Throw a pretty error if the hooks cannot be sorted

If the hooks form a cycle, we now throw an error with the hooks in
question and show their `before` and `after` fields to help identify the
cycle.

Author: sandydoo

modules/pre-commit.nix

@@ -35,7 +35,32 @@ let
         (a: b: builtins.elem b.id a.before || builtins.elem a.id b.after)
         (builtins.attrValues enabledHooks);
     in
-    builtins.map (value: value.raw) sortedHooks.result;
+    if sortedHooks ? result then
+      builtins.map (value: value.raw) sortedHooks.result
+    else
+      let
+        getIds = builtins.map (value: value.id);
+
+        prettyPrintCycle = opts: cycle:
+          lib.pipe cycle [
+            (builtins.map (hook:
+              lib.nameValuePair hook.id { before = hook.before; after = hook.after; }
+            ))
+            lib.listToAttrs
+            (lib.generators.toPretty opts)
+          ];
+      in
+      throw ''
+        The hooks can't be sorted because of a cycle in the dependency graph:
+
+          ${concatStringsSep " -> " (getIds sortedHooks.cycle)}
+
+          which leads to a loop back to: ${concatStringsSep ", " (getIds sortedHooks.loops)}
+
+        Try removing the conflicting hook ids from the `before` and `after` attributes of these hooks:
+
+          ${prettyPrintCycle { indent = "  "; } sortedHooks.cycle}
+      '';
 
   configFile =
     performAssertions (