flake-utils -> flake-parts (#20)

README.md

@@ -12,7 +12,7 @@ Haskell project template optimized for a fully reproducible and friendly develop
 
 First-time setup:
 
-- [Install Nix](https://nixos.org/download.html) & [enable Flakes](https://nixos.wiki/wiki/Flakes)
+- [Install Nix](https://nixos.org/download.html) (>= 2.8) & [enable Flakes](https://nixos.wiki/wiki/Flakes)
 - Run `nix develop -c haskell-language-server` to sanity check your environment 
 - [Open as single-folder workspace](https://code.visualstudio.com/docs/editor/workspaces#_singlefolder-workspaces) in Visual Studio Code
     - When prompted by VSCode, install the [workspace recommended](https://code.visualstudio.com/docs/editor/extension-marketplace#_workspace-recommended-extensions) extensions
@@ -30,6 +30,7 @@ Open `Main.hs`, and expect all HLS IDE features like hover-over tooltip to work
 Renaming the project:
 
 ```sh
+# First, click the green "Use this template" button on GitHub to create your copy.
 git clone <your-clone-url>
 cd your-project
 NAME=myproject

flake.lock

@@ -16,19 +16,23 @@
         "type": "github"
       }
     },
-    "flake-utils": {
+    "flake-parts": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
       "locked": {
-        "lastModified": 1652776076,
-        "narHash": "sha256-gzTw/v1vj4dOVbpBSJX4J0DwUR6LIyXo7/SuuTJp1kM=",
-        "owner": "numtide",
-        "repo": "flake-utils",
-        "rev": "04c1b180862888302ddfb2e3ad9eaa63afc60cf8",
+        "lastModified": 1653510071,
+        "narHash": "sha256-KoBs0USqNunyxaBOYx/trSREMf8f3fA+msDfADiMI5o=",
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "rev": "c7d1a3d10117247892df7db45cef562bf62789d5",
         "type": "github"
       },
       "original": {
-        "owner": "numtide",
-        "ref": "v1.0.0",
-        "repo": "flake-utils",
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
         "type": "github"
       }
     },
@@ -51,7 +55,7 @@
     "root": {
       "inputs": {
         "flake-compat": "flake-compat",
-        "flake-utils": "flake-utils",
+        "flake-parts": "flake-parts",
         "nixpkgs": "nixpkgs"
       }
     }

flake.nix

@@ -2,29 +2,24 @@
   description = "haskell-template's description";
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
-    flake-utils.url = "github:numtide/flake-utils/v1.0.0";
-    flake-utils.inputs.nixpkgs.follows = "nixpkgs";
+    flake-parts.url = "github:hercules-ci/flake-parts";
+    flake-parts.inputs.nixpkgs.follows = "nixpkgs";
     flake-compat.url = "github:edolstra/flake-compat";
     flake-compat.flake = false;
     flake-compat.inputs.nixpkgs.follows = "nixpkgs";
   };
 
-  # Consider this to be a function producing Flake outputs for the given system
-  # and inputs; viz.:
-  # 
-  #   mkOutputsFrom :: Set Inputs -> System -> Set Outputs
-  #   mkOutputsFrom inputs system = { ... }
-  #
-  # We use eachDefaultSystem to allow other architectures.
+  # We use flake-parts as a way to make flakes 'system-aware'
   # cf. https://github.com/NixOS/nix/issues/3843#issuecomment-661720562
-  outputs = inputs:
-    inputs.flake-utils.lib.eachDefaultSystem
-      (system:
+  outputs = { self, nixpkgs, flake-parts, ... }:
+    flake-parts.lib.mkFlake { inherit self; } {
+      systems = nixpkgs.lib.systems.supported.hydra;
+      # The primed versions (self', inputs') are same as the non-primed
+      # versions, but with 'system' already applied.
+      perSystem = { self', inputs', pkgs, system, ... }:
         let
           name = "haskell-template";
 
-          # Because: https://zimbatm.com/notes/1000-instances-of-nixpkgs
-          pkgs = inputs.nixpkgs.legacyPackages.${system};
           inherit (pkgs.lib.lists) optionals;
 
           # Specify GHC version here. To get the appropriate value, run:
@@ -54,7 +49,7 @@
                 # Use callCabal2nix to override Haskell dependencies here
                 # cf. https://tek.brick.do/K3VXJd8mEKO7
                 # Example: 
-                # > NanoID = self.callCabal2nix "NanoID" inputs.NanoID { };
+                # > NanoID = self.callCabal2nix "NanoID" inputs'.NanoID { };
                 # Assumes that you have the 'NanoID' flake input defined.
               };
               modifier = drv:
@@ -75,17 +70,13 @@
           apps = {
             default = {
               type = "app";
-              program = "${inputs.self.packages.${system}.default}/bin/${name}";
+              program = "${self'.packages.default}/bin/${name}";
             };
           };
           # Used by `nix develop ...`
           devShells = {
             default = project { returnShellEnv = true; withHoogle = true; };
           };
-          # For compatability with older Nix (eg in CI)
-          devShell = inputs.self.devShells.${system}.default;
-          defaultPackage = inputs.self.packages.${system}.default;
-          defaultApp = inputs.self.apps.${system}.default;
-        }
-      );
+        };
+    };
 }