unread [nfc]: Factor out deleteFromListMap.

Should be helpful when converting over the other parts of the
unread state.

src/unread/unreadModel.js

@@ -136,6 +136,28 @@ function deleteFromList<V>(
   return list.filterNot(id => toDeleteSet.has(id));
 }
 
+const emptyList = Immutable.List();
+
+/**
+ * Remove the given values, given where to find them; and prune.
+ *
+ * That is, for each entry in `toDelete`, we apply `deleteFromList` with the
+ * given list to the corresponding entry in `state`.  When the resulting
+ * list is empty, we prune that entry entirely.
+ */
+function deleteFromListMap<K, V>(
+  state: Immutable.Map<K, Immutable.List<V>>,
+  toDelete: Immutable.Map<K, Immutable.List<V>>,
+): Immutable.Map<K, Immutable.List<V>> {
+  // prettier-ignore
+  return state.withMutations(mut => {
+    toDelete.forEach((msgIds, key) => {
+      updateAndPrune(mut, emptyList, key, list =>
+        list && deleteFromList(list, msgIds));
+    });
+  });
+}
+
 /**
  * Delete the given messages from the unreads state.
  *
@@ -176,18 +198,11 @@ function deleteMessages(
     });
 
   const emptyMap = Immutable.Map();
-  const emptyList = Immutable.List();
   // prettier-ignore
   return state.withMutations(stateMut => {
     byConversation.forEach((byTopic, streamId) => {
       updateAndPrune(stateMut, emptyMap, streamId, perStream =>
-        perStream && perStream.withMutations(perStreamMut => {
-          byTopic.forEach((msgIds, topic) => {
-            updateAndPrune(perStreamMut, emptyList, topic, perTopic =>
-              perTopic && deleteFromList(perTopic, msgIds),
-            );
-          });
-        }),
+        perStream && deleteFromListMap(perStream, byTopic),
       );
     });
   });