Type Lists and Recursion at Compile Time
One firm's engineering blog published an exercise on removing adjacent duplicates from a compile-time vector using template metaprogramming, and variadic templates are probed directly elsewhere. The framing that makes all of it tractable is this: a parameter pack is a list, the type system is a purely functional language, and every operation is written the way you would write it in one.
The vocabulary
A variadic template takes a pack of types, and a class template holding one is a list.
template <class... Ts> struct TypeList {};
using Book = TypeList<Order, Level, Trade>;
There are no loops here and no mutation. There is pattern matching, in the form of partial specialisation, and recursion. So every algorithm has the same shape as its equivalent in a functional language: a base case for the empty list, and a step that peels the head off and recurses on the tail.
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