Use cabal2nix for flake expression

flake.nix

@@ -3,33 +3,24 @@
 
   outputs = { self, nixpkgs }:
     let
-      pkgs = nixpkgs.legacyPackages.x86_64-linux;
+      system = "x86_64-linux";
+      pkgs = nixpkgs.legacyPackages.${system};
+      opium = pkgs.haskellPackages.developPackage {
+        root = ./.;
+        modifier = drv:
+          pkgs.haskell.lib.addBuildTools drv [
+            pkgs.cabal-install
+            pkgs.haskellPackages.implicit-hie
+            pkgs.haskell-language-server
+          ];
+      };
     in {
-      apps.x86_64-linux.cabal = {
+      packages.${system}.opium = pkgs.haskell.lib.overrideCabal opium {
-        type = "app";
+        # Currently the tests require a running Postgres instance.
-        program = "${nixpkgs.legacyPackages.x86_64-linux.cabal-install}/bin/cabal";
+        # This is not automated yet, so don't export the tests.
+        doCheck = false;
       };
-      devShells.x86_64-linux.default = pkgs.mkShell {
-        packages = [
-          pkgs.cabal-install
-          pkgs.haskellPackages.implicit-hie
-          (pkgs.ghc.withPackages (hp: with hp; [
-            attoparsec
-            containers
-            bytestring
-            hspec
-            postgresql-libpq
-            text
-            time
-            transformers
-            vector
-          ]))
 
-          pkgs.haskell-language-server
+      devShells.${system}.default = opium.env;
-        ];
-        shellHook = ''
-          PS1="<opium> ''${PS1}"
-        '';
-      };
     };
 }