FFI, safe vs unsafe

[Simon Marlow]

What are the conclusions of this thread?

I think, but correct me if I'm wrong, that the eventual outcome was:

 - concurrent reentrant should be supported, because it is not 
   significantly more difficult to implement than just concurrent.

 - the different varieties of foreign call should all be identifiable,
   because there are efficiency gains to be had in some implementations.

 - the default should be... concurrent reentrant, presumably, because
   that is the safest.  (so we need to invert the notation).

So, can I go ahead and update the wiki?  I'll try to record the
rationale from the discussion too.

I'd like to pull out something from the discussion that got a bit lost
in the swamp: the primary use case we have for concurrent reentrant is
for calling the main loop of a GUI library.  The main loop usually never
returns (at least, not until the application exits), hence concurrent,
and it needs to invoke callbacks, hence reentrant.

Cheers,
	Simon