Delegate most Nix to srid/haskell-flake (a flake-parts module for Haskell dev) (#22)

* Move Haskell stuff into separate module

cf. https://github.com/hercules-ci/flake-parts/blob/main/modules/apps.nix

* Parametrize the haskell part

* Allow user to override default attrs

* Add baseBuildTools (so they can be overriden)

* Don't hardcode non-essential build tools

* Avoid types.anything

* Merge into one buildTools, using sets

* getExe and type are redundant

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

* Use docstring wherever possible

* Use ${self} as default for root

* Allow defining multiple Haskell packages

The downside: the user is forced to define 'default' flake outputs.

* Automatically create 'default' attr if a singleton projects set

* Make haskell.nix its own repo

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

flake.lock

@@ -36,6 +36,21 @@
         "type": "github"
       }
     },
+    "haskell-flake": {
+      "locked": {
+        "lastModified": 1653916420,
+        "narHash": "sha256-RQSbdhZCeDqZkYJN+3g5tSh+TmW2cXXadX7yGNuajg8=",
+        "owner": "srid",
+        "repo": "haskell-flake",
+        "rev": "812ca2cdbc7be6026541125da09ec7e7971b76ec",
+        "type": "github"
+      },
+      "original": {
+        "owner": "srid",
+        "repo": "haskell-flake",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1653326962,
@@ -56,6 +71,7 @@
       "inputs": {
         "flake-compat": "flake-compat",
         "flake-parts": "flake-parts",
+        "haskell-flake": "haskell-flake",
         "nixpkgs": "nixpkgs"
       }
     }

flake.nix

@@ -1,5 +1,5 @@
 {
-  description = "haskell-template's description";
+  description = "srid/haskell-template: Nix template for Haskell projects";
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
     flake-parts.url = "github:hercules-ci/flake-parts";
@@ -7,74 +7,26 @@
     flake-compat.url = "github:edolstra/flake-compat";
     flake-compat.flake = false;
     flake-compat.inputs.nixpkgs.follows = "nixpkgs";
+    haskell-flake.url = "github:srid/haskell-flake";
   };
 
-  # We use flake-parts as a way to make flakes 'system-aware'
-  # cf. https://github.com/NixOS/nix/issues/3843#issuecomment-661720562
-  outputs = { self, nixpkgs, flake-parts, ... }:
+  outputs = { self, nixpkgs, flake-parts, haskell-flake, ... }:
     flake-parts.lib.mkFlake { inherit self; } {
       systems = nixpkgs.lib.systems.flakeExposed;
-      # The primed versions (self', inputs') are same as the non-primed
-      # versions, but with 'system' already applied.
-      perSystem = { self', inputs', pkgs, system, ... }:
-        let
-          inherit (pkgs.lib.lists) optionals;
-
-          # Specify GHC version here. To get the appropriate value, run:
-          #   nix-env -f "<nixpkgs>" -qaP -A haskell.compiler
-          hp = pkgs.haskellPackages; # Eg: pkgs.haskell.packages.ghc921;
-
-          # Specify your build/dev dependencies here.
-          shellDeps = with hp; [
-            cabal-fmt
-            cabal-install
-            ghcid
-            haskell-language-server
-            fourmolu
-            hlint
-            pkgs.nixpkgs-fmt
-            pkgs.treefmt
+      imports = [
+        haskell-flake.flakeModule
       ];
-
-          project =
-            { returnShellEnv ? false
-            , withHoogle ? false
-            }:
-            hp.developPackage {
-              inherit returnShellEnv withHoogle;
-              name = "haskell-template";
-              root = ./.;
-              overrides = self: super: with pkgs.haskell.lib; {
-                # Use callCabal2nix to override Haskell dependencies here
-                # cf. https://tek.brick.do/K3VXJd8mEKO7
-                # Example: 
-                # > NanoID = self.callCabal2nix "NanoID" inputs'.NanoID { };
-                # Assumes that you have the 'NanoID' flake input defined.
+      perSystem = { self', pkgs, ... }: {
+        haskellProjects.haskell-template = {
+          buildTools = hp: {
+            # TODO: Use https://github.com/numtide/treefmt/pull/169
+            inherit (pkgs)
+              treefmt
+              nixpkgs-fmt;
+            inherit (hp)
+              cabal-fmt
+              fourmolu;
           };
-              modifier = drv:
-                pkgs.haskell.lib.overrideCabal drv (oa: {
-                  # All the Cabal-specific overrides go here.
-                  # For examples on what is possible, see:
-                  #   https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/haskell-modules/lib/compose.nix
-                  buildTools = (oa.buildTools or [ ]) ++ optionals returnShellEnv shellDeps;
-                });
-            };
-        in
-        {
-          # Used by `nix build ...`
-          packages = {
-            default = project { };
-          };
-          # Used by `nix run ...`
-          apps = {
-            default = {
-              type = "app";
-              program = pkgs.lib.getExe self'.packages.default;
-            };
-          };
-          # Used by `nix develop ...`
-          devShells = {
-            default = project { returnShellEnv = true; withHoogle = true; };
         };
       };
     };