Check Emacs versions

flake.nix

@@ -67,5 +67,31 @@
           emacsPackages)
       );
     });
+
+    checks = eachSystem (
+      pkgs:
+        lib.mapAttrs' (
+          name: emacsPackage:
+            lib.nameValuePair
+            "${name}-version" (pkgs.runCommand "check-${name}-version" {
+                buildInputs = [
+                  emacsPackage
+                ];
+              } ''
+                EXPECTED="GNU Emacs ${emacsPackage.version}"
+                ACTUAL="$(emacs --version | grep -F 'GNU Emacs' | head -1)"
+                if [[ "$EXPECTED" = "$ACTUAL" ]]
+                then
+                  echo >&2 "OK: $ACTUAL"
+                  touch "$out"
+                  exit 0
+                else
+                  echo >&2 "NG: $EXPECTED (expected) vs. $ACTUAL (actual)"
+                  exit 1
+                fi
+              '')
+        )
+        emacsPackages
+    );
   };
 }