Add code from haskell-template

.gitignore

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+.direnv
+result

flake-module.nix

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+# A flake-parts module for Haskell cabal projects.
+{ self, config, lib, flake-parts-lib, ... }:
+
+let
+  inherit (flake-parts-lib)
+    mkSubmoduleOptions
+    mkPerSystemOption;
+  inherit (lib)
+    mkOption
+    mkDefault
+    types;
+  inherit (types)
+    functionTo
+    raw;
+in
+{
+  options = {
+    perSystem = mkPerSystemOption
+      ({ config, self', inputs', pkgs, system, ... }: {
+        options.haskellProjects = mkOption {
+          description = "Haskell projects";
+          type = types.attrsOf (types.submodule {
+            options = {
+              haskellPackages = mkOption {
+                type = types.attrsOf raw;
+                description = ''
+                  Which Haskell package set to use
+
+                  You can effectively select the GHC version here. To get the appropriate value, run:
+                    nix-env -f "<nixpkgs>" -qaP -A haskell.compiler
+                '';
+                default = pkgs.haskellPackages;
+              };
+              root = mkOption {
+                type = types.path;
+                description = ''Path to the Cabal project root'';
+                default = "${self}";
+              };
+              overrides = mkOption {
+                type = functionTo (functionTo (types.attrsOf raw));
+                description = ''Overrides for the Cabal project'';
+                default = self: super: { };
+              };
+              modifier = mkOption {
+                type = functionTo types.package;
+                description = ''
+                  Modifier for the Cabal project
+
+                  Typically you want to use `overrideCabal` to override various
+                  attributes of Cabal project.
+              
+                  For examples on what is possible, see:
+                  https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/haskell-modules/lib/compose.nix
+                '';
+                default = drv: drv;
+              };
+              buildTools = mkOption {
+                type = functionTo (types.attrsOf (types.nullOr types.package));
+                description = ''Build tools for your Haskell package (available only in nix shell).'';
+                default = hp: { };
+                defaultText = ''Build tools useful for Haskell development are included by default.'';
+              };
+            };
+          });
+        };
+      });
+  };
+  config = {
+    perSystem = { config, self', inputs', pkgs, ... }:
+      let
+        projects =
+          lib.mapAttrs
+            (name: cfg:
+              let
+                inherit (pkgs.lib.lists) optionals;
+                hp = cfg.haskellPackages;
+                defaultBuildTools = with hp; {
+                  inherit
+                    cabal-install
+                    haskell-language-server
+                    ghcid
+                    hlint;
+                };
+                buildTools = lib.attrValues (defaultBuildTools // cfg.buildTools hp);
+                mkProject = { returnShellEnv ? false, withHoogle ? false }:
+                  hp.developPackage {
+                    inherit returnShellEnv withHoogle name;
+                    inherit (cfg) root overrides;
+                    modifier = drv:
+                      cfg.modifier (pkgs.haskell.lib.overrideCabal drv (oa: {
+                        buildTools = (oa.buildTools or [ ]) ++ optionals returnShellEnv buildTools;
+                      }));
+                  };
+              in
+              rec {
+                package = mkProject { };
+                app = { program = package; };
+                devShell = mkProject { returnShellEnv = true; withHoogle = true; };
+              }
+            )
+            config.haskellProjects;
+        # Inject a 'default' attr if the attrset is a singleton set.
+        withDefault = attrs:
+          let xs = lib.attrValues attrs; in
+          if builtins.length xs == 1
+          then attrs // { default = mkDefault (builtins.head xs); }
+          else attrs;
+      in
+      {
+        packages =
+          withDefault (lib.mapAttrs
+            (_: project: project.package)
+            projects);
+        apps =
+          withDefault (lib.mapAttrs
+            (_: project: project.app)
+            projects);
+        devShells =
+          withDefault (lib.mapAttrs
+            (_: project: project.devShell)
+            projects);
+      };
+  };
+}

flake.nix

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+{
+  description = "A `flake-parts` module for Haskell development";
+  outputs = { self, ... }: {
+    flakeModule = ./flake-module.nix;
+  };
+}