Lisp like C++ function calling with std::tuple and fold expressions

Question

The most basic thing I can think about Lisp is how to call functions. To those who doesn't know how to do that in Lisp, is something like that:

(fun1 a b (fun2 c d))
; c and d are parameters to the function fun2,
; and a, b and the result of fun2 are parameters
; to the function fun 1

Well, I want to do something like that in C++ using std::tuple and fold expressions. Basically, I have a tuple with 2 elements, the firts is a function that i want to call, and the second is a another tuple with the arguments. I was able to make this concept works with 1 function and putting the "Lisp tuple" in another tuple with the following code:

 auto add = [](auto ... args){ return (args + ...); };
 auto t1_args = std::make_tuple(10, 5);
 auto t1 = std::tie(add, t1_args);
 auto t_m = std::tie(t1);

 auto result = std::apply([](auto ... args){
     return (std::apply(std::get<0>(args), std::get<1>(args...)), ...);
 }, t_m);

 std::cout << result << std::endl; // prints 15

But I can't do that with multiple functions, multiple "Lisp tuples", one inside the other. Someone here could help me with that? Perhaps with another way to make the "Lisp tuple", but I really don't know how to make this works.

Answer 1

You have to evaluate each argument recursively, something like:

template <typename T>
auto eval(T t)
{
    return t;
}

template <typename ...Ts>
auto eval(std::tuple<Ts...> tuple)
{
    return std::apply([](auto... args){ return std::invoke(eval(args)...); }, tuple);
}

Demo.