Re: [boost] [review] **NEXT WEEK** Review of Outcome (starts Fri-19-May)

Subject: Re: [boost] [review] **NEXT WEEK** Review of Outcome (starts Fri-19-May)
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2017-05-15 11:29:20

>> Also, to construct some object, it takes time. error_code is just two
>> words, but why waste time on setting them if you will be overriding
>> them in the course of the function call.
> 
> Overly focusing on outcome<>'s runtime performance is odd given that
> it's usually returned by functions that take millions of cycles to do
> their job.

That may not be the case in some use cases. I've tried very hard to make
Outcome's runtime overhead statistically indistinguishable from C
integer error code returns (see FAQ for graph).

> But either way, it's possible to optimize the default
> construction to still set a bit and only create the error_code on demand
> when it's retrieved.

Outcome does do this, but in a different situation. If you try to
retrieve a .error() from an excepted Outcome, you get back an error_code
of monad_errc::exception_present with category monad_category.

>> And, in the current state you can perform a loss-less conversion (or
>> "upgrade") form `option<T>` to `outcome<T>`.
> 
> option<T> is only present because of the empty state; no empty state, no
> option<T>. Duplicating opional<T> is questionable, upconverting it to
> outcome<T> is, too. If you call a function that optionally gives you an
> object T, and you have to return outcome<T>, you need to provide a
> reason for the missing T, and the silent upconversion allows you to
> punt. Error context is lost because only you know why your
> implementation detail failed to provide the T you asked of it.

I think you might be underestimating the bug detecting value of an
outcome<T> having an unhandled empty state. It traps failure to handle
option<T> correctly very nicely.

Also, don't forget option<T> is the only one from result<T> and
outcome<T> which is usable in constexpr programming. One therefore ends
up using option<T> or expected<T, E> in constexpr and feeding the output
to result<T> or outcome<T>.

Niall

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