Re: [boost] Futures

Subject: Re: [boost] Futures
From: Gottlob Frege (gottlobfrege_at_[hidden])
Date: 2015-01-13 12:57:26

Let's see if I've captured everything:

- we want to get a value from (likely) another THREAD, and WAIT if it
is not yet ready UNTIL it is ready, (whatever THREAD WAIT UNTIL mean)
- a future (boost:: or std:: or hpx:: etc) is a (high-level!) wrapping
of that concept
- to be inter-operable we need the lower level concepts (in ALL-CAPS) exposed

THREAD:
we actually don't need to completely define what "another" THREAD is,
we only need a partial definition of our OWN THREAD. ie the *thread of
execution* of the current code,
because any code (that we assume at some point runs) runs in some
"thread of execution" - whether user mode thread, kernel thread, HPX,
whatever...
What we really need is access to the _controller_ of that thread of
execution - ie typically the scheduler, but not a full scheduler
interface.  In recent C++ proposals this has been called the
"execution agent", although I've been pushing for EXECUTION CONTEXT -
the C++ object/interface that controls the context of the execution of
the current set of instructions.
WAIT:
The running code needs to be able to get at its EXECUTION CONTEXT in
order to be able to ask it WAIT.
For a given EXECUTION CONTEXT, WAIT means stop "running" - stop using
CPU, etc. No further PROGRESS (which turns out to be a hard term to
define, but the committee is working on that)
UNTIL:
Some *other* EXECUTION CONTEXT needs to be able to tell the
aforementioned EXECUTION CONTEXT to RESUME.

Note that I've left off the actual value inside the future, and the
"waitable" object (kernel or otherwise).

The value is orthogonal - the value is just a memory location (unless
we abstract that away!) and can be set via atomic operations or
whatever. Once the value is set, you then call RESUME on the execution
agent.  From a value point-of-view, what you want is just an
ObservableValue.  ie when value is changed, call this callable
(function, lambda, whatever).  Given an ObservableValue, and an
ExecutionAgent you can build a promise.

Tony