Re: [boost] [next gen future-promise] What to call themonadicreturn type?

Subject: Re: [boost] [next gen future-promise] What to call themonadicreturn type?
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2015-05-29 17:46:06

On 29 May 2015 at 11:05, Peter Dimov wrote:

> > If so, I was going to have .then() cope with a R(T) callable, but that 
> > requires metaprogramming to examine the input and determine the overload.
> 
> You're not going to be able to tell the difference between .then and .next 
> when given [](auto x) { ... }.

Surely with Expression SFINAE you can?

But sure, it's sounding slower than a simple overload. And I'd like 
to retain VS2015 compatibility.

> > I think I saw you didn't like Vicente's catch_error(E)? Was there a reason 
> > why? Is there anything wrong with a .next(R(E))?
> 
> It's not that I don't like it, I don't see sufficient motivation for it.

Oh okay. In my situation, T and error_code are considered equals, 
only exception_ptr is exceptional.

> I'm 
> also not quite sure how it's to be used. Vicente's wording in N4048 is a bit 
> unclear.
> 
>     future<X> f1 = async( ... );
>     future<Y> f2 = f1.catch_error( []( exception_ptr e ) { return Y(); } );
> 
> What happens when f1 has a value? 

Surely for catch_error() one must always return the same type of 
future as the input? Otherwise it couldn't work when f1 has a value.

Niall

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