list processing in modern c++

lisp.cc

#include <utility>

namespace demo {
    template <typename Hd, typename Tl>
    using cons_t = std::pair<Hd, Tl>;

    constexpr auto nil = nullptr;

    constexpr auto cons(auto hd, auto tl) {
        return std::make_pair(hd, tl);
    }

    template <typename Hd, typename Tl>
    constexpr auto car(const cons_t<Hd, Tl>& c) {
        return c.first;
    }

    template <typename Hd, typename Tl>
    constexpr auto cdr(const cons_t<Hd, Tl>& c) {
        return c.second;
    }

    constexpr size_t length(nullptr_t) {
        return 0;
    }

    template <typename Hd, typename Tl>
    constexpr size_t length(const cons_t<Hd, Tl>& c) {
        return 1 + length(cdr(c));
    }

    constexpr auto reverse(nullptr_t, const auto& c) {
        return c;
    }

    template <typename Hd, typename Tl, typename R = nullptr_t>
    constexpr auto reverse(const cons_t<Hd, Tl>& c, const R& r = nullptr) {
        return reverse(c.second, cons(c.first, r));
    }
}

using namespace demo;
int main(void) {
    constexpr auto a = cons(3.343434, cons("hello", cons(1, nil)));
    constexpr auto a_length = length(a);
    constexpr auto b = reverse(a);
    constexpr auto b_length = length(a);
    constexpr auto b_0 = car(b);
    
    return 0;
}

gcc-explorer