[Haskell-cafe] Execution Contexts

[Ian.Stark at ed.ac.uk]

On Sat, 27 Nov 2004, Benjamin Franksen wrote:
> I would think that with ACIO we have a nice mathematical
> characterization for the IO actions that would be "safe" even at the
> top-level. ("Safe" meaning mainly that we do not open a can-of-worms
> with regard to execution order.) I don't know how easy or hard it is to
> prove of a certain IO action that is in fact in ACIO.

Hard, because it depends on observational equivalence of IO effects, and
for that you need a semantics for the RealWorld.

Maybe a better way to treat it is that whereas doing an IO action puts it
in an execution trace at a specific point, doing an ACIO action is simply
"perform this some time, maybe, if required".  Giving something like
newUnique an ACIO type indicates that semantics is sufficient; whereas for
readIORef it typically isn't, and you want the stronger guarantee of an IO
type.

> This breaks down as soon as the IO action does a forkIO.

Isn't sharing global variables the correct semantics for forkIO ?
That explicitly creates a 'lightweight' thread, which shares execution
context with its invoker.

I agree that forkOS, with its own local context, is harder.  I suspect
that, yes, as soon as you want to have more than one execution context
simultaneously, then you need to manage them.  For which XIO seems
to do the job.

Ian

--
Ian Stark                               http://www.ed.ac.uk/~stark
LFCS, School of Informatics, The University of Edinburgh, Scotland