On 7/10/07, skaller <[EMAIL PROTECTED]> wrote:
> So actually we have not got this right yet.
>
> The last version of Felix added typeclasses, and it
> changed the whole way we approached making libraries:
> we start to use typeclasses instead of open overloading,
> but typeclasses make you think much harder about your
> abstractions, which is good. But the conversion isn't
> finished because we're still confused.. this isn't
> a Felix language issue but a library issue.
>
> It needs to be driven by what users expect tempered
> with a nice theoretical model .. so ..
>
> What do you want?
Consider an I/O model based on promises/futures with continuations. By
this, I mean that all I/O calls return a promise. When the code needs
the results of the I/O to complete, it invokes a "when(promise) { ...
}" which waits until the promise/future has settled to a value, ie.
the I/O has completed, then continues the computation with that value.
In essence, "when" just takes a continuation for the rest of the
computation, and queues it on the promise. So, in pseudo OCaml:
let processFile fd =
let bytes = (read fd 1024) in (* read() returns a promise for a string *)
let otherstuff = (* do some other processing which does not depend
on bytes *) in
when bytes do
print bytes;
(* do something else with bytes *)
;;
is auto-transformed into:
let processFile fd =
let bytes = (read fd 1024) in
let otherstuff = (* do some other processing which does not depend
on bytes *) in
when bytes (fun bytes:string -> print bytes; (* do something
with bytes *))
;;
When 'bytes' settles to a value, the anonymous function is invoked,
but the pre-transformation code still looks sequential and quite
straightforward. I believe this achieves optimal asynchrony with very
little mental overhead. It's the approach I plan to use in my VM.
You can take this even further and make all computation via such
calls, and concurrency reduces to safe event-loops. See "Robust
Composition: Towards a Unified Approach to Access Control and
Concurrency Control" [1].
Sandro
[1] http://www.erights.org/talks/thesis/
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