Dan Weston wrote:
In the GHC docs: http://www.haskell.org/ghc/docs/6.4.1/html/users_guide/sec-ffi-ghc.html#usin...
"There can be multiple calls to hs_init(), but each one should be matched by one (and only one) call to hs_exit()[8]."
What exactly happens with nested calls? Is there only one runtime created, with a simple counter to know which hs_exit should shut it down?
Yes. But the runtime doesn't currently support restarting (hs_exit() followed by hs_init()).
If so, is there a way of having multiple interpreters open safely at the same time?
I'm not sure exactly what you mean by "multiple interpreters". The runtime is only designed to support a single instance of itself (it uses global static storage everywhere).
Or does each hs_init() create a new separate concurrent runtime (the preferable alternative)?
Nope, see above. To do that you'd need a "runtime handle" returned by hs_init() and passed to every foreign exported function, somehow.
And what is the cost of creating and destructing the GHC runtime anyway?
Not much.
Can the Haskell interpreter be in a Linux shared-object library, so long as I make sure to call hs_init() after loading and hs_exit() before unloading it? My experiments so far show this working flawlessly, but I vaguely remember an e-mail thread saying GHC couldn't be linked in dynamically.
Perhaps, although the shared library won't really be shared - it'll be linked in place each time you use it, because we don't currently have support for PIC everywhere (well, we have partial support and there's a SoC project to finish it off). Cheers, Simon.