Function eval

sample/eval.cpp:5–18

template <typename T1, typename T2>
constexpr auto eval(std::tuple<char, T1, T2> const &expr)
{
  using namespace matchit;
  Id<T1> i;
  Id<T2> j;
  return match(expr)(
      pattern | ds('+', i, j) = i + j, pattern | ds('-', i, j) = i - j,
      pattern | ds('*', i, j) = i * j, pattern | ds('/', i, j) = i / j,
      pattern | _ = []
      {
        assert(false);
        return -1;
      });
}

#if __cplusplus > 201703L
constexpr auto result = eval(std::make_tuple('*', 5, 6));