Add a default.nix for nix-build

byg/byg.cabal

@@ -1,4 +1,4 @@
-cabal-version:      3.12
+cabal-version:      3.8
 name:               byg
 version:            0.1.0.0
 synopsis:           The website builder for mad

byg/default.nix

@@ -0,0 +1,2 @@
+with import ./nix/common.nix;
+haskell.callCabal2nix "byg" ./. { }

byg/nix/common.nix

@@ -0,0 +1,9 @@
+let
+  sources = import ./sources.nix;
+  pkgs = import sources.nixpkgs {};
+  haskell = pkgs.haskell.packages.ghc910;
+in
+{
+  pkgs = pkgs;
+  haskell = haskell;
+}

byg/shell.nix

@@ -1,12 +1,7 @@
-let
-  sources = import ./nix/sources.nix;
-  pkgs = import sources.nixpkgs {};
-  haskell = pkgs.haskell.packages.ghc910;
-in
-pkgs.stdenv.mkDerivation {
-  name = "byg";
-  buildInputs = with pkgs; [
+with import ./nix/common.nix;
+pkgs.mkShell {
+  buildInputs = [
     haskell.ghc
-    cabal-install
+    pkgs.cabal-install
   ];
 }