Re: Announcing 8sync: an asynchronous programming language for Guile

[William ML Leslie]

On 6 December 2015 at 01:58, Christopher Allan Webber
<address@hidden> wrote:
> Amirouche Boubekki writes:
>> 8sync has two types of async-request:
>>
>> ** run-requests, which implements kind of a *coroutine* behavior.
>>
>> It pause the execution of the current procedure and schedule
>> the provided lambda to be run soonish; This doesn't exists in
>> async.scm. The only thing useful this can do, is break the callstack
>> to allow deeper recursion.

An aside on concurrency models: being able to do this - pause
execution of one turn and allow another to run - creates a strictly
more powerful model, but with very different properties for the
purposes of analysis and auditing.  It means that any time you call a
function, you need to ensure that if there are properties that a turn
must maintain - such as there being a constant amount of money in the
system, for example - that this property is maintained /before/ the
call.

This feature is known as 'stale stack frames' in the literature
(particularly in E circles), and in an ideal async world, we'd never
need to do this.  There's a really accessible post at
https://glyph.twistedmatrix.com/2014/02/unyielding.html about what can
happen as codebases grow in a system with the potential for stale
stack frames.  I also think
https://wingolog.org/archives/2012/02/16/unexpected-concurrency is
good to review when discussing this, even though it's dealing with a
slightly different cause.

I'm not saying that it's a bad idea to expose it in 8sync, btw.  One
great thing about Guile is that it gives you options about how you
want to do concurrency.  But libraries that use this feature (or may
decide to use it in the future) should come with a large, flashing
warning - calling these functions may allow other events to run!

-- 
William Leslie

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