[boost] future/packaged_task Sean Parent design

Subject: [boost] future/packaged_task Sean Parent design
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2015-06-23 19:09:00

Hi,

Sean Parent suggests (See [1] Better Code: Concurrency) to remove the 
promise and left the packaged_task serve as single provider of a future. 
Both are created at once using the package factory.

His future library doesn't include a shared_future as the future 
contains a value that can be observed several times.
The continuation takes the value and not the future. So a future can 
have several continuations. If the store type is movable only, then the 
future can have only one continuation.
Not really strange coming from Sean, future and packaged_task are 
copyables :)
One more thing, there is not get no wait, but a get_try that return an 
optional<T>.

Scales better. IIUC the implementation allows to implement when_all on 
top of future/packaged_task. The trick is that that the packaged_task 
can store some context.

async has always an Scheduler as parameter.
Scheculers are just Copyable Callables having this signature 
void(void()). There is no need for function add/spawn.

when_all has also a callable as argument.

I find the Sean's design clean and simple (See the code [2]). What do 
you think of it? Do you see any issues on it?
* I don't understand why when_all continuation has the futures as 
parameters (I suspect that this is a type on the documentation).
* I would rename get_try to try_get. This function could return 
optional<expected<T>>.

What are you missing ?
* catch_error/recover that is already planed.
* when_any

Best,
Vicente

[1] 
https://github.com/sean-parent/sean-parent.github.io/wiki/Papers-and-Presentations
[2] https://github.com/stlab/libraries/tree/develop