important news: refocusing discussion

[Simon Marlow]

On 28 March 2006 00:24, Ross Paterson wrote:

> On Mon, Mar 27, 2006 at 09:36:28AM +0100, Simon Marlow wrote:
>
>> The portable interface could be Control.Concurrent.MVar, perhaps.
> 
> As Malcolm pointed out, using MVars requires some care, even if you
> were just aiming to be thread-safe.

I don't really understand the problem, maybe I'm missing something.  I
thought the idea would be that a thread-safe library would simply use
MVar instead of IORef.  So instead of this:

   do 
      x <- readIORef r
      ...
      writeIORef r x'

you would write

   do
      modifyMVar_ r $ \x ->
        ...
        return x'

actually the second version is not only thread-safe, but exception-safe
too.

Malcolm's objections:

> But Q2 explicitly raises the issue of whether a non-concurrent
> implementation must still follow a minimum API.  That could be a
> reasonable requirement, if we fleshed out the detail a bit more.
> The specific suggestion of requiring MVars makes me a tiny bit
> worried though.  After all, MVars capture the idea of
> synchronisation between threads, and I assume that since a
> non-concurrent implementation has only one thread, that thread will
> be trying to MVar-synchronise with something that does not exist,
> and hence be blocked for ever.  I can imagine that there are
> situations where synchronisation is entirely safe
> and free of blocking, but how to specify when it would be unsafe?

There's no synchronisation, because we're not writing multi-threaded
code here.  Just code that doesn't have any race conditions on its
mutable state when run in a multi-threaded setting.

Maybe you could elaborate on what problems you envisage?

Back to Ross:
> How about STM (minus retry/orElse) and TVars as the portable
> interface? They're trivial for a single-threaded implementation, and
> provide a comfortable interface for everyone.

Now that's a rather good idea.   It does raise the bar for the
concurrent implementations, though, and STM is not nearly as mature and
well-understood as MVars.  There do exist implementations of STM in
terms of MVars (at least two I know of).

Cheers,
	Simon