Function main

example/string/foldable.cpp:15–26

int main() {
    auto sum_string = [](auto str) {
        return hana::fold_left(str, hana::int_c<0>, [](auto sum, auto c) {
            constexpr int i = hana::value(c) - 48; // convert character to decimal
            return sum + hana::int_c<i>;
        });
    };

    BOOST_HANA_CONSTANT_CHECK(
        sum_string(BOOST_HANA_STRING("1234")) == hana::int_c<1 + 2 + 3 + 4>
    );
}