> Filters are used all the time in C++. The problem is that you need a vastly more complex machinery to cover all the various ways to avoid copying/reference c

> Filters are used all the time in C++. The problem is that you need a vastly more complex machinery to cover all the various ways to avoid copying/reference counting and then still don't have a general solution when you can't avoid multiple ownership (unless you count std::shared_ptr with its very high overhead). As I said before, it's not that you can't do it, it's that there are costs associated with it. > If you need both the original list and the the filtered list, in a functional language you need to allocate new cons cells anyway. You can filter arrays also and allocating cons cells for a list is pretty cheap with a modern GC, as discussed before.