flake.nix

{
  description = "A VTE-based terminal emulator configurable in Haskell";

  # Nixpkgs / NixOS version to use.
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";

  outputs = { self, nixpkgs }:
    let

      # Generate a user-friendly version numer.
      version = builtins.substring 0 8 self.lastModifiedDate;

      # System types to support.
      #
      # TODO: Since callCabal2nix uses IFD, adding multiple systems to
      # supportedSystems doesn't currently work with flakes.  Commands like
      # `nix flake check` and `nix flake show` don't work.
      #
      # https://github.com/NixOS/nix/issues/4265
      #
      # Termonad is likely to also work aarch64-linux, but you'll have to edit
      # this line to enable it.
      supportedSystems = [ "x86_64-linux" ];

      # Helper function to generate an attrset '{ x86_64-linux = f "x86_64-linux"; ... }'.
      forAllSystems = f: nixpkgs.lib.genAttrs supportedSystems (system: f system);

      # Nixpkgs instantiated for supported system types.
      nixpkgsFor = forAllSystems (system: import nixpkgs { inherit system; overlays = [ self.overlay ]; });

    in

    {

      # A Nixpkgs overlay that defines Termonad.
      overlay = final: prev:
        let
          overlays = import ./.nix-helpers/overlays.nix;
        in
        prev.lib.composeManyExtensions overlays final prev;

      # Provide some binary packages for selected system types.
      packages = forAllSystems (system:
        let
          pkgs = nixpkgsFor.${system};
        in {
          termonad = pkgs.termonad;
        }
      );

      # The default package for 'nix build'. This makes sense if the
      # flake provides only one package or there is a clear "main"
      # package.
      defaultPackage = forAllSystems (system: self.packages.${system}.termonad);

      devShell = forAllSystems (system: nixpkgsFor.${system}.termonadShell);

      defaultApp = forAllSystems (system: self.apps.${system}.termonad);

      apps = forAllSystems (system: {
        termonad = {
          type = "app";
          program = "${self.packages.${system}.termonad}/bin/termonad";
        };
      });
    };
}