Hana Caclulator

calc.cpp

#include <boost/hana.hpp>

#include <iostream>
#include <string>
#include <type_traits>
#include <utility>
#include <memory>
#include <cassert>
using namespace boost::hana;
using namespace boost::hana::literals;
using namespace std::literals;

#define RETURNS(...) -> decltype(__VA_ARGS__) { return __VA_ARGS__; }

template<char C>
struct fun;

template<>
struct fun<'+'>
{
    template<class T, class U>
    constexpr auto operator()(T&& x, U&& y) RETURNS(x+y);
};

template<>
struct fun<'-'>
{
    template<class T, class U>
    constexpr auto operator()(T&& x, U&& y) RETURNS(x-y);
};

template<char C>
using digit = std::enable_if_t<
    (C >= '0' && C <= '9'), 
    _integral_constant<int, static_cast<int>(C - 48)>
>;

BOOST_HANA_CONSTEXPR_LAMBDA auto evaluate = [](auto x, auto op, auto y) 
RETURNS(fun<decltype(op)::value>()(digit<decltype(x)::value>()(), digit<decltype(y)::value>()));

auto try_evaluate = overload_linearly(
    evaluate,
    always(nothing)
);

template<char... Cs>
auto try_evaluate_c() RETURNS(try_evaluate(char_<Cs>...));

int main() {
    BOOST_HANA_CONSTEXPR_CHECK(evaluate(char_<'1'>, char_<'+'>, char_<'2'>) == 1 + 2);

    BOOST_HANA_CONSTANT_CHECK(try_evaluate_c<'?', '+', '2'>() == nothing);
    BOOST_HANA_CONSTANT_CHECK(try_evaluate_c<'1', '?', '2'>() == nothing);
    BOOST_HANA_CONSTANT_CHECK(try_evaluate_c<'1', '+', '?'>() == nothing);
    BOOST_HANA_CONSTANT_CHECK(try_evaluate_c<'?', '?', '?'>() == nothing);
}