On 29 March 2006 14:35, John Meacham wrote:
On Wed, Mar 29, 2006 at 02:05:35PM +0100, Simon Marlow wrote:
What you are suggesting is that there may be implementations that do not support reentrant/blockable, but do support the others. And in that case, of course you really need to know the difference between blockable and reentrant. I'm just not sure the standard should allow such implementations.
It would be a very odd thing to disallow, seeing as how it would rule out or at least place fairly large implementation restrictions on yhc,hugs and jhc and not a single foreign import in the standard libraries needs to be 'blockable reentrant' as far as I can tell.
though, I do need to look at hugs and yhcs source more carefully to know whether that is the case for sure. it depends on a lot of implementation details and how they handle OS-level threads and blockable in general.
Ok, let's explore how difficult it really is. Take a single-threaded implementation that forks OS threads for concurrent foreign calls. Let's call the OS thread running Haskell code the "runtime thread". An OS thread wanting to call a foreign export must make an RPC to the runtime thread. You could do this by: - have a channel for RPC requests - the callee creates a condition var for the result, and sends the call details down the channel with the condition var. - the runtime thread picks up the request in its event loop and forks a Haskell thread to handle it - when the Haskell thread completes it places the result in the right place and signals the condition variable - the callee picks up the result, frees the resources and continues on its merry way can't be more than a couple of days work, unless I'm missing something? It's not particularly fast, but then call-ins in GHC aren't fast either. Cheers, Simon