migrated all languages to devshell

modules/dev/haskell.nix

@@ -4,23 +4,45 @@
   pkgs,
   ...
 }:
+let
+  packages = builtins.attrValues {
+    inherit (pkgs)
+      haskell-language-server # LSP server for Haskell
+      cabal-install # Standard Haskell build tool
+      hlint # Linter for Haskell source code
+      ;
+    inherit (pkgs.haskellPackages)
+      hoogle # Haskell API search engine
+      ;
+  };
+in
 {
-  options.my.dev.haskell.enable = lib.mkEnableOption "enable";
-  config = lib.mkIf config.my.dev.haskell.enable {
-    users.users.jawz.packages = builtins.attrValues {
-      inherit (pkgs)
-        haskell-language-server # LSP server for Haskell
-        cabal-install # Standard Haskell build tool
-        hlint # Linter for Haskell source code
-        ;
-      inherit (pkgs.haskellPackages)
-        hoogle # Haskell API search engine
-        ;
-    };
-    environment.variables = {
-      CABAL_DIR = "\${XDG_CACHE_HOME}/cabal";
-      STACK_ROOT = "\${XDG_DATA_HOME}/stack";
-      GHCUP_USE_XDG_DIRS = "true";
+  options = {
+    my.dev.haskell.enable = lib.mkEnableOption "Install Haskell tooling globally";
+    devShells.haskell = lib.mkOption {
+      type = lib.types.package;
+      default = pkgs.mkShell {
+        inherit packages;
+        name = "haskell-dev-shell";
+        shellHook = ''
+          echo "λ Haskell dev environment"
+        '';
+      };
+      description = "Haskell development shell";
     };
   };
+  config = lib.mkMerge [
+    (lib.mkIf config.my.dev.haskell.enable {
+      users.users.jawz = {
+        inherit packages;
+      };
+    })
+    {
+      environment.variables = {
+        CABAL_DIR = "\${XDG_CACHE_HOME}/cabal";
+        STACK_ROOT = "\${XDG_DATA_HOME}/stack";
+        GHCUP_USE_XDG_DIRS = "true";
+      };
+    }
+  ];
 }