C++: Expand array elements as parameter of a function using boost::hana

Question

I discovered two months ago boost::hana. Seems very powerfull so I decided to take a look. From the documentation I saw this examples:

std::string s;
hana::int_c<10>.times([&]{ s += "x"; });

that is equivalent to:

s += "x"; s += "x"; ... s += "x"; // 10 times

I'd like to know if it is possible (and if yes how) to write smthg like:

std::string s;
std::array<int, 10> xs = {1, 3, 5, ...};
hana::int_c<10>.times([&](int i){ s += std::to_string(xs[i]) + ","; });

a sort of "unpacking" at compile time, or even:

myfunction( hana::unpack<...>( xs ) );

Answer 1

Your question seems twofold. First, the title of your question asks whether it is possible to expand the elements of an array as the parameters of a function. It is indeed possible, since std::array is Foldable. It suffices to use hana::unpack:

#include <boost/hana/ext/std/array.hpp>
#include <boost/hana/unpack.hpp>
#include <array>
namespace hana = boost::hana;


struct myfunction {
    template <typename ...T>
    void operator()(T ...i) const {
        // whatever
    }
};

int main() {
    std::array<int, 10> xs = {{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}};
    hana::unpack(xs, myfunction{});
}

Secondly, you ask whether it is possible to do something like

std::string s;
std::array<int, 10> xs = {1, 3, 5, ...};
hana::int_c<10>.times([&](int i){ s += std::to_string(xs[i]) + ","; });

The answer to this is to use hana::int_c<10>.times.with_index:

hana::int_c<10>.times.with_index([&](int i) { s += std::to_string(xs[i]) + ","; });    

Equivalently, you could also use hana::for_each:

hana::for_each(xs, [&](int x) { s += std::to_string(x) + ","; });