On 30 March 2006 22:01, Claus Reinke wrote:
the point being: the FFI says something about how to integrate foreign and Haskell memory management; should it also say something about threadability of foreign code (wrt to scheduling, and wrt thread-safety)? If Haskell' includes concurrency then of course it must say something about the behaviour of foreign calls with respect to concurrency (if that's what you mean by "threadability"). Is that what you're asking?
yes. I'm all for Haskell' having both ffi and concurrency, but that means that the feature interactions have to be investigated. I know you made a start on that, so I was just pointing out some further corners that may need looking into:
- if foreign code is used in Haskell' scheduling, what properties of the foreign code need to be specified, how does long-running foreign code interact with the scheduler, and are the current annotations sufficient for all that?
- if Haskell' code involving concurrency is foreign exported, what does that mean? does it work out of the box, or what are the limitations?
Rather than answer these directly, I'll point to this paper which addresses these questions and more: http://www.haskell.org/~simonmar/papers/conc-ffi.pdf GHC implements exactly what is in that paper. For Haskell', I expect we won't require all of it, some will be relegated to extensions.
I was looking for a practical example involving concurrency and ffi in such a way as to expose some of the questions that need to be answered. you're mentioning forkIO, so that's an example of such questions: why shouldn't we be able to foreign export that?
The (slightly glib) answer to this question is "because it doesn't have a legal FFI type". If you want to foreign export forkIO, you'll have to give me a version that I can foreign export. And that involves some important decisions: just where do you want to run the IO() action, on the server, or the client? If it's on the server, then how to I get the IO() in the first place? YOu must provide some more API functions to build a FunPtr(IO()), or something. (by server I mean the library proferring the forkIO API, by client I mean the client of this API). Cheers, Simon