Re: [boost] Really dumb off topic question regarding variant

Subject: Re: [boost] Really dumb off topic question regarding variant
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2019-02-22 23:20:30

> 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?
The reason is that there are different tradeoffs at work. I cover this a
bit in https://ned14.github.io/outcome/history/#outcome-v2 where I
explicitly mention variant2, but to summarise:

1. Outcome has a simple implementation with minimum SFINAE, a fixed ABI
and aims for low impact on build times in order to maximise usefulness
in public interfaces in large codebases.

2. variant2 has a less simple implementation, enabling more flexibility,
better code density, but at the cost of build time impact.

3. A fully conforming Expected can be implemented easily using either
Outcome or variant2. See
https://ned14.github.io/outcome/faq/#how-far-away-from-the-proposed-std-expected-t-e-is-outcome-s-checked-t-e.

4. Optional is an interesting one. Outcome uses an internal optional
implementation because reusing Optional wasn't efficient. Specifically,
I needed more than a single bit discriminant, and Optional wasn't
reusable for that with doing UB.

Niall