(list)+filter function

This generates a new list, and appends to it every time the current
list value satisfies the condition.  It then recursively calls the
filter on the next value.

list.cpp

@@ -75,6 +75,16 @@ T reduce(List<T> *lst, T (*reducer) (T, T), T init = 0)
   return reduce(lst->next, reducer, init);
 }
 
+template <typename T>
+List<T> *filter(List<T> *lst, bool (*f)(T), List<T> *new_lst = nullptr)
+{
+  if (!lst)
+    return new_lst;
+  if (f(lst->value))
+    new_lst = append(new_lst, lst->value);
+  return filter(lst->next, f, new_lst);
+}
+
 template <typename T>
 std::ostream& operator<<(std::ostream& ostream, const List<T> *lst)
 {