nix: Add checks

answers.nix

@@ -0,0 +1,6 @@
+{
+  day1 = {
+    part1 = "53194";
+    part2 = "54249";
+  };
+}

flake.nix

@@ -34,6 +34,7 @@
         ghc94Pkgs = pkgs.haskell.packages.ghc94.override {
           overrides = ghcOverrides;
         };
+        answers = import ./answers.nix;
       in rec {
         packages = rec {
           dev-env = ghc94Pkgs.shellFor {
@@ -43,10 +44,42 @@
               (pkgs.haskell-language-server.override {supportedGhcVersions = ["948"];})
               pkgs.cabal2nix
               pkgs.ormolu
+              pkgs.shellcheck
             ];
           };
-          aoc2023 = ghc94Pkgs.aoc2023;
+          aoc2023-haskell = ghc94Pkgs.aoc2023;
+          aoc2023-bash =
+            let bashDir = pkgs.runCommand "aoc2023-bash" {} ''
+              mkdir $out
+              cp -r ${./bash} $out/bin
+              chmod -R +w $out
+              patchShebangs $out
+            '';
+            in pkgs.writeShellApplication {
+              name = "aoc2023";
+              runtimeInputs = [ pkgs.util-linux ];
+              text = ''
+                ${bashDir}/bin/main.sh "''${@}"
+              '';
             };
+        };
+        checks =
+          let mkCheck = (prefix: exec: day: parts: {
+                name = "${prefix}-${day}";
+                value = pkgs.testers.testEqualContents {
+                  assertion = day;
+                  expected = pkgs.writeText "expected" ''
+                    Part 1 Answer: ${parts.part1}
+                    Part 2 Answer: ${parts.part2}
+                  '';
+                  actual = pkgs.runCommand "actual" {} ''
+                    "${exec}" "${day}" < "${./input}/${day}" >$out
+                  '';
+                };
+              });
+
+          in pkgs.lib.attrsets.mapAttrs' (mkCheck "haskell" "${packages.aoc2023-haskell}/bin/aoc2023") answers
+          // pkgs.lib.attrsets.mapAttrs' (mkCheck "bash" "${packages.aoc2023-bash}/bin/aoc2023") answers;
         defaultPackage = packages.dev-env;
       });
 }