Does anyone plan to add support for multiple OS threads to Hugs or NHC?
I think it will depend a bit on the complexity so let me sketch how I think it can be implemented.
First let me outline my current understanding of what 'bound' means. Consider the following scenario:
Haskell program is running in OS thread 't1' Haskell program calls C function 'foo'. 'foo' forks a new OS thread 't2'. In parallel: 't1' calls Haskell function 'f1' and 't2' calls Haskell function 'f2' 'f1' calls C function 'g1' 'f2' calls C function 'g2'
My understanding is that 'bound' requires that 'g1' be executed by thread 't1' and that 'g2' be executed by thread 't2'. It would be nice if 'f1' and 'f2' could run simultaneously but the ffi is not going to impose that on us. If 'f1' were to block on an MVar, 'f2' could start running and vice-versa. While 'g1' is running, 'f2' can run and while 'g2' is running, 'f1' can run.
Yes, if the Haskell functions 'f1' and 'f2' are both exported using "bound", that is
foreign export bound "f1" f1 :: something foreign export bound "f2" f2 :: something
Based on this understanding, I believe that single-threaded runtimes could easily implement 'bound' by doing nothing more than using a lock to ensure that at most one OS thread executes Haskell code at once.
That was the general intention.
Things might get more complex though if you want non-blocking foreign calls (a.k.a. "threadsafe"). Then you could either always create one OS thread for every Haskell thread, or use a slightly more complex scheme (as GHC does, but I'm sure there are other ways of doing this). Implementing "threadsafe" is basically independent from implementing the bound threads proposal, i.e. you can implement one without implementing the other (GHC currently has "threadsafe", but "bound threads" are still science fiction).
Cheers,
Wolfgang
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