Implement lenses for std::tuple, std::variant and std::optional about lager

[arximboldi]

Yes, all those lenses and cursors transformations are indeed useful and may be added soon. Thanks! 👍

[arximboldi]

You did this, hero! @Siapran

[Siapran]

I need to sort out licensing with my managers but I have a working solution for handling unfocused lenses using optionals: sample unit tests

[Siapran]

got an answer on licensing. I'll submit a PR for review soon.

[arximboldi]

Awesome! Those tests are looking good, I like that approach. Looking forward to look at that code 🙂

[Siapran]

I'm trying to make a smart lens for automatically lifting anything that doesn't handle optionals, but I'm having trouble detecting this properly:

struct functor_wrapper_t {
    template <typename T>
    const_functor<T> operator()(T&& t) const noexcept;
};

template <typename Lens, typename T>
using viewed_t = decltype(
    std::declval<Lens>()(functor_wrapper_t{})(std::declval<T>()).value);

template <typename T>
struct smart_lens<std::optional<T>>
{
    template <typename U>
    static auto make(U &&u) {
        // don't lift lenses that can already handle optionals
        if constexpr (zug::meta::is_detected<viewed_t, U, std::optional<T>>::value) {
            return std::forward<U>(u);
        } else {
            return ::lager::lens::optlift(smart_lens<T>::make(std::forward<U>(u)));
        }
    }
};

TEST_CASE("zooming, unfocused")
{
    state<std::vector<person>> st{{person{{5, 4}, "juanpe"}}};
    auto p  = st[0];
    auto n1 = p[&person::name];
//    auto n2 = p[lens::attr(&person::name)]; // detected as able to handle optional
    auto n3 = p[lens::fallback(person{{1, 1}, "NULL"})];
    auto n4 = p[lens::optlift(lens::attr(&person::name))];
}

I might need some help to finish implementing this...