Re: [boost] [next gen futures] Lightweight monad ready for inspection

Subject: Re: [boost] [next gen futures] Lightweight monad ready for inspection
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2015-06-29 06:35:37

On 29 Jun 2015 at 20:08, Gavin Lambert wrote:

> > There is also another potential design problem in that get_error()
> > returns a null error_code if the monad is not errored even if it is
> > excepted. I did it that way to match get_exception() as
> > boost::future<T>::get_exception_ptr() returns a null exception_ptr if
> > there is no exception. However there is a discrepency, because if the
> > monad is errored not excepted and you call get_exception(), you'll
> > get an exception_ptr to the system_error for the error_code, and it's
> > not the same the other way round.
> 
> Perhaps there should be an error code that represents "has an 
> exception"?  (Either from a custom category or finding an appropriate 
> system error to hijack for this purpose.)

I already have a monad_category and a monad_errc which can do this.

> I don't like the idea of someone writing code that only tests the 
> error_code (because they weren't expecting exceptions), but which claims 
> there is no error because there really was an exception instead. 
> Exceptions are errors are exceptions, and neither is an ordinary result 
> value.

It's definitely a very valid concern. I'll sleep on it and make a 
decision.

Thanks for commenting on this problem, it's a particular worry of 
mine.

Niall

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