[boost] expected<T, E...> (Was: [outcome] High level summary of review feedback accepted so far)

Vicente J. Botet Escriba wrote:

You have taken some deviations from the proposal that could merit discussion

* missing sizeof...(E) >0 assertion

expected<T> is valid. Its purpose is to make expected{ expr } valid (the equivalent of make_expected.)

* unexpected_ type is not an explicit and different type

Could be done a different type, if this simple alias is determined to not work for some reason.

* inheritance of bad_expected_access<T> from from bad_expected_access<> :)

That enables you to catch all bad_expected_access errors. Without a base, you have to enumerate every possible E, and some of them may not be known until run time (because dlopen.)

* Additional throw_on_unexpected

This translates the E types into exceptions, as I explained in the doc.

https://github.com/pdimov/variant2/blob/master/include/boost/variant2/expect... I suspect that this is is_move_constructible

Correct, copy/paste mistake.

https://github.com/pdimov/variant2/blob/master/include/boost/variant2/expect... has_error/error should be defined only if sizeof... (E) is 1

In principle. I haven't gotten this right yet, hence the static_assert. The problem is that adding a template parameter interferes with has_error<E>/error<E>.

* unexpected()/error() should have a narrow contract (but this needs to be discussed in Toronto)

Please no.

I don't see how get<I> can work here.

https://rawgit.com/pdimov/mp11/develop/doc/html/mp11.html#mp11.reference.alg...

* ::then seems to be the monad::bind, could you confirm?

It's a monadic bind, yes. I'll remove .then in favor of operator>>, as documented.

* Wondering from constructors from convertibles expected from expected.

I'm not sure yet whether to enable value conversion. Pretty sure we don't want error conversions outside remap_errors.

* Factories are missing

Since we have argument deduction now, expected{ expr } and unexpected_{ expr } can be used for the purpose, and I saw no need for separate factory functions.

* I don't know if expected has been implemented

Not at the moment, although I intend to add it. It will basically change all T references throughout to void.