
          "9.6.3"
          "9.8.1"
          "9.10.1"
          # "ghcHEAD" # doctest doesn’t work on current HEAD
        ];

        ## The versions that are older than those supported by Nix that we
        ## prefer to test against.
        nonNixTestedGhcVersions = [
          ## NB: Haskerwaul relies heavily on `QuantifiedConstraints`, so GHC
          ##     versions prior to 8.6 won't work.
          "8.6.1"
          "8.8.1"
          "8.10.1"
          "9.0.1"
          "9.2.1"
          "9.4.1"
          "9.6.1"
          ## since `cabal-plan-bounds` doesn’t work under Nix
          "9.8.1"
          "9.10.1"
        ];

        ## However, provide packages in the default overlay for _every_
        ## supported version.
        supportedGhcVersions = system:
          self.lib.testedGhcVersions system
          ++ [
            "9.2.6"
            "9.2.7"
            "9.2.8"
            "9.4.6"
            "9.4.7"
            "9.4.8"
            "9.6.4"
            "9.6.5"
            "9.8.2"
          ];
      };
    }
    // flake-utils.lib.eachSystem supportedSystems
    (system: let
      pkgs = import nixpkgs {
        inherit system;
        ## NB: This uses `self.overlays.default` because packages need to
        ##     be able to find other packages in this flake as dependencies.
        overlays = [self.overlays.default];
      };
    in {
      packages =
        {
          default =
            self.packages.${system}."${self.lib.nixifyGhcVersion self.lib.defaultGhcVersion}_all";
        }
        // flaky-haskell.lib.mkPackages
        pkgs
        (map self.lib.nixifyGhcVersion (self.lib.supportedGhcVersions system))
        cabalPackages;

      devShells =
        {
          default =
            self.devShells.${system}.${self.lib.nixifyGhcVersion self.lib.defaultGhcVersion};
        }
        // flaky-haskell.lib.mkDevShells
        pkgs
        (map self.lib.nixifyGhcVersion (self.lib.supportedGhcVersions system))
        cabalPackages
        (hpkgs:
          [self.projectConfigurations.${system}.packages.path]
          ## NB: Haskell Language Server no longer supports GHC <9.2, and 9.4
          ##     has an issue with it on i686-linux.
          ## TODO: Remove the restriction on GHC 9.10 once
          ##       https://github.com/NixOS/nixpkgs/commit/e87381d634cb1ddd2bd7e121c44fbc926a8c026a
          ##       finds its way into 24.05.
          ++ nixpkgs.lib.optional
          (
            (
              if system == "i686-linux"
              then nixpkgs.lib.versionAtLeast hpkgs.ghc.version "9.4"
              else nixpkgs.lib.versionAtLeast hpkgs.ghc.version "9.2"
            )
            && nixpkgs.lib.versionOlder hpkgs.ghc.version "9.10"
          )
          hpkgs.haskell-language-server);

      projectConfigurations =
        flaky.lib.projectConfigurations.haskell {inherit pkgs self;};

      checks = self.projectConfigurations.${system}.checks;
      formatter = self.projectConfigurations.${system}.formatter;
    });

  inputs = {
    ## Flaky should generally be the source of truth for its inputs.
    flaky.url = "github:sellout/flaky";

    flake-utils.follows = "flaky/flake-utils";
    nixpkgs.follows = "flaky/nixpkgs";
    systems.follows = "flaky/systems";

    flaky-haskell = {
      inputs.flaky.follows = "flaky";
      url = "github:sellout/flaky-haskell";
    };
  };
}