[boost] Really dumb off topic question regarding variant

Subject: [boost] Really dumb off topic question regarding variant
From: Robert Ramey (ramey_at_[hidden])
Date: 2019-02-22 22:54:55

On 2/22/19 9:48 AM, Peter Dimov via Boost wrote:
> I'd like to request a formal Boost review for the Variant2 library,
> https://github.com/pdimov/variant2. Variant2 contains an implementation 
> of a
> variant<T...> type that is an almost conforming std::variant, except it
> doesn't require C++17 and supports C++11 and above.
> 
> It's also never valueless and has a few other extensions.
> 
> The same library also contains an expected<T, E...> type, which is like the
> proposed std::expected<T, E>, but supports more than one error type.
> expected<> is not yet production-ready and has no test suite, but I will
> finish it if the library is accepted.

I see optional, expected, outcome and .. (monad?) as just special cases 
of variant. for example

template<typename T>
using optional = variant<false_type, T>;

Why is it necessary to have all these types separately implemented? 
Can't there be some sort of "base" type which can be used to implement 
all these others?  Wouldn't this approach make things much simpler to 
review, maintain, and use?

Robert Ramey