(list)+reduce function

Basically accumulates values, check
https://en.wikipedia.org/wiki/Fold_(higher-order_function) for more
detail.

list.cpp

@@ -63,6 +63,18 @@ void map(List<T> *lst, U (*f)(T))
   map(lst->next, f);
 }
 
+template <typename T>
+T reduce(List<T> *lst, T (*reducer) (T, T), T init = 0)
+{
+  if (!lst)
+    return init;
+  if (!init)
+    init = lst->value;
+  else
+    init = reducer(init, lst->value);
+  return reduce(lst->next, reducer, init);
+}
+
 template <typename T>
 std::ostream& operator<<(std::ostream& ostream, const List<T> *lst)
 {
@@ -82,6 +94,7 @@ int main(void)
   std::cout << lst << std::endl;
   map<int, int>(lst = reverse(lst), [](int x){ return x * 2; });
   std::cout << lst << std::endl;
+  std::cout << reduce<int>(lst, [](int a, int b) { return a + b; }, 0) << std::endl;
   delete lst;
   return 0;
 }